Does the HLA work on all web browsers?
Currently the HLA is supported onFirefox,Safari (version 3), andInternet Explorer (version 7). You may find that the HLA interface mostly works on other modern browsers, but you may find some aspects that do not work correctly. We are interested in feedback on your experience using other browsers. We may not be able to fix problems quickly in browsers other than Firefox and Safari. We will also endeavour to maintain Internet Explorer compatibility, but due to the many differences between IE and the other browsers, it is possible that some future features will first appear for Firefox and Safari and only later will be supported on IE. We encourage the use of standards-supporting browsers.
In the future we plan to support Google Chrome, but at the moment the HLA interface does not work in that browser.
To fully realize all of the HLA's functionality, one must have cookies and popups enabled.
Are data from all HST instruments available via the HLA?
Yes, with the exception of data from the FOC, HSP, and WF/PC (the pre-1994 aberrated camera), all the HST instruments are represented in the HLA. All instruments are shown in the search results (including the footprints view), and data from all instruments are available for download. Some instruments have enhanced data products developed for the HLA while others have the standard products available from the HST archives. The matrix below gives more detail:
| Instrument/Product | Source | HLA Enhanced Products 1 |
Download
Format |
Interactive
Display? |
|
|---|---|---|---|---|---|
Notes:
|
|||||
| ACS/combined images | STScI | ~95% 2 | FITS | ✔ | |
| ACS/source lists | STScI | ~70% (e.g., not very crowded fields) | Ascii | ✔ | |
| ACS/grism extractions | ST-ECF | 10% 3 | FITS | ||
| ACS/mosaic images | STScI | small prototype sample | FITS | ✔ | |
| WFPC2/combined images | CADC | ~95% | FITS | ✔ | |
| WFPC2/source lists | STScI | ~70% | Ascii | ✔ | |
| NICMOS/grism extractions | ST-ECF | ~80%, 1-D & 2-D spectra | FITS | ||
| NICMOS/images | STScI | 30% 4 | FITS | ✔ | |
| STIS/images and spectra | STScI | FITS | ✔ | ||
| FOS/spectra | STScI | Tar | |||
| GHRS/spectra | STScI | Tar | |||
| ACS,WFPC/Contributed Products | Community | FITS | ✔ | ||
Note that footprints (but not previews or the data itself) are available for proprietary WFPC2 and NICMOS data, but are NOT available for proprietary ACS data (for historical/technical reasons).
How accurate are the HLA data?
Please note that while the HLA images and source lists will be sufficient for many people's science, by necessity these products are developed for general usage rather than being "tuned" for a specific scientific goal, hence in many cases the optimal science can be achieved by going back to the original STScI calibration pipeline data and processing on a chip-by-chip basis. Also, for some projects it is desirable to use data at their original pixellation; HLA-produced data have been resampled to a uniform grid to correct for geometric distortions. Nonetheless, these are in fact science-quality images that are suitable for use in many research projects.
More details are available regarding the accuracy of the HLA astrometry and photometry.
How good is the astrometry for the images and the source lists?
HLA processing attempts to correct the astrometry of most images by matching sources with one of more of three catalogs, namely the Guide Star Catalog 2 (GSC2), 2MASS, and the Sloan Digital Sky Survey (SDSS) catalog; results are recorded in the astrometry keywords in the image header. The absolute astrometry is adjusted to match that of SDSS if a sufficient match is available, otherwise to GSC2; 2MASS is used to assess the quality of the solution. An analysis of the astrometric correction applied and its internal consistency for a subset of ACS data is presented in astrometry_all.summary.
When a correction is possible, the typical absolute astrometric accuracy of the HLA-produced images is ~ 0.3 arcsec in each coordinate. About 80% of the ACS WFC images have corrected astrometry. This astrometric correction is not possible for many images because of crowding, lack of matching sources (especially for instruments with small field of view, such as ACS/HRC and NICMOS), or sources that are unresolved from the ground. In such cases, the absolute astrometry is typically accurate to ~1-2 arcsec in each coordinate, as for the standard pipeline products.
The SDSS, 2MASS, GSC2, FIRST and GALEX catalogs are available for overlaying on the images in the interactive display and provide an excellent way to check the absolute astrometry for a particular image. The astrometry accuracy varies for the external catalogs and also varies from source-to-source. The approximate accuracy of the catalogs is given below:
| Catalog |
Positional Accuracy (RMS, arcsec) |
Description |
|---|---|---|
| SDSS | 0.15 | Sloan Digital Sky Survey DR5 catalog |
| 2MASS | 0.15 | Two Micron All Sky Survey Point Source Catalog |
| GSC2 | 0.25 | Guide Star Catalog 2.3.2 |
| FIRST | 0.50 | Faint Images of the Radio Sky at 20 cm |
| GALEX | 1.0 | Galaxy Evolution Explorer GR4 catalog |
How do I find moving targets (e.g., planets, asteroids, ...) in the HLA?
There are two ways to find moving targets. Under advanced search, click the box to the right of "Moving Targets only" and then do an all-sky (0 0 r=180) search. (Note that when the Moving Target box is checked, an all-sky search is done by default when the search box is empty.) The search will return only observations where HST was tracking the target during the exposure. You can restrict the choice of instruments, filters, etc. using the other search parameters.
The other approach is to use the ACS moving target list and the WFPC2 moving target list. These lists provide information on the target name, detector, filters, proposal ID, etc., in a compact tabular format. They also include links enabling easy HLA searches for particular observations and can be a convenient way to browse the moving target data.
How can I find a particular proposal ID number?
There are two ways to find data from a particular proposal ID. The first is to use the Proposal ID text box in the advanced search options. An all-sky search with a specified proposal ID (either by leaving the search box blank or entering "0 0 r=180") will return all data from that proposal. The second method is to do an all-sky "0 0 r=180" search and then filter the results by the desired proposal ID using the blank box under the PropID column in the inventory view. The first method is usually preferred since it is considerably faster. Note that the second method can only be used for one instrument at a time, while the first can be used with multiple instruments selected. The filtering approach will be more convenient only if the region of the sky being searched is already fairly restricted, in which case data from other proposals may also be of interest.
Note that the footprint view is not available when doing all-sky searches. In order to enable the footprint view for the data from a specific proposal ID, one can run the search again specifying a region of the sky suitable to include data from that proposal.
Can I search for a list of targets? What formats are allowed for the user-defined search lists?
It is possible to use your own coordinate list to search the HLA, using the "Position List File Upload" feature after clicking the advanced search button.
Lists can be delimited in various ways (whitespace,
commas, tabs, etc.) and can have positions given in
degrees or in sexagesimal format (hr min sec deg min
sec, with the fields separated by blanks or colons).
