thank you very much for your answer`s . I understand , that i have a collimationserror and a tiltproblem . The 1,820mm focal length calculation came from PI and the PhotometricColourCalibration Tool and my scope is Meade 10 " ACF . What would you think , useful or not ?
Next - if the ACF is not flat as you say, i don`t understand now, why i don`t need a additionally flattener ?
Todo list for me :
1. Collimation first,
2. Evaluation of focal length with the reducer - but how to do ? AP _for CCDT67 : 305-distance + 16 mm / 305 is the F-ratio , for SCT`s to ? d would be than if i have the ratio d=321-305*F for CCD T67
3. Remove of any tilt in the optic train - I have a tiltadapter from ZWO - all elements in the train are threaded adapters .
4.Shorter exposure times to see what is more collimationserror / trackingerror ? - i will looking for short exposures and show you the images .
I hope i have understand ?
Best wishes and thank you for your help !!!!!
Not certain where PI is getting the 1,820mm focal length, but it doesn't really matter as long as you are only doing comparison from before/after as opposed to comparisons with other scopes. Is your scope the f/10 version or the f/8? I believe Meade currently offers both.
In any event, the reason most people don't need an additional flattener with the ACF is that it is "flat enough". Stars in the corners are usually pretty good as long as your camera has a small enough chip and as long as you aren't trying to use too much focal reduction. I can't tell you, though, that you'll be fine as long as your chip is smaller than 'x' mm or as long as your focal length is greater than 'y'mm. First, I don't own this particular scope so I would just end up looking up other people's results just as you can, and second because everyone's standards for what constitutes "good enough" is different. None of my telescopes--not one--has stars that are as good in the extreme corners as in the center of the field. That includes OTA's as expensive as a $20,000 Astro-Physics Riccardi-Honders. But all of them meet the standard of 'good enough'. My personal standard of 'good enough' is that FWHM in the extreme corners--all four--should be within about 5% of the center of the field. That's enough for me to stop caring. So if I can get 1.9" stars in the center of the field, I would be fine with 2" stars in the corner, but I would work on improving things if I were at 2.1" or 2.2". Improving things might mean choosing a different focus point not in the center (what I did with an RC I had), or tweaking the spacing behind a flattener, or getting that last little bit of tilt out of the optical train. You'll have to set your own standards. I can tell you, though, that there is always more that could be done. At some point you need to say, "I'm satisfied for now, and I want to start making pictures not optimizing my scope."
You list of "to do's" seems reasonable. Don't stress too much on the focal length. It only matters if you want to compare your results with other people's. Personally, I would recommend you put the spacing back at the AP spec of 90mm or so (assuming you have the room) and assume it's working at 0.67x. The only way to get it exact is with a plate solve, and that's overkill for now.
Keep in mind I was recommending the shorter exposure times just for evaluating the optics. Not for making your actual images. Short exposures will remove the tracking variable from the equation so you can evaluate just the optical alignment and quality.