20100920UT

These are (left-to-right) bias frames PreCam_2813.fits, PreCam_2817.fits, and PreCam_2818.fits from http://sdssdp62.fnal.gov/precam/Raw/20100919/. Note that 47 Tuc (the bright blob of stars) is clearly visible halfway up on the right side of each image, even though these were 0-sec dark (a.k.a., "bias" or "zero") exposures. Could this be persistence (we observed 47 Tuc before these images were taken), or just a lingering effect from the shutter problems earlier that night (which we just fixed a few exposures beforehand; see night log here)?

(Note: similar problems appear in images from the night of 20100921UT and other nights.)