The position file must have one position per line,
with the RA and Dec (J2000) being the first two
columns on the line. Lines beginning with a hash
symbol (#) are treated as comments and ignored.
Trailing information on the line is ignored.
How can I find some interesting looking HLA images?
The target list upload feature can be used to find interesting images, e.g., from the NGC or Messier catalogs. Here are some lists that we generated:
Interesting WFPC2 images, Messier objects, Interesting ACS images, Harris globular clusters, Abell clusters, and Spitzer SINGS galaxies
Are there any Help movies available?
Yes, there are currently five movies:
How
to search with the HLA (~5 min)
An Introduction to HLA Footprints (~4 min)
Field Guide to ACS Image Anomalies (~4 min)
How
to use the Interactive Display (~5 min)
How
to use the shopping cart (~3 min)
How can I tell if the astrometry has been corrected for a given image and whether the image has been matched to the SDSS, GSC2, or 2MASS catalog?
World Coordinate System (WCS) coordinates are encoded in the CRVAL1 (right ascension of the "reference pixel") and CRVAL2 (declination of the "reference pixel") keywords in the header of all FITS images. You can view the image header using the "imheader" command in IRAF (or the equivalent command in other reduction packages) or by using your normal text editor.
The absolute astrometry for HST images typically has errors in the range 1 - 2 arcsec, due to uncertainties in the postions of the guide stars. We attempt to correct for these errors in the HLA by comparing the positions of stars in the images with stars in one of three catalogs, 1) the Sloan Digital Sky Survey (i.e., SDSS), 2) the Guide Star Catalog 2 (i.e., GSC2), and 3) the Two Micron All Sky Survey (2MASS).
At present, the order of preference is SDSS, then GSC2. 2MASS solutions are also considered (and included in the header, see below), but are not propagated into the active CRVAL even if there is no SDSS or GSC2 solution. This is likely to change in the near future.
The original CRVAL parameters, as well as the equivalent values based on comparisons with the GSC2, SDSS, and 2MASS, are all stored in the headers of HLA images. Here is an example for imagehst_05993_01_wfpc2_f606w_wf (near Abell 1689).
This is the active astrometric zeropoint:
CRVAL1 = 197.9593583858333 / right ascension of reference pixel (deg) CRVAL2 = -1.325279720833333 / declination of reference pixel (deg)
These are the original CRVAL parameters from the "raw" image (i.e., before drizzling and rotation to north-up; hence mainly useful as an historical record since the values are for a different frame of reference than used for the HLA images).
OCRVAL1 = 197.9451240351 / right ascension of reference pixel (deg) OCRVAL2 = -1.325601571138 / declination of reference pixel (deg)
This is the original astrometric zeropoint after drizzling and rotation to north-up, but BEFORE an astrometric correction has been made based on matching to SDSS, GSC2, or 2MASS, as described above.
O_CRVAL1 = 197.9592647 O_CRVAL2 = -1.32533505
This is the solution, offset, RMS, and number of stars used in the solution versus Guide Star Catalog 2
G_CRVAL1 = 197.9593676566667 / GSC2 CRVAL1 G_CRVAL2 = -1.325229256111111 / GSC2 CRVAL2 G_DRA = 0.370644 / GSC2 delta RA in arcsec G_DDEC = 0.380858 / GSC2 delta DEC in arcsec G_RMSRA = 0.545364 / GSC2 rms in arcsec G_RMSDEC = 0.269053 / GSC2 rms in arcsec G_NMATCH = 15 / GSC2 number of sources
This is the solution, offset, RMS, and number of stars used in the solution versus the Sloan Digital Sky Survey
S_CRVAL1 = 197.9593583858333 / SDSS CRVAL1 S_CRVAL2 = -1.325279720833333 / SDSS CRVAL2 S_DRA = 0.337269 / SDSS delta RA in arcsec S_DDEC = 0.199185 / SDSS delta DEC in arcsec S_RMSRA = 0.204462 / SDSS rms in arcsec S_RMSDEC = 0.158806 / SDSS rms in arcsec S_NMATCH = 38 / SDSS number of sources
This is the solution, offset, RMS, and number of stars used in the solution versus the Two Micron All Sky Survey
T_CRVAL1 = 197.9593402116667 / 2MASS CRVAL1 T_CRVAL2 = -1.3252723075 / 2MASS CRVAL2 T_DRA = 0.271842 / 2MASS delta in arcsec T_DDEC = 0.225873 / 2MASS delta in arcsec T_RMSRA = 0.251613 / 2MASS rms in arcsec T_RMSDEC = 0.06729300000000001 / 2MASS rms in arcsec T_NMATCH = 3 / 2MASS number of sources
Hence in the example above, the SDSS solution has been used to define the astrometric solution for the HLA image, since the CRVAL and S_CRVAL values match. Note that the best way to check the absolute astrometry is to overlay the three catalogs using the interactive display tool in the HLA.
Why do searches sometimes return observations that fall outside the search radius?
Sometimes the search results include observations that fall outside the requested search area. This is the result of a shortcut in the search logic, which makes the searches as fast as possible but results in some inaccuracies. The results are guaranteed to include all observations that should be included; it never omits data that ought to be included. But it can also include additional observations due to a fuzzy outer search radius that depends on the orientation of the observation. For a 1 degree search radius, it is possible for observations as far as 1.4 degrees from the search position to be included.
Note that zero-radius searches are accurate. A search using r=0 finds observations that actually cover the search point, and the approximation in the search logic does not affect that case.
In the future we plan to fix this, assuming that we can identify an efficient algorithm that does not degrade the search performance. Until then you should be aware that HLA searches could include some extraneous observations and that the footprint view may list more observations than actually appear within the displayed field of view. One convenient way to find the location of a particular observation is to select it (by clicking on it) in the Inventory or Images view, and then to go to the Footprints view, where the selected observations are highlighted. Observations off the edge of the field will not be visible even when selected
Why do all STIS filenames end with _drz.fits?
Due to an oversimplification in the shopping cart mechanism and the complexity of STIS data, which can be taken in different modes (resulting in flt, x2d, sx2, crj, etc. files), file names for STIS data added to the cart from the Inventory and Images tabs are incorrect when delivered by the shopping cart. All of the downloaded STIS file names end in _drz.fits regardless of the file extension (e.g., x2d which should end _x2d.fits). The correct filename and extension are listed in the FITS header. Note files added from the More link within the Images tab are correct. We are working on a fix for this and will update the HLA as soon as that is available.ch point, and the approximation in the search logic does not affect that case.
What are the data product levels? What does "Best Available" mean?
