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 is based on Leif Wilden's fits2web Javascript program.

• 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 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.
• 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 adjusting the scale, binning options, and units.
  

  Currently plotting does not work for HLSP (high-level science products). In the future we plan to enable this capability.

• 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 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.
  
• 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.