The MegaCam Data Graphical Search Tool lets you search for multiple MegaCam exposures taken on the same patch of sky. It lets you make queries such as "Show me where there are more than 100 exposures in any band" or "Show which part of the sky has been observed in the g r and i-bands". It also shows the outlines of the images on the sky, allowing you to determine if the coverage meets your needs. The MegaCam Data Graphical Search Tool also provides access to the stacked images produced by MegaPipe and Terapix. "Stacked" here means that multiple input images have been astrometrically and photometrically calibrated and then added together to form a single, deeper output image.
Four kinds of data are available through this tool:
The main part of the tool shows outlines of MegaCam data. The different data sources are rendered in different colours. Individual images are shown in black, unstacked groups of images are shown in blue, MegaPipe stacks are shown in red and Terapix images are shown in orange. The "Plate-Carrée" projection is used; this will cause some distortion near the celestial poles. The map uses the publically available Google Maps API. The controls work much the same as the Google Maps, although Google's version is more recent and typically has more features. You can click and drag to move the map (or use the arrows) and use the slider to control the zoom level. Double clicking recentres the map on the postion of the mouse. Your mouse scroll wheel controls the zoom. At the highest zoom level the screen spans about half a degree; at the lowest, the screen covers the whole sky.
Recentering the map can be done in one of four ways:
This section has a series of check boxes. Check or uncheck them to change the kind of images that you wish to see. For example, if you only want to see MegaPipe images, leave the top box checked and uncheck the others.
These inputs control the group/stack criteria. Enter the minimum exposure time (in seconds) and number of images you require in each filter. Ticking one of the boxes implies at least some data in that band is required but doesn't specify how few exposures (although obviously the minimum is 1 exposure) or how little the total exposure time (although in practice, it will be 1 second). The tick boxes also control which of the individual images will be displayed. How the criteria for each band are combined is set by the OR/AND buttons at the bottom of this section. For example, if you want only stacks with 5000 seconds exposure time in both the u and r bands, enter "5000" in the "Minimum Exp. Time" boxes under u and r then click on "AND". Or if you want images taken with any one of the gri filters, click these checkboxes, unclick u and z, an click on "OR". There is not currently a way to exclude filters. Criteria within a band (the minimum exposure time and minimum number of images) are always combined with a Boolean "AND", regardless of how the different band criteria are combined. Leaving a box blank is equivalent to setting that parameter to 0.
|List images, groups or stacks|
Once the criteria have been set, click on one of the "List..." buttons. This will open a new page which will list all the images or groups or stacks shown on the screen which match your criteria. From this page, you will be able to retreive the images or stacks either directly via http links or using the CADC downloadManager. Only the images whose centres appear on the main map will be on this list, unless you also specific positional requirements as described in the next section.
You can confine the positional requirements to only a portion of the screen. The first step is to click on "Switch to Select mode". Click once on map to start selection. Click again to end selection. (Many people's instinct is to click and drag, the way one selects text in most text editors. However, click and drag here just moves the map.) As you pause in your selection or after click the second time a green box will show you your selection. To select a different region, repeat the process. Subsequent queries using the "List..." will only include images whose centres are in this region. To get rid of the positional requirements click on "Switch to Normal mode". The drawing of the box is currently a bit slow.
This box tells you the RA and Dec of the location underneath the mouse.