There are several levels of data in the HLA:
The Advanced Search options can be used to select the level of data included and searches. The options also include All, which selects all level 1-5 data, and Best Available, which selects the data product that is generally the most useful (e.g., the combined image or HLSP product if it exists, rather than the single exposure or color image). Currently the Best Available list includes all image products of level 2 or above, and includes level 1 data only for exposures that are not used as part of level 2 or 3 products. However, there are cases where it is useful to set the level manually using the "advanced search" option, for example to diagnose why the combined image looks unusual, by selecting exposures (level 1) or All.
How do I search for a particular instrument or spectral element? Can I sort or filter my results?
Select the Advanced Search option, and you will see check boxes for each instrument in the HLA. You can also select specific spectral elements (e.g., F555W, F814W).
After the initial search, you can also filter and sort the results for a set of spectral elements using the boxes under the Detector column in the inventory. To sort, click on the field name to sort by that field (first click sorts alphabetically, second click sorts inverse alphabetically). To filter, type your text in the box beneath each keyword. For text fields, you can use an * as a wildcard (e.g., in the Spectral Element field, G230* will find all STIS observations using the G230L or G230LB grating). For numeric fields, you can use the > and < symbols (e.g., in the ExpTime field, >600 will find all exposures with >600s exposure time).
How can I change the order of the columns displayed in the Inventory table? How can I change the number of columns displayed in the Inventory table?
At the bottom of the Inventory page is a "Columns" table with the column definitions. You may use the mouse to drag any column definition up or down to change its location in the Inventory table.
By default not all of the (many) table columns are displayed. The columns may be added or removed one at a time by clicking on the arrows (« ») at the right edge of the main table headings. To add or subtract many columns at once, in the "Columns" table drag the gray bar that says "Columns below are hidden" up or down in the list. Columns above the bar are shown and those below are hidden. Similarly, dragging individual column definition in this table above or below the bar will show or hide that column. Drag the bar to the bottom of the table to show all columns. Rows corresponding to images that have been selected in the footprint view will be highlighted in green. However, the table will not be resorted, by this operation, thus it may be necessary to scroll down the page or to following pages to see the selected images.
What is the difference between an "exposure" (i.e., level 1) and a "combined" (i.e., level 2) image? How can I find out what exposures are used to construct a combined image? How are the HLA combined images different than the associations that come out of the normal HST pipeline?
An "exposure" (i.e., level 1) is a single readout from the telescope (e.g., still containing cosmic rays etc.). It has been through the normal HST calibration pipeline (bias subtracted, flat fielded, ...). For ACS, WFPC2, and NICMOS images the differences from the exposures that come out of the normal calibration pipeline are:
These steps are in preparation for building the combined image, which uses the the same multidrizzle software as used in the normal calibration pipeline. All the exposures with the same filter, same camera, and within the same visit are combined. The HLA pipeline also combines some images that are not combined in the normal HST pipeline (e.g., when POS TARG is used to define offsets.)
STIS images have NOT been astrometrically corrected, nor aligned north up.
Data for FOS and GHRS come from MAST through the HSTonline system, which provides immediate downloads of tar files containing FITS data.
For some ACS, WFPC2, NICMOS exposures which have not been processed though the HLA, the data come from the DADS system, which requires separate request submissions and retrievals. These include proprietary data which can only be retrieved through DADS with an authorized user account. See the STScI Archive Manual for further information on using the MAST archive.
You can see what individual exposures went into a combined image by using the "More..." button in the bottom right when using the image mode to look at previews, or by looking in the header of the combined image.
What is the difference between a preview and a cutout (when using the advanced search option)?
A preview image (i.e., what you are looking at by default when you select the "Image" tab) shows the entire field of view, binned down and turned into a JPEG image to make it available very quickly. The only control you have over these previews is that you can select a larger size (512 pixels instead of 256 pixels) in the advanced search options.
On the other hand, a cutout image is a view of a small portion of an image centered at the RA/Dec position specified in the search. A cutout may be either a JPEG image (viewable in the browser) or a regular FITS file containing the true pixel values. You can switch between Previews and Cutouts using the advanced search controls. If you select cutouts, an additional option to set the size of the region is available (the default size is 12.8 arcsec, corresponding to 256x256 WFC pixels.)
If there are multiple images available (for example, using different filters), then there will be a cutout shown for each in the "Image" view. Note that since the cutout is centered at a specific RA and Dec position, all the cutouts will show the same area of the sky regardless of the pointing centers for the individual images. If the search position falls outside the image,a blank image is shown with the message "Cutout position is outside image".
The cutout view is especially useful when searching images centered on a specific object of interest; then the object will be centered in each cutout, and images that do not overlap the search position will appear blank. For a large area or all-sky search, the search position is likely to be located outside of some or all of the cutouts, resulting in many blank cutouts.
The cutout view is also very useful in conjunction with target lists. In that case the cutout is centered at the matching position from the list for each dataset. This is a quick way to upload a list of source positions and get a snapshot of the HST observations for all of the sources that have been observed by ACS or WFPC2.
The size of the cutout can be selected to be a small region around an object of interest (which will then download very fast) or a larger region which could be as big as the full image itself (which will generally be much slower to generate and will take several minutes to download if it is an ACS image).
How do I download images and source lists? Can I download several files I have selected at the same time?
Yes. You need to have cookies enabled to download multiple files through the shopping cart interface. A "cart" tab has been added on the main search page which shows how many files have been selected. Click on the cart tab to see what has been selected, to modify the selections, or to fetch the data. The files will be bundled into a zip file for transport. Most browsers will then decode the HLAData.zip on arrival and put the uncompressed files into a folder called HLAData or named after the visit(s) that included the image(s).
The images can be placed in the cart from the Inventory (far left column) or the Images (bottom of each image). The source lists can only be placed in the cart from the Images page. The download will not start until you click the "fetch" button on the cart page. Note that even in compressed format images can take a while to transfer.
If you do not have cookies enabled, then you can only download individual files. Click on the file link in the Inventory or Images Tab and a filesave dialog box will appear to save the file.
What are all the files listed using the "More..." feature in the image mode?
The More... link in the bottom right corner of the Images view gives access to an often large number of files associated with the image. These are the working files used to make the images and source lists, other filters from the same visit, etc. Here is a brief description of some of the more useful files.
The first line provides a link to the MAST (Multimission Archive at Space Telescope) page for the proposal. This includes the proposal abstract, links to papers using the data, and a form used to pull data out of the standard HST archives (STDADS).
The next lines show what images were combined to make the image. The first entry in a line (using the "IPPPSSOOT" name starting with J for ACS, U for WFPC2, and N for NICMOS) is the gzipped calibrated (level 0) image from the archives (STDADS), at the original pixellation. The second entry in a line (starting with HST_...) is the corresponding level 1 (single exposure) image produced by the HLA multidrizzle pipeline. This image has been astrometrically corrected (see FAQ), and regridded with north up in preparation for the combination step using multidrizzle.
