Chandra Footprint Service Frequently Asked Questions

FAQ - General

  1. Does the footprint service work on all web browsers?

    The footprint service has been tested on Firefox (version 3.6+) and Safari (version 4). You may find that the interface mostly works on other 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. We encourage the use of standards-supporting browsers.

  2. Are data from all CXC instruments available via this site?

    Yes, all CXC instruments are represented in the CDA. This site will return searches for any observation which is part of the public Chandra archive. In the case of the Footprints view, you can also distinguish between the standard archive or the observations contained within the Chandra Source Catalog. You may also access public data for both Chandra Source Catalog and non-Source Catalog observations via the Inventory or Image view of this site, ChaSeR or our public FTP archive.

    Please note that the footprint service and the public FTP archive do not contain proprietary observations. If you would like to retrieve data for an observation that is still proprietary, please use ChaSeR.

  3. How can I find a specific proposal ID number?

    The easiest way to find a specific proposal is to search within the Inventory or Footprints view for the target or coordinates desired and then filter the resulting view by proposal ID.

    Alternatively, use ChaSeR to look up the proposal directly.

  4. How can I find some interesting looking CXC images?

  5. 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.


  1. What are the data levels?

    There are two levels of data in the CDA:

  2. How do I search for a particular instrument? Can I sort or filter my results?

    After the initial search for a target or coordinates, you can filter and sort the results by instrument using the boxes under the Instrument column in the Inventory and Footprint views. 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 Instrument field, ACIS* will find all ACIS-I and ACIS-S observations. For numeric fields, you can use the > and < symbols (e.g., in the Exposure Time field, >600 will find all exposures with >600ks exposure time).

  3. 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/Footprint 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.


  1. How do I download images? Can I download several files I have selected at the same time?

    To download an observation, you may either download the image and all supporting products by selecting the observation and pressing the "Download Selected ObsIDs" button, or you can use the individual links in the Inventory/Footprints tables to download individual JPEG/FITS files for an observation.

  2. What is all of the information listed using the "Observation Details" feature in the Image view?

    The Observation Details link at the bottom of the Images view gives access to categorized information about an observation including details about the observation, instrument setup, the proposal abstract, etc.

  3. What is the purpose of the Preview Image Size list in the Image Preferences section of the Search Options?

    The Preview Image Size list contains options for how large to display the preview images on the Preview Images tab of the website.

  4. How can I download the preview images? Can I download full size jpeg images?

    The 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 Images view to download a Low-Resolution (full frame) JPEG image of an observation. Similarly, you can download a high-resolution center image by clicking on the "High-Resolution Preview" link.


  1. 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 "Search Options" link (near the top), changing the radius to the desired amount, and then selecting 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.

  2. 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). Similarly, if you select an observation in the table, the aperture will be highlighted (yellow outline) in the footprint view.

  3. How do I choose footprints that have a small instrument aperture?

    There are two ways to choose a footprint when the aperture is small in the overlay.