Chandra Footprint Service Help

GETTING STARTED

The goal of this website is to provide a visual interface to the data captured in the Chandra Data Archive by:

  1. Providing a table-based and image-based view of the existing CXC data products and CXC SIAP service
  2. Adding a footprint (sky coverage) and visualization service to make it easier to browse, select, and download Chandra data
  3. The footprint service is available for two distinct components of the Chandra Data Archive: the archive of all processed public observations (through Level 2); and the subset of data included in the Chandra Source Catalog (Level 3)

This website provides direct access to images; for downloading complete datasets (including event files) it refers the user to services provided in conjunction with ChaSeR. The services that are being accessed use data that are in the public Chandra Data Archive. Proprietary observations are not included in the returned tables or the Footprint overlay. If you need to access data for your proprietary observation, you will still need to use ChaSeR to access your data.

Browser requirements: Currently the Footprint interface is supported on Firefox 3.x and Safari (version 4).

HOW TO SEARCH

Type the name or coordinates of an object of interest into the "search box" at the top of the page and click on the Search button. If a name is used, the request is sent to the name resolver at SIMBAD (using NED as a fallback), which returns the coordinates and an image size that are used for the search. Some examples:

M101 Most objects have a default search radius of 0.2 degrees
M87 r=1.0d You can explicitly change the default search radius
Antennae The resolver recognizes many names
14h03m12.6s +54d20m56.7s If no radius is given a default of 0.2 degrees is used
14 03 12.6 54 20 56.7 Units are optional; equinox must be ICRS
14:03.210 54:20.945 Many formats are accepted
180.468 -18.866 r=0.4d Default units are degrees in this case
122 Entering a single obsid results in the same results as entering the target name or coordinates
122,22,1741-1743,1739 If multiple obsids are entered, only results for those obsids will be returned. Obsid ranges are allowed. Appropriate image center and search radius are automatically calculated. Any input search radius will be ignored.
0 0 r=180d An all-sky search. Some restrictions exist.

There is an Search Options button to the right of the search box that reveals more options for selecting the data of interest, e.g., by directly entering the RA, Dec, and/or Search Radius of your search. If you already have performed a search for a target, changing the radius in the Search Options will reload data for the new radius without re-resolving the target coordinates. If you are encountering difficulty resolving a target name, or your coordinates are not being recognized correctly by the resolver, you may directly enter the RA, Dec, and Radius here in decimal degree format (ICRS).

The Image Preferences button exposes options can be used to increase the size of the small preview images which are shown on the Preview Images tab of the web page.

BROWSING THE SEARCH RESULTS

The tabs just below the search box provide three basic ways to view the results from the search:

FOOTPRINTS - Footprints are the outlines of the region of the sky associated with each Chandra observation. In this view, footprints are shown overlaid on a Digitized Sky Survey (DSS) image, as well as listed in tabular form.

Selectable options for the footprint page include:

A table with various information about each image is provided below the footprint image. You can either click on a table row to see the corresponding footprint, or on a footprint to highlight the corresponding table row. Selected datasets can also be highlighted through the Image Inventory and Preview Images views, and will remain highlighted in all three views (Caveat: The Footprint tab uses a different data source than the Image Inventory and Preview Images tabs, so the table results will not always be identical). There is also a textbox that displays the total exposure time of the selected rows.

Note that filtering in the Image Inventory view does not affect the datasets shown in the footprints view. We plan to rectify this shortcoming in a future version.

FILTERING - You can display only a subset of the Footprints and Inventory by using the empty boxes under each column heading to filter out certain rows of the table (e.g., HRC* will select only HRC-I and HRC-S images).

There are two types of filters supported:

Numeric columns support filtering using specific values (e.g. 3450), ranges (e.g. 3450..3455), or limits (using >, >=, =, <, or <=).

Character fields support only exact matches and matches with wildcards (*). Examples: HETG or *ETG
If no wildcards are specified the string must match completely. Append a '*' to match select all rows matching at the beginning (i.e., '3C*' matches '3C273'). Matches are case-insensitive (i.e., '3c273' matches '3C273'). A '!' at the beginning selects all rows that do not match. Move the cursor over an empty box for instructions on the filter formats supported by a given column.

Chandra Source Catalog - If you are interested in only viewing results for the Chandra Source Catalog, you can add the 'level' column to the Footprints tab's table of observations. By placing a filter to only show level 3 observations (enter 3 in the filter box), you can view only footprints and observations which are in the Chandra Source Catalog.

To see the smallest footprints you may need to increase the scale. This is done by changing the search radius, either by adding "r=value" in the search box or by changing the radius box value in the Search Options. The minimum radius used in the footprints view is 0.01 degrees. The background DSS image is not particularly helpful for very small fields, but the ability to see how the HST observations relate to one another can still be helpful.

An important point to keep in mind when using footprints: While we try to make the footprints as complete as possible, some observations may be missing. For example, this site does not include planned observations or observations which are still within their proprietary period. As observations become public, they will be added to this tool.

Click on a table column heading to sort by that value (e.g., "ObsID"). Clicking again on the same heading reverses the sort order. If there are more than 20 images found, there are controls to go through the table by pages or to jump to the first or last page. You can also change to 10, 50, 100, or 500 results per page (Note: displaying 500 results per page may slow your browser).

Brief definitions of the table columns are provided at the bottom of the page; you can also get tool tips by placing your cursor on the column heading. By default not all of the (many) table columns are displayed. See instructions on the bottom right to add, remove, or reposition columns.

