My new ZWO ASI 533 MC Pro camera arrived at the beginning of December. As I've previous mentioned its my first real astro camera and will be replacing the Canon 600D modified DSLR I've been using for the last 2 years. I'll be updating my Equipment pages on this site shortly with the new rig setup as I took the opportunity to tidy up the rig a little.
Anyway the first week of December although clear was plagued with a full Moon. Actually this worked to my advantage as it allowed me to create the new Profile in N.I.N.A, updating the necessary configurations. Also, as I had adjusted the rig I recalibrated the PHD2 settings and generally played with the new camera without there being any pressure to grab great pictures.
One of my biggest concerns of was getting the back focus correct, the distance between the last pice of glass in the optical train and the camera sensor. I had done this with the DSLR but it was early in my journey and given the extenders that came with the Zenith Star 61 I had almost accidentally hit upon a correct distance, with the Adjustable field flattener being used to add on a missing 12.5mm. Anyway, new camera so I tried to take a more methodical approach to this. Long story short, after reading more forum posts I came to a 12.5mm distance once again if I used all the spacers that came with the ZWO camera, with a minor (perhaps 2mm) modification due to the 2" Optolong l-enhance filter. So how accurate do you need to be with this, obviously eyeballing the 12.5 mm mark on a screw thread and locking down to that distance is not going to be anything better than a +/- 0.5 mm. I therefore adjusted left and right in the field a few millimetres and even tried the N.I.N.A. Hocus Focus plugins capability with its aberration inspector mode.
My conclusion, on the small 1 inch sensor at my fairly wide focal length the eyeballed setting seem fine. Adjusting a rough mm in each direction really showed no difference. Perhaps on longer focal lengths and definitely if I was to use a larger sensor then I have a feeling this would have been more important to get correct. If I may I'll relate something that this exercise emphasised to me:-
Forums are undoubtedly useful sources of information, BUT use your own judgement based on the images you have. In my case with a full frame camera it may be necessary to run focus tests and by micrometer callipers to get back focus, but with the equipment I have why chase a theoretical optimal setting if you cannot see the end result.
Anyway, a couple of pictures of the new rig and in particular the position in the field adjuster - you can see after being screwed down the 5mm mark is just showing.
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