Is vignetting a problem due to the fast focal ratio of the HyperStar system?
Vignetting will only be noticeable with the largest CCD chips such as those in the ST-10XME and ST-2000XME cameras. Even then, the amount of vignetting is minimal and is easily removed using standard flat-field techniques. In fact, none of the images on this website required flat-fielding. Another concern is that the mirror used in SBIG self-guiding CCDs will block a portion of the main CCD. Test show that vignetting affects about 30 rows of the CCD and that the light loss is less than 10%. It is rare that 30 rows of pixels are not cropped from the final composite image anyway! The worst case scenario for vignetting is for the off-axis guide chip in an ST-10XME camera. Tests using this setup show only about 15% of the guide chip is sacrificed and that guide stars are still easy to find. Consider also that unguided exposures of up to 120 seconds are feasible at the short focal length produced by the HyperStar system and that self-guiding is much less critical using this setup.
