The Canadian Astronomy Data Centre Herzberg Institute of Astrophysics

Acknowledgments

This page describes HST image groups,why they are relevant, how they are produced and how to search for them.

A "group" is a list of two or more HST images that cover more or less the same area of the sky. The images are not necessarily taken in the same filter, at the same time or same orientation. The images have not necessarily been added together to form a visit or an association, but this may have taken place.

The main reason that image grouping is useful is that it allows users to search for patches of sky that have been covered by HST in multiple bands. With the previous search tools, it was possible to search for all images taken in any given set of bands, but it was up to the user to sort through the list to which images lay one on top of each other. Other than certain well-known fields (e.g. the Hubble Deep Fields) it was difficult to find areas with multi-band coverage.

The images are grouped together based on position and size. The "size" of an image is just its maximum extent in RA and Dec. ACS images are about 3.5 arcminutes on a side; the WFPC2 images are about 2 arcminutes. The images are first sorted by RA into a list. Initially, each image is in a group containing just one member. The grouping program loops over the the groups looking for other groups that are "nearby". If "nearby" group is found, the two groups are merged. The members of the second group are moved into the first group and the second group is discarded. The group's position is adjusted (to the average of the positions of the original two groups) as is its size (to the larger of the sizes of the original two groups). After the groups are merged, the iteration continues. The process ends when no new groups are found to be "nearby" to each other, which takes about 5 iterations.

Groups are deemed to be "nearby" if their positions are within a quarter of the sum of their sizes. Very roughly, this means the images overlap by at least a half. This criterion represents a compromise. One could define two images to be "nearby" if they overlap even slightly. However, this would generate a number of groups that contained images that only overlapped by a few arcseconds. This would be of limited usefulness to most researchers unless that spot contained a particularly interesting indivdual source (but in this case the researcher is more likely to do a positional search anyway). Using this marginal criterion could be useful in order to find areas of the sky that have been tiled by HST (e.g. the GOODS fields or the Extended Groth Strip), but in general, these areas are already well-known. Alternatively, one could restrict the definition of "nearby" to images whose positions are within a few arcseconds of each other. This, however, would exclude a number of potentially interesting groups with partial overlaps. The current definition of "nearby" is not so restrictive, but does ensure that most groups have about 1 square arcminute, which is large enough to be useful,

So far, only images from the WFPC2 and ACS cameras have been grouped. Data from other cameras may be added in the future.

The main search page allows you to search for groups. The form allows you to specify a minimum number of exposures in each band as well as a mininum total exposure in each band. Click on "Submit Query" to find groups meeting all of these criteria.

The search will return only groups meeting all the specified criteria. The criteria are combined with a Boolean "AND". There is not currently a mechanism for combining criteria with "OR". Nor can one search for groups containing a "generic" colour, e.g. "search for UV data". If you leave the a box blank (or containing "0"), it will be ignored.

Mousing over a filter name will give you a short description of that filter (assuming you have JavaScript enabled). Clicking on a filter name will give more details about that filter.

The figure below shows a typical search result. It gives the Group ID, RA and Dec of each group. For each filter specified in the main search page, the table gives the total exposure time (labelled "Time") and the number of exposure in that filter (labelled "Num").

The table may be re-ordered by clicking on the small blue arrows in each column. If this feature, fails to work, enable "JavaScript" on your browser.

Clicking the "List" link for each row will give you a list of the images in that Group. Clicking the "GSky" link for each row will give you a the footprint or layout of the images.

This page lists all the images in a group and gives you mechanisms to retrieve them. It provides only a minimum of metadata (centre, exposure time, filter and instrument). For more detailed information, click on the "Metadata" link for an image. To get the data itself, click on the "Data" link for that image. This option is useful if you only want one or two of the images. A more convient way of getting all the images is to click the "MarkAll" button to select all the images and then click the "Request Marked Datasets". This will take you to a page that will allow you to download all the images simultaneously using the CADC Download Manager.

If you came to this page from the "GSky" page described below, it may contain two section. The first will list all the images in the group, and second will list images that do not belong to the group, but which lie nearby.

The GSky interface lets you view the footprints of the the images of a given group. It also lets you see the images in the neighbourhood of that group. This allows to include more images for download if the grouping algorithm was sub-optimal. The interface is based on the Google Maps technology. It will not work unless you have enabled JavaScript on your browser.

The following picture shows a typical screen shot of the GSky. A "Plate-Carree" projection is used; this will cause some distortion near the celestial poles. Descriptions of the features labeled in red are below the image.

1. These are the outlines of the images in the group. They are shown as heavy lines. The WFPC2 images are shown in red and the ACS images are shown in black.
2. These are the outlines of the images that are not in the group. They are shown as faint lines. The WFPC2 and ACS images are shown with the same colour scheme as above, and the ACS Visits (which are stacked images) are shown in blue. The ACS Visits are never included in the groups because they contain multiple individual ACS images.
3. These buttons allow you to pan the map and to control the zoom level. You can also move the map by clicking on it and dragging.
4. These areas show the current map centre and zoom level. You can change them by entering new values and clicking "Reset Centre". The RA and Dec entries will accept hh:mm:ss notation as well as dd.dddd notation
5. This area allows you to control the filters that currently displayed. By default, all the images from the group with any of the filters you selected in the initial query are shown. By ticking the checkboxes, you can display only a subset of the filters from the initial query.
6. This area allows you to control the kinds of images that are displayed. Click on the check boxes to display or hide images from the WFPC2, the ACS or the ACS Visit images.
7. These two buttons allow you go back to list images in a group. The upper button ("List images in group H...") will just list the image belonging to the group (the same as if you had selected "List" instead of "GSky" from the list of groups page. The second button ("List all visible images") will list all the images on the screen. Zoom out to include more images.
8. Click this button to toggle between "Normal" and "Select" mode. In "Select" mode, you can select a sub-section of the screen. Click once to start the selection, then click a second time at the opposite corner. A green box will be drawn indicating your selection. Note that the more intuitive selection method (click and drag) is of course already used to move the map. When you have made a selection, the "List all visible images" button will change to "List all selected images". Clicking on it will list all the images inside the box, whether or not the belong to the original group.
9. This area shows the RA and Dec coordinates of the location of the mouse when it is over the map.