I agree with Joe that the most important piece of equipment for imaging is generally the mount, and that should get the biggest chunk of the budget - at least when building a rig from scratch. On the other hand, if your AZ GTi is working ok for you at this point, and what you want most to improve is the lack of color correction on your 102, you can do that by going with a decent ED scope. It could be an ED doublet, or something with more elements, depending on how much you want to spend. The main thing will be to keep the weight at or under your current weight, otherwise you may suddenly become unhappy with the mount's performance.
When thinking about the optics, you'll get a major improvement in color correction over your 102 going to even a decent ED doublet. The color improvement from a doublet to a triplet or better will be much less. (It will also depend on the type of ED glass used, but that's still fairly small differences compared to the improvement from using any ED glass at all.)
If you go to a corrected astrograph like the FF65, the biggest additional benefit (which you are definitely paying for) will be a flatter field over a wider imaging circle. Since you're using a 533, which has a modest sensor, you won't notice that right now. If you're thinking about getting a larger (perhaps APS-C size) sensor camera and want to future proof, this would be more important to think about. (Although you can generally add a field flattener later if you need to.)
And Petval designs, which the FF65 is and which you also pay for, will make it unnecessary to worry about backfocus.
Edited by scanner97, 01 November 2024 - 10:11 AM.