If you hit the "Show all ... associated files" a large number of files appear for all the relevant images in that visit, not just the filter you clicked. Here are some generic types of files to look for.
ACS images
WFPC2 Images
Other Instruments - The files are the normal files that come from the HST calibration pipeline. See the corresponding Data Handbook for the various instrument details.
HLSP - The listing includes link to the MAST page with detailed information on the project that produced the HLSP. It is actually positioned above the proposal link, so it is no longer true that the proposal link is the first line. Note that if the product used data from multiple proposals, there is no proposal link. And the details of the included files are different for the HLSP and vary depending on the particular product.
Is there a summary of known anomalies in the ACS and WFPC2 data?
HLA Instrument Science Report 2008-01 [pdf] decribes known anomalies with the HLA ACS data. There is also anInstrument Science Report for WFPC2 anomalies as well as a shortaddendum [PPT] on HLA WFPC2 anomalies.
How can I look at a STIS spectrum with the HLA? How accurate is the wavelength scale?
With the 2-d spectral image displayed in the Interactive Display window, you can do both column (c) and line (l) plots. Place your cursor on the spectrum and hit the l key to make a line plot (you may want to expand the scale of the image to properly place your cursor on the spectrum).
The plotting tool enables scaling, binning and smoothing, and to use various units.
For slitted, first order observations, the wavelength scale should be good. However, for Echelle observations (which displays multiple orders in one image), the wavelength scale is not useful.
Also, for STIS slitless observations (e.g., 50CCD/25MAMA aperture), the wavelength scale is only valid for those objects centered in the (direct) image (i.e., objects that would be centered if a slit were used).
Why do moving targets (e.g., planets, asteroids, ...) look strange?
The primary reason is because there is no easy way to align the images. In general, the single exposure images are scientifically much more useful (e.g., they have been geometrically and astrometrically corrected when possible). To preview the single exposures, use the "advanced search" button and set the "Image Type" to "All" or "Exposure".
The images look funny because the cosmic ray removal software thinks that part of the moving target that does not overlap in separate exposures is a giant cosmic ray, since the pixels are bright in one image and dark in another (like a cosmic ray). The software then uses the lower value as the truth, and removes a large part of the moving targets.
Why are some of the color previews such strange colors?
The best-looking color images use a combination of three colors that are well spread out in wavelength. A common example is F435W (B), F606W (V) and F814W (I). However, in most cases the available filters used to make the color preview images were not selected to make a "pretty picture", hence the spacing is not optimal and the resulting color is sometimes not very good (e.g., a F435W + F775W + F814W combination).
Another example is when only two filters are available, so that the middle color used to provide the green image (in the normal red-green-blue or RGB method of making color images) is populated by combining the blue and red colors. Hence there are not three independent colors. These do not always provide very pleasing color images.
Still another example is when the WF4 anomaly is present for chip 4 of the WFPC2 (i.e., data from the past few years where the bias level has been suppressed). The non uniformity of the image leads to a wide range of colors from the color lookup table in these cases. Bias jumps on chip 4 can also lead to colorful striping patterns. See theWFPC2 anomalies presentation [PPT] for examples.
These unattractive color images are retained in the HLA because they are often still very useful scientifically (e.g., to differentiate between a cosmic ray and a real source), even if they are not always so pleasing to look at.
Color previews are available only for enhanced HLA images (ACS, WFPC2, NICMOS) and for the high level science products.
How can I download the color preview images? Can I download full size jpeg images?
The color preview images seen after clicking on the Images tab can be downloaded the normal way you download files from your browser (e.g., right click on the image and use the "save image as" feature).
To download a full size image in JPEG format you can use the HLA image cutout service athttp://hla.stsci.edu/cgi-bin/fitscut.cgi, which is the web service that generates all the HLA JPEG images. You will need to add parameters specifying the size of the region to extract and the dataset to use for each color (red, green, and/or blue). If a single band is specified then a gray-scale image is generated; if two or three bands are specified then a color image is created.
Here are some examples:
(1) To get a full-size grayscale JPEG of an image in a single filter:
http://hla.stsci.edu/cgi-bin/fitscut.cgi?red=HST_10188_10_ACS_WFC_F814W&size=ALL
(2) To get a full-size color JPEG using 3 filters:
http://hla.stsci.edu/cgi-bin/fitscut.cgi?red=HST_10188_10_ACS_WFC_F814W&green=HST_10188_10_ACS_WFC_F550M&blue=HST_10188_10_ACS_WFC_F435W&size=ALL
(3) To get a grayscale JPEG that is exactly 800 pixels in its longest dimension (width or height):
http://hla.stsci.edu/cgi-bin/fitscut.cgi?red=HST_10188_10_ACS_WFC_F814W&size=ALL&output_size=800
Can I get "HLSP" that have been contributed to MAST (e.g., UDF, COSMOS, GOODS, ...)?
Yes, many of the contributed high-level science products (HLSP) that include HST images can be accessed through the HLA (in the inventory, images, and footprints views, and in the interactive display). They appear in the inventory tables asLevel 5 (where Level 1 = exposures, Level 2 = combined images, etc.)
The HLSP are automatically included using the default search settings (Data Product = "Best Available"), or the Data Product menu (in the advanced search options) can be used to search only for "Contributed HLSP (Level 5)". You may also access these data through the MAST HLSP interface, which in some cases provides access to additional data products such as catalogs. The More... page accessed through the Images view includes a link to the relevant MAST HLSP page for each project.
A quick way to view HLSP data is to select "advanced search", enter "0 0 r=180" in the position search box, set the "Data Products" to "Contributed HLSP (Level 5)", select the ACS instrument only, then click the "Search" button. This will return all of the ACS HLSP data. There are also HLSP datasets for WFPC2 and NICMOS.
Note that many of these products are very large (e.g., the COSMOS images are nearly 2 GB), so downloads may be slow. We have made some improvements to the performance of the interactive display to make it more useful for these images, particularly when the image is zoomed out so that the entire field is visible. To reduce download times, we recommend using the interactive display to assess data quality and using image cutouts where appropriate to avoid downloading the entire image if a small portion of it will suffice.
Also, note that the criteria by which these products were produced vary, so contributed products do not have the standardization that HLA produced products have (i.e., north may not be up, columns missing, missing headers, astrometry may or may not be improved, etc.)