SELECTING - Table rows can be selected by clicking on them, and selected rows are highlighted. Clicking a selected row toggles the selection, so the row becomes unselected (and unhighlighted). Observations can also be selected in the Image Inventory and Preview Images views described below; Observations selected in any view are also selected and highlighted in the other views. The Reset selection button unselects all the selected rows.

The row selection can also be used for sorting and filtering the table. By default the selected rows are mixed in with the unselected rows, with the sort order determined by other parameters, but there are four possible choices:

When the selected data are shown first and the table is sorted by a column, each group of rows (selected and unselected) is sorted separately.

When rows are selected, they are highlighted but do not immediately move (e.g., to the top of the table when the selected rows are shown first). But when you move to a new page, the selected rows do get resorted and moved. So when selected rows are being shown first, you may notice that when you click a row, move forward a page, and then move back, the selected row has moved. This is even more noticeable when only selected or non-selected rows are shown; then rows may disappear altogether after paging forward and back. If you find this behavior disconcerting, you can use the default "Mixed" view, where whether a row is selected does not affect its position or visibility in the table.

Note: When switching between views the selection method (First, Mixed, etc.) will be reset.

Observation selection is integrated across the Image Inventory, Preview Images and Footprint views, so observations selected in one view are highlighted in all views.

Image Inventory - Shows the results from the official CXC SIAP service in a table format. The operation of the table sorting/filtering is identical to the operation of the table in the Footprints view. This table uses a different data source (official CXC SIAP VO service) than the Footprints view, so there may be a discrepency between the columns in the table, and which observations are displayed returned.

Preview Images - Shows preview images of the search results, using the same sorting, filtering, selection, paging mechanisms as the other views. The Preview Images and Image Inventory views always contain an identical set of observations, and selected observations in one will remain selected in the other. This generally holds true between the Footprints view and the other two views, but because they draw from two different data sources, there may be some inconsistencies between the Footprints view and the other two views. Each Preview Image is annotated with the Observation ID, which should help to identify observations that are relevant to your search.

The Images view offers a few additional options for data access through the associated "Observation Details" links. On the observation details page you can view additional details about the observation, such as the instrument configuration, proposal abstract, high/low resolution preview images, primary/secondary products, etc). The "Observation Details" page will open in a new window.

DOWNLOADING OBSERVATIONS

Downloading of full observational data is done by requesting a package of selected data. There is a Download Selected ObsIDs button on each tab of the footprint service web site which will allow you to batch download all of the observations that you have currently selected on the footprint service web page. If you would like to download only the primary or secondary products for an individual observation, you may do so by selecting the "ObsID" link on the Footprints tab or by selecting the "Observation Details" link for the observation on the Preview Images tab of the footprint service. From the page that opens, you can request a download package of the products desired.

VO SERVICES / Get VOTable

The CDA provides access to the data used to generate the footprint service overlay via a Virtual Observatory (VO)-compatible Simple Image Access (SIA) service. The VO SIA protocol allows retrieving image data from a variety of astronomical image repositories through a uniform interface. A query defining a region on the sky is used to select candidate images. The service returns a list of candidate images formatted as a VOTable. For each candidate image an access reference URL may be used to retrieve the image. Images may be returned in a variety of formats including FITS and various graphics formats.

To access the SIA service on the CDA Footprint server, use the URL http://cxc.harvard.edu/cgi-gen/cda/footprint/get_vo_table.pl?strict=1 with a region expressed as the right ascension and declination of the field center and a search radius, both specified in decimal degrees. For example:

http://cxc.harvard.edu/cgi-gen/cda/footprint/get_vo_table.pl?strict=1&POS=40.669,-0.013&SIZE=0.1

This query will return a VOTable of data that overlap the user input region. The client then can access each image via the URL field in the VOTable.

Note that the CDA Footprint service returns observations for which the actual sky coverage footprint of the observation overlaps the search circle. This differs from other SIA implementations, which usually return images with a reference position that falls within the search box. Specify SIZE=0 to retrieve data only for images that cover the specified RA/Dec point on the sky.

CAVEAT: The VOTable that is queried will contain all matching data that is found in the search region. The filters that are set in the table and the instrument check boxes on the footprint overlay will not filter data in the VOTable that is downloaded. You may query for specific instruments of interest by removing the unwanted instrument(s) from the search query url. In the future, we hope to handle instrument-specific VOTable queries automatically based on the current user settings.

LIMITATIONS AND KNOWN PROBLEMS

Filtering between Views:
When switching between the Footprints view and either the Image Inventory or the Preview Images view, any filters that you have set will be removed. This is due to a difference in the available list of columns available in the data sources for the Footprints view and the other two views. If you switch between the Image Inventory and the Preview Images view, your filters will stay in place. It is only when the switch involves the Footprints view that the filters are cleared.

Downloading Data:
The page that is opened when downloading data contains a plain-text URL that needs to be copied and pasted into a browser tab or wget/curl session. We hope to resolve this in the future.

All-sky searching:
All-sky (0 0 r=180) searches can lead to very long searches with a large load both on our servers and your browser. It is very likely that your browser will time out while waiting for an all-sky search to complete. If you need to find all of the results for a search, it would be advised to fetch the data as a VOTable directly by querying http://cxc.harvard.edu/cgi-gen/cda/footprint/get_vo_table.pl?pos={RA,DEC}&size={RADIUS}&inst=ACIS-S,ACIS-I,HRC-S,HRC-I . You may remove unwanted instruments from that search URL, and the RA/DEC should be in decimal degrees (ICRS) with search radius also in degrees.

Footprints in the vicinity of RA = 0 or a pole:
Footprints that contain a segment of the RA = 0 longitude or are near a pole may be drawn incorrectly. This will be fixed in the future.

FUTURE ENHANCEMENTS