You can look at an image or 2-dimensional spectrum more carefully, and overlay HLA source lists or external catalogs, by using the interactive display (accessible via either the inventory or images view). The HLA interactive display was originally based on Leif Wilden's fits2web Javascript program.

General functions

• The image can be moved either by dragging using the mouse or by using the arrow buttons in the upper left (< > ^ v). The o button recenters the view.
• The + and - buttons zoom in or out. The zoom bar shows the current zoom level; you can also click on the zoom bar to jump directly to a different zoom. Initially the image is displayed at its natural scale, with one image pixel per display pixel. The zoom changes by a factor of two between levels and ranges from 4 times larger (1 image pixel per 4 display pixels) to a size where the entire image is smaller than 256 pixels on each side.
• The coordinate box in the upper left shows the current mouse position both in X,Y pixels and in world coordinates. The world coordinates are either the position on the sky (RA and Dec) or, for 2-dimensional spectra, the wavelength and position along the slit. Note that the wavelengths are not accurate for echelle and slitless spectra because they do not have FITS header WCS keywords that describe the mapping between position, order and wavelength on the image.
• The third line of the coordinate box shows the actual pixel value from the image at the mouse position when the mouse stops moving.
• Type the S or s key on your keyboard to copy the current cursor position into the search box on the HLA search form. You can use this in conjunction with the image cutout options to get quick previews from other HLA images at the same sky position.
• Click the save icon to save the current view. This creates a JPEG image covering the region currently visible in the browser using the current contrast settings. This generates a "clean" image: catalog overlays and other graphics (e.g., the crosshair and coordinate box) are not shown. If you want to capture the graphics as well, use your computer's screen capture function (e.g., the Grab utility on a Macintosh).
• Click the printer icon to print the current view. You can also use the usual print shortcut for your browser.
• The help button opens a new window showing this page.

Contrast and colors

• The Lighter and Darker buttons change the contrast. Note that the image tiles must be reloaded from the server using the new contrast setting, so the results are not instantaneous. Most browsers cache the images, however, which makes changing back to a previous contrast setting happen quickly.
• The advanced contrast controls link displays a more complex set of controls. For color images, the the Lighter and Darker buttons can be used to adjust the color balance between the filters. The Saturation and Sky + and - buttons allow adjusting the contrast parameters independently at the bright and faint ends of the pixel values. Experimentation is the best way to get a feel for what these buttons do; their effect varies depending on the contents of the image. The Reset button resets the contrast parameters to their original values. Clicking the advanced contrast controls link again will hide these controls.
• For color images, the filters are shown as color drop-down menus. You can change the assignment of filters to the red/green/blue bands using these menus. If the filter to color mapping is changed, the image tiles are reloaded from the server.

Plotting

• Type the L or l key on your keyboard to plot a horizontal line from the image at the current mouse position, or type the C or c key to plot a vertical column. The plot pops up in a new window. Note that the plot takes some time to appear because it comes from the HLA web server (since the actual image pixel data are not loaded into the browser.) There are controls for zooming in on the plot, adjusting the scale, binning and smoothing options, and units. Simple measurements (e.g., equivalent width estimates) are also possible.

Catalogs

• Click the appropriate checkbox to overlay source lists from the DAOPHOT and/or SExtractor catalogs constructed from the image. Note that the response of the display can be somewhat sluggish if the source list is large. The source list can be hidden by clicking the checkbox again. Keep in mind that the SExtractor sources actually use elliptical isophotes rather than circular apertures. It is also possible to download the segmentation image that shows the actual apertures used for the photometry.
  
• Click the appropriate checkbox to overlay catalogs from SDSS (Sloan Digital Sky Survey), 2MASS (Two Micron All Sky Survey), GSC2 (Guide Star Catalog 2), FIRST (Faint Images of the Radio Sky at Twenty Centimeters) or GALEX (the Galaxy Evolution Explorer ultraviolet survey). This can be especially useful for checking the absolute astrometry of the image.
  
• You can also overlay sources from the Hubble Source Catalog (HSC), which merges source lists from individual HLA images into a master catalog. Note that this catalog is still in active development and is currently in beta release. The current version has several limitations which will be remedied in future releases; See the HSC Frequently Asked Questions for details.
• The advanced HSC catalog controls provide some options when overlaying Hubble Source Catalog objects. The Apply astrometric shift checkbox corrects the astrometry of the HLA image using corrections that are derived by the process that creates the HSC from individual HLA catalogs. The corrected astrometry is used for other external catalogs that are overlaid on the image (e.g., 2MASS). This option is available only if a shift has been determined for the image. The Require NumImages>1 option filters the HSC to include only objects detected in at least two images. This removes many of the spurious sources from the catalog but also removes real sources (e.g., in fields that have been observed in only a single filter, all objects have NumImages=1). In some fields with many observations, it may be helpful to increase the limit to a value larger than 1.
• Click on sources to pop up a window with information on source positions, magnitudes, etc. If there are multiple sources nearby or sources from several catalogs, information will be shown on all of them. Use the All columns button above the catalog listing to see more information, or the Main button to get back to a limited set of columns.