The current HLA release does not yet include all contributed HLSP products. There are currently more than 4500 HLSP images (approximately 700 ACS, 3300 WFPC2, and 500 NICMOS images, including about 500 color images) available from the following projects:
| Project Name | PI | Instruments 1 | Reference Position | Number of Products |
|---|---|---|---|---|
Notes:
|
||||
| ANGST (ACS Nearby Galaxy Survey) | Dalcanton | ACS | Multiple targets | 114 |
| COMA (ACS Treasury Survey of Coma Cluster of Galaxies) | Carter | ACS | 12:59:49.45 27:55:21.1 r=1.2d | 69 |
| COSMOS (Cosmic Evolution Survey) | Scoville |
ACS, WFPC2,
NICMOS |
10:00:27.85 02:12:03.5 r=1.0d | 999 |
| GEMS (Galaxy Evolution from Morphologies and SEDs) | Rix | ACS | 03:32:30 -27:48:20 r=0.4d | 187 |
| GOODS (Great Observatories Origins Deep Survey) | Giavalisco | ACS |
North: 12:36:55 62:14:15 r=0.3d
South: 03:32:30 -27:48:20 r=0.3d |
175 |
| Hubble Heritage | Noll, others |
ACS, WFPC2,
NICMOS |
Multiple targets | 91 |
| STAGES (Space Telescope A901/902 Galaxy Evolution Survey) | Gray | ACS | 09:56:02.16 -10:05:20.5 r=0.4d | 80 |
| UDF (Ultra Deep Field) |
Beckwith,
Stiavelli, Thompson |
ACS, WFPC2,
NICMOS |
03:32:29.45 -27:48:18.4 r=0.1d | 20 |
| APPP (Archive Pure Parallels Program) | Casertano | WFPC2 | Multiple targets, all sky | 2858 |
| SGAL (Spiral Galaxies) | Holwerda | WFPC2 | Multiple targets | 96 |
See the more detailed description of the High Level Science Products currently accessible through the HLA, organized by instrument and project, for more information.
What do I need to know about the WFPC2 combined images in the HLA?
The WFPC2 combined (level 2) images were produced at the Canadian Astronomical Data Centre (CADC). The same basic software (i.e., multidrizzle) was used to produce both the ACS and WFPC2 images, hence they are as similar as possible (e.g., they both have north up, are astrometrically corrected when possible, etc.) However, due to the intrinsic differences in the two cameras, there are some important differences. These are summarized below:
1. There are two sets of images for each WFPC2 observation:
A) WFPC2 - Includes both the three Wide Field (WF) chips and the single Planetary Camera (PC) chip. The PC chip is resampled to match the scale of the WF chips (i.e., 0.10"/pixel).
B) WFPC2-PC - Includes only the PC, using a scale of 0.050"/pixel, hence retaining the better spatial resolution of the PC. The DEFAULT is to NOT retrieve these along with the WFPC2 images. You must use the Advanced Search button and click the WFPC2-PC box to access these products.
2. The units are in ELECTRONS/SEC (like ACS) rather than DN (Data Number), hence adjustments will need to be made in zeropoints for the gain (DN/ELECTRONS) and the exposure time. Some prototype catalogs are available, along with more details on the appropriatezeropoints to use.
3. A comparison between ACS and WFPC2 relative astrometry shows up to 0.1 to 0.2 arcsec differences between the two in the corners of the image. We believe this is probably related to chip-to-chip movement in the WFPC2 over the years. New software is being developed in the STScI calibration pipeline to correct this, and will be incorporated in the next (DR4, Fall 2009) HLA release.
4. The quality of the absolute photometry appears to be quite good (e.g., RMS compared to ground-based STETSON photometry is ~ 0.03 mag). However for projects that require the optimal photometry people should consider working on the individual chips, as in the past, rather than the multidrizzle combined images in the HLA. More specifically, if Charge Transfer Efficiency (CTE) is an important consideration (i.e., faint sources, low background, several years into the mission, ...), observers would need to work on the individual chip data from the STScI calibration pipeline to obtain the highest accuracy. When WFPC2 source lists become available (DR3), there will be estimates of CTE corrections for each source.
Thispresentation [PPT] has some comparisons between photometry done on the HLA multidrizzle combined images and various other ground- and space-based observations.
Why do some color images show misalignments? Is there a list of these cases?
Some ( ~1 %) images show a clear misalignment between different filters, especially for the WFPC2. An example is 09634_9o (47 Tuc), where the green stars (F450W) are offset from the red stars (F814W) by about 1". Note that the blue stars (F300W) are offset still further, but are harder to see since they are dimmer. These misalignments are generally caused by a failure to reacquire the guide stars after an earth occultation. We leave the color image in for these cases to help alert users that there is a misalignment between filters. The individual filter images are generally fine, just not aligned with each other. We also remove the total or detection images in these cases to keep people from using misaligned data (and because we cannot make source lists for these images in any case). There are also much smaller misalignments such as 09259_01 (03:38:16.91 19:36:05.1). These are generally due to a slight drift between observations, often because they were taken with one guide star tracking. We have generated alist with the known misalignments.
It is also possible to have misalignments for exposures with the same filter, although these are fairly rare. While clear cases are easy to catch, slight misalignments of this type are harder to see, hence observers should always take a careful look at the data to make sure the stars are circular. A careful look at the color image is a very sensitive way to catch misalignments between filters.
There are also cases of "planned" misalignments, such as moving targets (e.g., planets), where the color images can look rather bizarre. These are again retained to help alert people to the single exposures, which are generally scientifically much more useful.
Is there an example of how to use the plotting tool for quick-look spectroscopy?
Here is an example using long-slit STIS observations:
Enter "antennae" in the search box and just get the STIS data. Go to image O5HE06010 and look at it with the interactive display. Move your cursor to a y value of 765 (e.g., .to show a spectrum of the of the region around X=295, Y=795 which happens to be the H alpha line of Hydrogen) and hit "L". You should get a line plot. You can manipulate coadding lines (I use 3), changing the x and y min and max values etc. Note that the real lines are around 6580 Angstroms (N), 6595 A (H_alpha), 6615 A (N), and 6750 and 6765 A (S). Most of the other "lines" are due to cosmic rays (this is the kind of thing that gets fixed in detailed reductions).
To get an approximate wavelength, blow up the x axis so you just get the H_alpha line (i.e., from say 6560 to 6610 for min and max X values ). The line center looks like about 6596 A. This line should be at 6563 A if the object were at rest. Hence it is redshifted 33 A. The formula to convert to velocity is delta_wavelength/wavelength = delta velocity/300,000 (speed of light). Hence 33/6563 = delta_velocity/300,000 or delta_velocity = 33/6563 * 300,000 = 1508. Hence this region is moving away from us at ~ 1500 km/sec.
