Footprints are shown for ACS, NICMOS, STIS, and WFPC2 images, as well as FOS and GHRS spectra. Are data from all instruments available via the HLA?
Yes. Some are enhanced data products
developed for the HLA while others are standard products available from
the HST archives. See the matrix below:
| Instrument/Product | Source | HLA Enhanced Products | Standard Products |
Download | Interactive Display? |
|---|---|---|---|---|---|
Notes:
|
|||||
| ACS/combined images | STScI | ~90% | |
FITS | ✔ |
| ACS/DAOPHOT source lists | STScI | ~50%, <~2004 | |
Ascii | ✔ |
| ACS/SExtractor source lists | STScI | ~10%, Beta product1 | |
Ascii | ✔ |
| WFPC2/combined images | CADC | ~Coming soon, Beta product1 | |
FITS | ✔ |
| WFPC2/single exposures | STScI | |
100% | DADS2 | |
| NICMOS/GRISM extractions | ST-ECF | ~80%, 1-D & 2-D spectra | |
FITS | |
| NICMOS images | STScI | |
100% | DADS2 | |
| STIS images and spectra | STScI | |
100% | FITS | ✔ |
| FOS spectra | STScI | |
100% | Tar | |
| GHRS spectra | STScI | |
100% | Tar | |
You should continue to use the usual
MAST interfaces (http://archive.stsci.edu)
to retrieve data that are not accessible through the HLA.
Does the HLA work only on Firefox and Safari (version 3)?
No, you may find that the HLA interface works well on other browsers (for example, most features work on Internet Explorer). However, there are some aspects that may not work well (e.g. several features do not behave correctly in Opera). 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, however.
How good is the astrometry for the images and the source lists?
In cases where there are several well defined objects in an images which can be matched with the Guide Star Catalog 2 (GSC2) 2 MASS, or SDSS, the absolute astrometry has been corrected, with typical uncertainties ~0.3 arcsec. However, there are cases (~20%) where the match is problematic (e.g., a very crowded region where the ground-based image is not able to resolve individual objects as well as HST), and uncertainties of 1 - 2 arcsec are still typical.
We attempt to identify these problem cases by defining three
categories
for ACS images:
A (Good Astrometry), B (Likely to have Good Astrometry), and C (Potential Astrometry Problem)
The file Astrometry
provides information about how
categories
A, B, and C are determined for ACS images, and information about the
fits for specific cases. The astrometry for the WFPC2 images has been
corrected in a similar way, but the quality of the fits have not been
quantified yet.
Note: The current astrometry file and plots are from the Early Data Release (July, 2007). The updated versions will be available ~ Feb. 15, 2008. We expect the results to be similar, but slightly better than the Early Data Release.
The astrometry for other instruments HAS NOT BEEN CORRECTED hence you
can expect typical uncertainties of 1 -
2 arcsec.
A manual spot check of the images show the following:
Figures Category A Astrometry, Category B Astrometry, Category C Astrometry show the scatter plot comparisons between the GSC2 and 2MASS fits.
How do I find moving
targets (e.g.,
planets, asteroids,
...) in the HLA?
The name resolver used by the HLA
search
box cannot provide coordinates for moving targets, therefore
making it hard to find these
objects. We have created a list of moving
targets that can help users find them in the HLA. Here you will
find moving target's associated proposal ID, filters, RA/Dec, visit
numbers and the list in a text file.
How can I find a
particular proposal ID
number? 
This FAQ
is related to an all-sky
radius problem
In the future, the
advanced search option will allow searches
by proposal ID. In the interim one can use "0 0 r=180d" (i.e. the
entire sky) in the search box, and then filter the results by the
desired proposal ID using the blank box under the PropID in the
inventory. One should generally restrict the search to one instrument
using the advanced search or this can take a very long time.
How can I find some interesting looking HLA images ?
Here is a list of some favorites:
Are there any Help movies
available?
Yes, there are currently three
movies:
What are the data product
levels? What does "Best Available" mean?
There are several levels of data in the HLA. Level 1 data are the individual exposures, Level 2 data are the combined images (exposures with the same filter, same camera, and within the same visit) and Level 4 data are false color images (a combination of two or three colors, depending on what is available). Level 0 (raw) and Level 3 (mosaics) are not yet available.
