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First image stack.
The image below is a stack of the best 26 (out of 37), unfiltered, one-minute CCD images taken by Joe Dellinger and myself at the George Observatory, using the Fort Bend Astronomy Club's 18-inch (46 cm) f/4.5 reflector and the Zilkha AP-8 camera. The image sequence began at 03:39:15 (UT) on 1 April 2002, a little more than 11 hours after the burst. The mid-time of the stack was about 04:07 UT. The image is roughly 40 arcminutes on a side. North is up, East is to the left.
Limiting magnitude is roughly equivalent to the DSS2 red image (about mag 20-21). The individual images were dark subtracted, flat fielded, hot/cold pixel removed before being averaged together.
I blinked the images against the DSS2 red image, and noted one very faint object inside the GRB error box in the DSS image at 13h 16m 51.97s, -17 53' 15" (pixel 468, 449), and another south of the error box at 13h 16m 29.36s, -18 03' 13.4s (pixel 596, 699). The second object is brighter, PinPoint gives a magnitude of 21.3 using the GSC-ACT catalog. Given that no filters were used, and the GSC ACT has well-known problems, this number is rather dubious.
Of course, these objects may have other explanations rather than GRB afterglows (e.g., noise cluster, variable stars, different spectral sensitivity of CCD vs film, etc), we will try to re-image the field as soon as possible. An attempt was made by Joe and Paul Garossino on April 5 UT, but they were only able to get 8 one-minute images before the clouds rolled in.
Note that we moved the telescope a few minutes of arc in RA and DEC so that on-camera flaws would have a triangular pattern. The most prominent can be seen in the upper right.
I did three sub-stacks of our images to identify moving objects. Five asteroid were found (with coordinates sent to the MPC), I don't believe any are in the GRB's error box.
Bill Dillon
