I finally took the next big plunge in imaging: I switched from a guide scope to off axis guiding (OAG). I was hesitant for a while, as I read a lot of horror stories about it being hard to find guide stars, and that they needed to be planned out for a target sometimes.
My guiding has been quite bad lately with the cheap 60mm guidescope that I was using. I carefully watched the guiding one night and noted how much the star shape was changing from frame to frame. I spent a good while attempting to improve focus, but the star shape never improved from about 12 pixels wide. And if I’m trying to guide at less than one pixel on a 12 pixel star, the profile better stay darn consistent. The cheap optics never let that happen, and the profile would vary wildly with seeing caused by the chromatic aberration. It was time to upgrade.
Since my cameras are ZWO, I decided to go with their standard OAG. It readily replaces a spacer that comes with their imaging cameras, so I was easily able to retain the 55mm of backfocus that I need for my coma corrector. It took just a few minutes to install it and the guide camera. I was not able to find a configuration that kept the prism away from the sensor, so this might be something I have to fiddle with more. I’m hoping that flats will correct the shadow from the prism.
So far, after just a few nights of playing around, I have not had a single problem finding a guide star. The huge aperture combined with the sensitive ASI178MM guide camera is definitely sufficient.
But the best part? My guiding is finally working! Over a few nights, I have averaged 0.4″ to 0.7″. The guiding is much much smoother, now. Much less variation in RA and declination. Declination mostly stays within seeing error, and the RA periodic error is now much more predictable for PHD2. Here is an example of guiding I never thought I would achieve:
