On Friday night I was out for a while, trying to find the right spacing arrangement for the focal reducer/field flattener on the 4". I never did find a combination that gave me the results I'm hoping for - it seems that no matter what I try, the field is not flat. Since this is the William Optics 0.8 FR/FF that is sold specifically to match the Zenithstar 105mm, I'm a bit disappointed to have so much difficulty getting it right. Although I'm having fun with the imaging so far, I would like to see less elongation of the stars in the corners, which this should correct.
I also tested out using a separate autoguider, which will be a big help when the guide star is too dim for certain filters. That part of my test night ended well although there was about an hour or two of frustration before it came together.
So, more testing needs to be done. In some ways I guess that's o.k. because part of the fun of this hobby is doing the engineering to make everything work right.
Anyway I couldn't go the whole night without something to show for it so here is a very short exposure (a few 30-second stacked exposures) of the Open Cluster M39. I calibrated it with dark frames but no flats. I used this field because it has a lot of stars across the field, making it easy to quickly determine whether the image has elongated stars in the corners.
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