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SBIG ST-7 Review

After much study and concentration, I just threw caution to the wind and bought the ST-7. I have been very satisfied with it and cannot imagine that I would have been able to do the work I've been doing with asteroid measurements without the guiding function built in. The guiding function works well even with my HUGE periodic error (+/- 1 arcmin or so). With this large problem, I am limited to tracking exposures of 5 secs or less, which limits my choice of guide stars. Nevertheless, I have been able to get by quite well. With a better telescope drive, I'm sure it would be nearly flawless.

The small pixel size is a curse/blessing. As with all CCDs you should try to optimize the focal length/pixel size combination to yield pixels that cover about 2 arcseconds of sky. With the ST-7 and its 9 micron square pixels, that implies a focal lenght of about 900 mm.

I used an Orion focal reducer with my 8" SCT to get an EFL of some 1400 mm, which yields pixels of 1.3 arcseconds. Typically for my asteroid work I use the 2x2 binning to get 2.6 arcsecond pixels. The advantage is that the larger pixels are more sensitive and permit deeper images for a given exposure time. The resulting FOV at 1400 mm is some 17 x 11 arcmin. The image file size for the 2x2 binned shots is about 100k (compressed).

Beginning in the Fall of 1997 I started using a MaxField .33 focal reducer from Optec that produces an EFL of some 660 mm, which implies 2.7 arcsecond pixels at the highest resolution. It also means a FOV of some 35x23 arcmin. The file sizes, however, jump to 400k (compressed) and download times are proportionally longer. The much larger field coverage (which raises the probability of discovering a new asteroid considerably) is worth the extra cost.

I didn't know what I was ordering and I got the anti-blooming option. It is true that I'm not bothered with those ugly blooming streaks from saturated regions, but I also suffer a reported 30% decrease in sensitivity. Given that most of my images have been of asteroid fields with hardly ever a star bright enough to saturate (and bloom), I plan to have the chip changed in the Summer of 1998.

All in all, I give the ST-7 high marks.