To smooth the plotted data along the axis of the column or line you are displaying enter the width (diameter) of the boxcar by which you wish to smooth in the box "Smoothing Diameter". To cooadd more lines or columns into the plot (in this case to add more rows to do a fast aperture extraction along the selected row) enter the diameter you wish to coadd in the "Coadditon Diameter" box.
For example, to smooth by 3 pixels we enter 3 in the Smoothing Diameter box. This smoothes the current data with a boxcar 3 pixels wide. To coadd 5 rows centered around the current row, we enter 5 in the Coaddition Diameter box. This adds the two lines before and two lines after the central line. There is no noise rejection, simply addition.
Please note that contributed products data do not support plotting tool functions.
What are mosaics? How do they differ from Level 2 combined images?
HLA mosaics, also referred to as Level 3 data, are images that combine data from multiple HST visits to cover a contiguous area of the sky. Level 2 combined images are restricted to data taken within a single HST visit, which means that the data can only cover a limited region of space (HST cannot change pointing by more than 2 arcmin within a single visit) and are obtained in consecutive HST orbits. In principle, mosaics can cover a very large area of the sky, such as the areas observed as part of the GEMS and COSMOS programs (0.25 and 2 square degrees, respectively). As mosaics can encompass data taken at different roll angles, with different guide stars, and at different times, alignment and handling of time-dependent effects (sensitivity, geometric distortion) pose greater challenges than for Level 2 combined images.
Only a small number of prototype mosaics have been released as part of DR3. You can do an all-sky search for level 3 data to get a complete list, or you can search for individual objects (Cl J0152-1357, 1Zw18, Tadpole, Cl J1138.2-1133, RDCS J0849+4452, RDCS J1252-2927, 1WGA J2235.0-2604, CL J1354.2-1230). These serve as examples of the type of data that will be produced in the future. User feedback will be very important in shaping future developments in this area; please let us know what is important to you by sending email to archive@stsci.edu.
What criteria are used to define mosaics? What is the naming convention?
Unlike visits, which are defined by the HST scheduling process and do not change, the definition of mosaics can change as more data are obtained in the same region of the sky. The definition of mosaics is based on "pointings", or groupings of image data that mutually overlap to cover a connected area of the sky. Pointings are instrument-specific; for simplicity, at this time they are identified by a single number. For this preliminary release, additional constraints have been imposed to simplify the processing: the data must be taken within a 4-month interval, individual visits must contain enough images to ensure that cosmic-ray identification can be carried out on for each visit separately, and all exposures must overlap each other to simplify the alignment.
A list of the mosaics for which data are available at the time of this release can be obtained by doing an all-sky search for Level 3 data for ACS (selection available under the Advanced Search options). All mosaic data contain the word MOS followed by an underscore and the numerical identifier of the pointing (e.g., ...MOS_597... for the files associated with Mosaic 597).
What steps are taken to produce the mosaics? What files are available?
In the current definition, mosaic processing requires three steps.
First, the data for each visit are combined with the standard multidrizzle processing (the same used for Level 2 combined data) in order to identify cosmic rays and other blemishes. The pixels in each exposure that are flagged will not be used in further combinations.
Second, the images are aligned across visits by matching source catalogs for each visit. The alignment is done using the same multifilter images that are also used to identify objects for the source lists. This step is required because images can only be successfully combined if they are registered to a relative accuracy of a fraction of a pixel (the goal is 0.1 pixels), much better than the residual uncertainty in the absolute astrometry even for images that have been astrometrically adjusted.
Third, the images for each filter are projected onto the final pixel grid using multidrizzle, with the adjusted astrometry but without additional processing to remove cosmic rays. Weight and exposure images are also produced for each filter.
Further refinements will be necessary in the future in order to process mosaics that do not fit the current restrictions. For example, cosmic ray rejection can be generalized to use all the data available, not just the data for each visit, which in turn would allow more data to be included in mosaic processing; a multi-tiered registration procedure may be needed for images that cover areas significantly larger than 5 arcmin on the side, for which binary image-to-image registration may entail errors that are larger than desired; and data covering a long time span may need to be more thoroughly scrutinized for time-dependent sources.
For each mosaic, the FITS-science file is a simple FITS file with the pixel data for the mosaic, while the Multi-extension FITS (MEF) file includes pixel data (in extension 1), weight (in extension 2), and the exposure image (in extension 3). These are the standard files produced by Multidrizzle.
The "More..." button exposes a list of all the exposures used in producing the mosaics associated with the pointing in question, regardless of filter, one line per file.
The button "Show all NN associated files", exposed in the "More..." view, gives access to other ancillary files produced during processing. These include the combined files for each filter and visit, as well as scaled jpg files used for quicker visualizationof the results.
What is the astrometric and photometric accuracy that can be expected for mosaics?
A full analysis of mosaic accuracy, by comparison with the individual input exposures, independent processing of the same data, and other data for the same area of the sky, will be carried out when more mosaics are available. A spot check for one of the mosaics currently released, compared with an independent processing of the same data, suggests that the photometric fidelity of the mosaic combination process is better than the intrinsic noise of the images, and the PSF FWHM is broader in the HLA mosaic by about 2.5%. This is to be expected, as the default parameters used for the mosaic combination will generally result in a slight broadening of the PSF compared with optimized processing. The absolute astrometric accuracy is expected to be comparable to that of the component images, or about 0.3 arcsec rms in each coordinate.
What source lists are available? How were they constructed?
At this time (Data Release 3), source lists
constructed using DAOPHOT and/or SExtractor
are available for almost all of the HLA Level 2
(combined) images for ACS and WFPC2. Source lists
for ACS were released in DR2, although in many
cases they were updated in DR3.
Source lists for WFPC2 are new for
DR3, and have been constructed using the same
criteria as for ACS. In future releases we
expect to produce source lists for NICMOS data,
as well as for Level 3 (mosaics) and Level 5
(contributed) products.
The source lists can be overlaid on an image using the interactive display, and can be downloaded via the Images view. Magnitudes are in the ABMAG system. The detection threshold is roughly 5 sigma, though there are cases where it is significantly less deep.
Please note that while the HLA images and source lists will be useful for science, by necessity these products are developed for general usage rather than being "tuned" for a specific scientific goal. Hence, in many cases, going back to the original STScI calibration pipeline data and processing on a chip-by-chip basis can achieve the optimal science. A more detailed description can be found in a future PASP paper.
Why do some sources appear to be blank for some filters?
The source lists are made from a "white light" image, also called the "Detection" or "Total" image (i.e., the combination of all the different wavelength filters available within a given visit). For this reason, the overlaid objects may appear not to correspond with a source in some cases (e.g., a very red source may not be visible in a blue image). Looking at the detection image will generally result in a better correspondence between the source list and the objects in an image for this reason.
