The TDRSS Finding Chart

One of the primary requirements for the Swift mission is to rapidly provide optical finding charts for transient sources.

The first TDRSS finding chart is an $8 \times 8$ arcmin, 150 s white image which is telemetered down in the earliest TDRSS packets. This is done to provide an optical finding chart for rapid ground-based follow-up observations. The finding chart data are usually obtained within 200 seconds of the detection of the burst and distributed to the community via the GCN within 270 seconds of the initial detection. In order to conserve TDRSS bandwidth the finding chart is constructed by making a list of the brightest sources in the image. Each source is parameterized into an (X,Y) location (detector coordinates) and a 16-bit intensity. These data are converted to a FITS format image and World Coordinate System (WCS) information is added to the file header. Transient objects in this image are identified by comparison to a catalog of known sources (currently the USNO B1.0 catalog) with a limiting magnitude of approximately V = 21.

The TDRSS link can only transmit a fixed number of bytes for finding chart data. Therefore the limiting magnitude of the finding chart will depend on the number of bright sources in the field of view. These sources will be sorted based on their observed v-band flux with the brightest sources being transmitted to the ground. The finding chart contains approximately 35 sources with a limiting magnitude of approximately v = 18 for uncrowded fields.

The current observing sequence (as of 7 October 2008) has three finding charts sent down via the TDRSS link. These finding charts are identical except for the exposure time and filter used. The three finding charts are

1. 150 s white
2. 250 s u
3. 150 s white

Finding charts may be truncated or not taken depending on the amount of observing time available in the current orbit.

In addition to the TDRSS finding chart a GeNIe corresponding to each finding chart is send to the ground early after a transient has been detected by the XRT. The GeNie is the $324 \times 324$ finding chart pixels around the XRT localization. These images will facilitate solving the question of ``dark'' GRBs. A GeNIe is sent immediately after each Finding Chart via TDRSS to the GCN.