APT Exposure Map

For several classes of observations, for example mosaics or programs that repeatedly cover the same region of sky, it is sometimes useful to make an exposure map to determine the amount of exposure time for a particular position in the region observed. APT has the capability of providing data that will allow the user to create such a map (outside of APT) via the Export Exposure Coverage option.

Once you have created the proposal, under the File menu, go to Export/Export Exposure Coverage data. This will export the exposure information for the entire proposal into the directory you last saved your proposal into. There will be 2 files.

PropID_coverage_catalog.csv file

This file provides a breakdown of the region covered by each exposure in the proposal, by recording all the positions of the corners of the detectors within each exposure. Each line corresponds to a reference point within a detector, thus one exposure will be represented by several consecutive lines.

The first field in each line is a unique "Name". The last five characters of the Name constitute the point identifier, which specify the shape point under consideration. Specifically, the point identifier has the form sh-vn, where 'sh' is the shape descriptor (a number followed by P for polygon and C for circle), and 'vn' is the vertex identifier (a V followed by the vertex number (origin 0)). For example, for a ACS/WFC exposure, you would have 2P-V0 for the first vertex of the second polygon (the WFC2 chip)

The correspondence between exposures and lines is as follows:

Each line in this file contains the following comma-separated fields:

This file is formatted such that it can be loaded into Aladin after replacing the commas with Tabs. Aladin will plot the vertices in a plane as if each vertex were a star in a catalog. When vertices are selected in the Aladin display, the column data is displayed at the bottom.

PropID_exposure_coverage.csv file

This file provides detailed exposure information in a more compact format, one line per exposure, and is recommended as the starting point to obtain an exposure map. The columns are the same as for the coverage_catalog.csv file, with the following exceptions:

In general, the coverage for an exposure is the union of the coverage for all shapes in the STC descriptor. The exception is the circle for WFC3/IR, which denotes an exclusion region and therefore must be *removed* from the coverage.

Using the APT Exposure Map files

In order to obtain the exposure time at a given location from the exposure-coverage file, users will need to carry out the following steps:

If an exposure map is desired, users will need to write a module capable of repeating the steps above for a grid of positions.