A related point is that these "white light" images may include more filters than are displayed in the color images. In such a case, there may still be a slight difference when overlaying the source list on the color preview and on the detection image. An example is the 47 Tuc ACS visit 10048_a2, which has a color preview constructed from three colors (F814W, F555W and F435W) but has a "white light" image that is actually constructed with all seven filters that are available.
Why are there both DAOPHOT and SExtractor source lists?
Two types of source lists are available via the HLA: DAOPHOT, which is optimized for point-like objects, and SExtractor, which is optimized for extended sources. The choice of which catalog to use depends on your science. Generally the photometry and morphology information from SExtractor is much more useful for galaxies, while the DAOPHOT photometry is more accurate for point sources.
How do I download the single-filter and the multi-wavelength source lists?
Note that there are single-filter source lists and multi-wavelength source lists for each visit where more than one filter is available. Both use the same master list of positions determined from the "white light" detection image. The single-filter source lists are available from the image view below each image with the appropriate filter. The multi-wavelength source lists are available under the detection and color preview images.
What are the differences between releases of the ACS source lists, and between WFPC2 and ACS lists? What are common artifacts to watch out for?
Preliminary ACS source lists were released as part of the Early Data Release (July 2007, designated Version Beta 3, or VB3) and of Data Release 1 (February 2008, designated Version 1.0). All such lists are superseded by the Version 2.0 lists, released as part of Data Release 2 in September 2008.
For many ACS images, a new version (V3.0) of
the SExtractor source lists has been released
to correct the photometric zero point used.
A significant fraction of the V2.0 SExtractor
source lists used incorrect zero points
because of a software problem. In addition,
the new lists use the most recent determination
of the ACS photometric zero points given at
http://www.stsci.edu/hst/acs/analysis/zeropoints.
Except for the change in zero point, these lists
are otherwise equivalent to the V2.0 lists.
No new DAOPhot lists have been released in DR3
for ACS images.
WFPC2 source lists have been released for the first time as part of DR3, and are designated Version 3.0. These lists are to be considered preliminary, as the algorithms developed to remove spurious sources for ACS have only limited validity for WFPC2; the detection threshold has thus been set high for WFPC2, especially for the SExtractor lists, in order to limit the number of false detections. We expect that future refinements of the detection and filtering procedures will result in a substantial increase in the number of sources reliably detected in WFPC2 images.
Fufor ACS to remove A future modification to WFPC2 source lists, or releases of new source lists will be designated with a new identifier based on the data release to which they correspond.
Unlike ACS, WFPC2 source lists explicitly include a correction for CTE based on the 2008 formulae from Dolphin. As a consequence, the lists differ in format from the ACS lists because they include additional columns with the information used for the CTE correction. Columns in common between ACS and WFPC2 lists have the same meaning, except that for WFPC2 the apertures used are 0.1 and 0.3 arcsec in radius, vs. 0.05 and 0.15 arcsec for ACS.
ACS DAOPHOT source lists use custom algorithms for the removal of spurious artifacts around bright stars, along the edges of the images, and in the gaps of images. These algorithms were only partially successful for VB3 source lists, were improved for V1.0, and have been improved still further for V2.0 (e.g., artifacts caused by bleeding from saturated stars have been greatly reduced). SExtractor source lists use essentially the same setup as used for the Ultra Deep Field. A common problem with SExtractor source lists is that they have trouble finding sources in regions of high background. Conversely, DAOPHOT is designed to work best with stars or very centrally concentrated objects; hence most faint extended objects (e.g., galaxies) will be missed.
In certain circumstances (e.g., around bright stars, in the gap between ACS detectors when only N=1 coverage exists) many real sources may be missed. This follows from our basic philosophy to minimize the number of artifacts, even if it means missing some real sources. These real sources can be separated from the artifacts manually, if necessary, by downloading the full/complete source list (rather than the trimmed version designated as ".trm") using the "More..." feature (located at the bottom right of each cutout on the images tab). Users should also be aware that bright stars are often saturated; hence their photometry will be inaccurate. These cases are identified by a flag value of 2 (potentially poor photometry) or 4 (saturated).
Other (generally rare) artifacts to be aware of are:
Why do you call them source lists rather than catalogs?
In most cases, an optimal catalog must be "tuned" or "optimized" for a specific scientific goal (e.g., point-sources, extended sources, faintest possible objects, crowded field, high backgrounds, etc.)
Our pipeline processing must be fully automated to handle tens of thousands of datasets. Hence we are not able to construct catalogs that are optimized for all conceivable projects. Instead, our approach is to provide a basic, general-purpose, first-look source lists. In many cases researchers will need to construct their own catalogs in order to optimize the science for their own specific goals.
This philosophy impacts our approach to making the source lists; for example, our adoption of the use of a "white light" detection image rather than making separate source lists for each filter. Because of this approach, the completeness for a given color is poorly defined. This is another reason for calling them source lists rather than catalogs.
How accurate and reliable are the source lists?
Some of the design goals are:
Spot checks (PPT) spot checks show that we are well within our design goals for the ACS source lists.
What are the basic parameters used to construct the source lists?
For the DAOPHOT source lists, circular aperture photometry is performed with radii of 1 and 3 pixels. The background is defined as the median in annulus from 5 to 10 pixels. A concentration index (CI) is defined as the difference between the magnitudes in the 1 and 3 pixel radii. This is used to define a flag value, which is 0 for point sources and 1 for extended sources. Aperture corrections for stellar sources are added to the point sources to estimate a total magnitude. For extended sources a value of 999. or -999. is placed in the total magnitude column. The information above, plus other useful information is included in the header of each source list.
For SExtractor source lists, we use the base set of parameters used to construct the catalogs for the Ultra Deep Field (UDF) with minor deviations, available at http://archive.stsci.edu/prepds/udf/udf_hlsp.html.
What is the key for the flag values in the Source Lists?
Here are the definitions for the ACS/WFC (also listed in the header of each source lists). Check the header of ACS/HRC catalogs for small modifications (e.g., different concentration index). There are currently eight values:
Note that the "trimmed" source lists (i.e., sources with flag values of < 5) are provided for download from the image mode. You can download the full catalog using the "More.." feature when using the image mode in the web interface. See the "More..." FAQ for details.
Where do I find the Segmentation, Background, and Aperture images that go with the SExtractor source lists?
It is important to realize that the "apertures" used by SExtractor are not the circles shown by the overlay in the Interactive Display, but are ellipses of different sizes, shapes, and orientations. The “aperture images" show the actual apertures used.
