Explorer Scientific N208/812 Newt (F3.9)
Thanks......I haven't found any specs on the corrector but I bought it & the scope from High Point. The corrector was at their recommendation. When first received I tried the different spacers that came with the scope & landed where I'm at today as this combination was the only one where I could achieve focus. Assumption was that I had it correct.
Thanks John & ks……………… I did have fits with collimation of this scope when first bought new. I was close to returning it as it had defective secondary vanes. They sent me replacements so I toughed it out. I do have a chelsire & a laser collimator & feel pretty confident in my set-up; however, I'll take another look at the star test as you've suggested at next opportunity.
One concern I still have is the offset of the secondary for this fast F3.9. Secondary is supposed to be 1/8" away from focuser & 1/8" closer to primary. I'm not confident that its there & have spent a bunch of time trying to tweak it. Any thoughts or experience with this? Could this be contributing to what I'm seeing?
I don't think the secondary is super critical to star shape. I think that's mainly about maximizing your illuminated area on the camera sensor and getting any vignettting centered. For now I think you can just make sure it points the laser beam into the middle of the primary, and come back to it later when you have sorted out the bigger issues.
You're making much faster progress than I did. For me the key was Chris Waters telling me I couldn't tell squat about the spacing issues or possible tilt in the system until I got the collimation dead on. And by that, I mean better than I had succeeded in getting it with my laser and Chesire. The star collimation was the beginning of progress for me. It made it obvious I hadn't gotten it that close up to that point. It may have also been telling me that perfectly collimated with my Chesire in the focuser wasn't perfect collimation with my camera in the focuser. I'm not smart enough to understand these things fully.
Once the collimation is dead on you can home in on the spacing as follows: If stars are stretched radially (zooming out from the center of the FOV), you are too close. If they swirl around the edge like a toilet being flushed, you are too far. If you get one of those things in some corners and the other in other corners, you have a tilt issue. That's usually solved by buying a tilt corrector and putting it between the corrector and the camera.
I'm not a super expert or anything, but I don't thnk what you have here looks half bad except for the lower left corner, where I think you probably have a tilt issue (assuming that your collimation is good). Gerd Neumann sells a very nice tilt corrector that allows you to adjust for tile in "real time". Its pricey but the less expensive ones make you take the corrector off the camera each time you want to adjust. Its pretty cumbersome. If you buy a tllt corrector, make sure its optical path isn't longer than your current spacing, which looks pretty close. I thnk the Gerd Neumann one is something like 17mm, and the shortest one I've seen is 9 mm https://www.firstlig...t-adjuster.html
If you are tight for backfocus distance, Baader and Teleskop Service sell very short optical path (1 - 2mm vs the usual 10 to 11mm) "T rings" for Canon cameras. One has a 48mm connection and the other is 42mm (complementary to the FLO tilt corrector)
Also, its easy to fret over star shape issues that turn out to be due to seeing. As you go through the optimization process, stop once in a while and actually stack some subs. In my case I find the stacked image generally looks better than the subs.
Edited by John Tucker, 22 January 2020 - 07:41 PM.