The Neil Gehrels Swift Observatory

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General Issues

Updated December 12, 2005

An overview of the UVOT observing sequence can be seen in the sw*uir.html file in the /log directory of any observation. It is a good top-level description of the event and image files available in the uvot data directories. The UVOT exposure report lists the times of the exposures, which filter, mode, window, and binning were used, exposure time and overall count rate. Users can get a graphical version of the UVOT observing sequence, showing which filters where in place as a fuction of time, by running the FTOOL uvotsequence.

The first UVOT image product available is a processed TDRSS message sent out via the GCN. We refer to this as the GeNI (GRB Neighborhood Image). Early GeNI images were binned onboard so that telemetered image pixel was 8x8 UVOT detector pixels. The current GeNI images are binned 2x2. The GeNI is 80 x 80 in image size. When the GRB afterglow is located within this sub-image, the product can be used to determine an accurate magnitude and position. If an XRT position for a GRB is not determined onboard Swift, the afterglow candidate may lie outside the GeNI image. It is then not possible to search for the afterglow in an image until the first Malindi pass data is processed by the SDC. A source list of stars found in the full-frame UVOT image is also sent trough TDRSS. Depending on how many sources are in the field, each star is sent down as either 1 pixel, a 3x3 or a 5x5 postage stamp. These source lists can also be searched for the afterglow but since they are not full images it is possible to sometimes get false positives and users should use caution in interpretting sources in this product.

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