One can access the Source Extractor (SE) auxiliary fits file from the "Images" view in the HLA web interface. By clicking on the "More..." link (located in the lower right corner of each image box), a separate web page will be opened. In the new web page you will find, and can click on the "Show all associated files" link, which will lead you to a link for the "se_check_files.tar.gz" file. Clicking on this link will add the file to your shopping cart. It is then necessary to go back to the original web page, click on the "Cart" tab near the top of the page, and then to click on the "Fetch All Marked Items" button to begin the download process. Once the file has been downloaded, you will need to unzip and untar the file per the methods designated for your specific operating system. The aperture image can be identified based on the word “aperture” in its name. The other fits files in the folder are intermediate products created by SExtractor (e.g., the background image subtracted from the original image before determining the apertures to use).
What do I need to know about photometric zeropoints for the ACS and WFPC2 images and source lists?
ACS
1. ACS images are in ELECTRONS/SECOND. Zero-points have been published in Sirianni et al. 2005, PASP, 117, 1049, Table 10; updated values are included in the ACS Instrument Handbook and can be found at http://www.stsci.edu/hst/acs/analysis/zeropoints. Zero-points can also be computed from the values of PHOTFLAM and PHOTPLAM in the header of the image file using the equation:
ABMAG_ZPT = -2.5 Log (PHOTFLAM) - 21.10 - 5 Log(PHOTPLAM) + 18.6921
Note that the HLA source lists are in units of ABMAG.
2. If performing aperture photometry, apply the relevant aperture correction. Published ACS zero points are for an infinite aperture; details of the aperture correction depend on filter and photometry procedure. See the Sirianni et al paper for more details.
3. Convert to the desired magnitude system. ACS zero-points are provided in three systems: ABMAG, STMAG, and VEGAMAG; see the ACS Data Handbook for more details on the systems. Note that the conversion between these systems depends on the filter and, to a lesser extent, on the system throughput curve; it DOES NOT depend on the source spectrum. The difference between magnitudes in the three systems is small around 550nm (0.05 mag or less for F555W) but it can be quite large (over 1 mag) in the UV and near IR.
4. Other adjustments may be necessary, e.g., for CTE losses. These are to date much smaller than for WFPC2, but they may need to be taken into account for high precision photometry. Consult the ACS Instrument and Data Handbooks and the Instrument Science Reports (all found at http://www.stsci.edu/hst/acs/documents) for the most up to date information on the calibration of the camera.
WPFC2
WFPC2 Level 1 (drizzled) and Level 2 (Combined) images produced by the HLA pipeline are also in units of ELECTRONS/SECOND, like ACS images. Note that this convention is different from that adopted by the Archive pipeline, which produces calibrated images (the *c0m.fits files) in total Data Number (DN). One DN is approximately 7 or 14 electrons, depending on the gain level chosen by the original observer. There are thus two differences in the images: HLA images express counts in electrons rather than DN, and they are divided by the exposure time to obtain a count rate rather than total counts.
The calibration information found in the WFPC2 documentation all refers to DN rather than electrons; therefore all zero-points must be adjusted to account for the gain. (Published zero points do assume that instrumental magnitudes are already corrected for the exposure time.) The following adjustments are therefore necessary for HLA WFPC2 images:
1. Convert published zero-points from DN to ELECTRONS using Table 5.1 of the WFPC2 Data Handbook. This is just the Gain, which is listed in Table 4.4 of the Instrument Handbook. This varies by about a percent for the different chip, but an average value of GAIN = 7.04 for the three WF chips should suffice for most purposes. Hence, this correction is 2.5 log(7.04) = 2.119. So for example, rather than using a zero-point of 22.557 (an average for the three WF chips from Table 5.1) you should use 22.557 + 2.119 = 24.676 mag.
NOTE: Unlike for ACS images, YOU SHOULD NOT USE THE VALUES OF PHOTFLAM AND PHOTPLAM from the header of the WFPC2 HLA images, since PHOTFLAM has not yet been adjusted from DN to electrons.
2. Ensure that the instrumental magnitude (-2.5 log(total pixel value)) determined by your photometric package uses an exposure time of 1 second. Some packages may read the value of EXPTIME from the image header and apply a correction of (-2.5 log (exposure time)) to the total pixel counts from the image; THIS WILL LEAD TO INCORRECT PHOTOMETRY. The value of EXPTIME in the image header is indicative of the total exposure contributing to the image, but it MUST NOT be used in computing the instrumental magnitude as the image units are already expressed in counts per second.
3. If performing aperture photometry, apply the appropriate aperture correction. WFPC2 zero-points are given by adding 0.1 mag to the counts within a 0.5 arcsec radius; thus if the photometry uses a radius of 0.5 arcsec, the aperture correction is 0.1 mag by definition. For other apertures, the correction can be determined directly from the data (e.g., by measuring an isolated bright star if available) or estimated using the encircled energy curves from Holtzman et al 1995, PASP 107, 156. If a very small aperture is used (less than 0.1 arcsec radius), the variation of aperture correction with focus may be significant, and thus a direct measurement from the data is preferable. See the WFPC2 Data Handbook for more information.
4. Convert from VEGAMAG to ABMAG or STMAG if desired. The same considerations apply as for ACS; however, WFPC2 zero-points are only provided in VEGAMAG, not in STMAG or ABMAG. The WFPC2 Data Handbook provides detailed examples on the steps needed to carry out the transformation using SYNPHOT.
5. Other adjustments may be necessary, CTE and contamination corrections being the most common. Please consult the WFPC2 documentation at http://www.stsci.edu/hst/wfpc2/documents for up to date information and for more details.
How do I "Zoom in" to have a close up look at the footprints?
There may be times when you want to manually increase or decrease the radius of the field for the footprints. This can be accomplished by clicking the "advanced search" button, changing the radius to the desired amount, and then hitting the search button. There is currently a lower limit of 0.01 degrees (i.e., 36 arcsec). At sizes larger than 1 degree the Digitized Sky Survey background image is not shown.
How do I tell which aperture in the footprint view corresponds to which observation in the table?
If you click on the aperture in the footprint view, the corresponding entries in the table at the bottom of the footprint page will be highlighted (in green) and brought to the top of the list. Similarly, if you select an observation in the table, the aperture will be highlighted (yellow outline) in the footprint view. Note that the same observations will also be highlighted in the inventory and image views. Conversely, if you click an image in the inventory or image view it will be highlighted in the footprints.
If you have a several pages of datasets, it may be difficult to find the selected, highlighted images. In that case try clicking theFirst button to move the selected images to the beginning of the table or the Only button to show only the selected images.
How do I choose footprints that have a small instrument aperture?
Small aperture instruments (particularly ACS and NICMOS grism spectra) have footprints that are too small to be selected at the current default search radius of 0.20. By 'zooming' in on the footprint you want, the footprint can be selected graphically. The footprints can be selected by clicking on rows in the table at any zoom resolution.