"Best Available" selects the data product that is generally the most useful (e.g., the combined image if it exists, rather than the single expsoure or color image). 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 level 1 or "all".
How do I search for a
particular
instrument?
Select the Advanced Search option, and you will see checkboxes for each instrument in the HLA. After the initial search, you can also filter the results for a particular instrument using the box under the Detector column in the inventory (See next FAQ).
Yes. 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.
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 and WFPC2 (available soon as a Beta prototype) 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 multi-drizzle 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 NICMOS, FOS, GHRS, and WFPC2 exposures (level 1) come from MAST. FOS and GHRS come from the HSTonline system, which provides immediate downloads of tar files containing FITS data; NICMOS and WFPC2 exposures come from the DADS system, which requires separate request submissions and retrievals. Note that proprietary data 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.
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 specified RA/Dec position. 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 region searched for images is large compared with the camera field of view, some (or even all) of the cutouts may be located off the edge of the image. In that case a blank image is shown with the message "Cutout position is outside image". (If you search using a radius of zero, all images will include the RA/Dec position used for the search and so none of the cutouts will be blank.)
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 a WFC image). All cutout images are created dynamically directly from the FITS images in the HLA, so generating large cutouts may be time-consuming.
Can I download several images I have selected at the same time?
Not at the present. We are developing a shopping cart feature that will allow this in the future.
How do I download images and source lists ?
The images can be downloaded from the Inventory (far left column) or the Images (bottom of each image). The source lists can only be downloaded from the Images.
What are all the files listed using the "more" feature in the image mode?
These are the working files used to
make the images and source lists. 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).
ACS images
STIS
WFPC2 Images
Is there a summary of known anomalies in the ACS data?
Yes there is a draft report on known anomalies with the HLA ACS data. There is also an Instrument Science Report for WFPC2 anomalies.
How can I look at a STIS spectrum with the HLA? How accurate is the wave length 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 line plot is currently 1 pixel wide, so if the spectrum is not aligned (such as in .flt data, and especially in echelle data), you will not be able to get the complete spectrum in one cut.
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.
These "false 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 presently only available for ACS and WFPC2 (available soon as a Beta prototype) images.
How can I download the color preview 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).
Can I get "high level products" that have been contributed to MAST (e.g., UDF, COSMOS, GOODS, ...)?
Not at the present time. You should continue to use the MAST HLSP interface to access these data.
How were the source lists constructed? Why are there both DAOPHOT and SExtractor source lists? Why do some sources appear to be blank for some filters?
See the brief description in the Getting Started section of the Help for some basic information about the source lists. A more detailed description can be found in a paper that will be available ~February 2008.
An important point is that the source lists are made from a "white light" image, also called the "Detection" 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 color preview 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 even the color preview. An example is the 47 Tuc 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.
Note that there are single-filter source lists and multi-wavelength source lists for each visit. 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 V1.0 source lists? What are common artifacts to watch out for?
The current (February 2008) source
lists have been designated
as
Version 1.0, in order to differentiate them from other future
versions. These superceed the VB3 (Version Beta 3) source lists
available as part of the Early Data Release (July 2007).
DAOPHOT source lists use an alogorithm for the removal of spurious artificats around bright stars, and along the edges and in the gaps of images. These algorithms were only partially successful for VB3 source lists, but are much improved for V1.0, hence there are now very few edge effects or spurious artifacts around bright stars. 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 have
N=1 coverage) many real sources are missed. This follows from our basic
philosophy to minimize the number of artifacts, even if it means
missing some real sources. Users should also be aware that
bright stars are often
saturated, hence their photometry will be inaccurate. These cases are
indentified by flag values =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 backbrounds, ...)
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 list. In most 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, in our adoption of the use of a "white light" detection image rather than making separate source lists for each filter. Beause 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:
Here are some examples of spot checks that show that we are well within our design goals.
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 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 last column of the DAOPHOT 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 CI criteria). 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. See the "more" FAQ for
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 image
corresponds to which
observation in the table?
If you click on the aperture in the
footprint view, the
corresponding entry in the table will be highlighted. Similarly, if you
select an observation, in the table the aperture will be highlighted in
the footprint view.