The Neil Gehrels Swift Observatory

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Tails in 100-s Finding Chart Exposure

These occur because the satellite is still settling when the finding chart exposure begins. The tails can be removed by screening out early data from the event list. Alternatively, an elliptical region file can be created so all photons from the early afterglow can be used in determining early magnitudes. The finding chart image is taken in Image/Event mode, where the event window is typically the full 2048x2048 while the Image window is 960x960 pixels. For data taken after the final drift correction upload, the image data from the finding chart exposure does not generally show tails.

For event data, detector positions are converted to sky positions based on the spacecraft attitude information. During a slew and just after settling, the source of attitude information switches from the gyroscopes to the locked star-trackers. This can introduce a slight jump in the resulting image such that there is a nearby ghost image for each real point source. The best way to look for this effect and decide how to proceed is for users to plot the spacecraft RA and Dec values during the time span of the finding chart exposure. If a jump in position information is observed, tools such as ximage can be used to extract two separate images from the event data: one from before the star trackers locked on, and the other from after.

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