Last weekend, I used CCDAP4 and had it suggest the dither amount. It recommended 10 pixels. I noticed a bad issue where after the guider had the star acquired, it would then dither and the star would be completely off the guider subframe. This would cause the entire guide sequence to fail. I had to drop down the dither to ~5 pixels and then it seemed to work OK.

ccdap20080928_211842.log (13 Kb, 25 downloads)

Hi Ozy,

Notice the log entry at 21:38:06. For some reason, CCDSoft did not report guide errors and the exposure continued. That is a problem with the CCDSoft automation interface. Try closing and reopening CCDSoft if you see that again.

A better dither number for your system is more like 6 pixels. To test guiding recovery, run a short session with three exposures. In the first three exposures, dithering swings over its maximum range so you can see how it will run unattended by making sure recentering after dither behaves properly.

BTW, since guiding algorithms determine centroids to at least a factor of 10, you might do better binning your guider 2x2. Reinitialization recommended if you do.

Actually that log entry was because the dither failed, and instead of hitting abort on CCDAP, I quickly hit stop on the autoguider, take image, reclicked on a guide star, and hit guide again as fast as possible. CCDAP didn't like that, but I didn't lose the shot.

I don't quite understand the details in what you mean by why binning 2x2 is OK. Any links to reading on that or easily explainable?

Most folks bin 2x2 because of increased SNR. 2x2 binning reads four pixels at once and only costs a single read and therefore one read noise contribution. For the same star, 1x1 binning takes 4 reads with twice the read noise. Camera centroiding algorithms are good to 1/10 to 1/20 of a pixel. So you lose nothing binning 2x2 with reasonable guide plate scale (yours is) and gain SNR.