AndreHackelbusch Can I assume that with gain 100 set in AAP I have reda nois of 1.5?
Yes. Gain 100 and above appears to be when High Conversion Gain is automatically enabled by that camera's firmware.
The ASI6200, 2600, etc don't allow you to enable HCG below gain of 100, and they don't allow you to disable HCG beyond gain 100.
Above Gain of 100, there is not much to be improved on since dynamic range decreases, while the read noise does not improve appreciably. You can test this out yourself by using something other than ASIAIR.
I believe the maximum gain of 100 is imposed by ZWO on the ASIAIR because the ASIAIR users may not understand the difference between dark current noise and read noise, and the trade-offs.
Re: vignetting, why don't you try to mount the camera without the filter wheel and with the camera closer to the metal back of the OTA to see if the vignetting becomes weaker?
Be sure to use M54 or larger diameter spacer tubes. M48 will probably lead to vignetting, and T2 (M42P0.75) spacers will certainly lead to horrifying vignetting.
Remember that the RedCat series of telescopes are Petzvals, with all 4 element optics fixed mounted relative to each other inside the optical tube on the objective side of the focuser. Because of that, back focus between the OTA's metal back and the camera sensor is of zero importance. Perfect optical distances are achieved at the same instant the sensor achieves focus.
So, you do not have to set the spacers to get to that 59mm number. That number which is given by William Optics is the largest back focus you can use, given the available helical focuser travel. With the RedCat, you can use any back focus distance that is shorter than 59mm.
Since the exit pupil of the Petzval may actually be smaller than the diagonal of a full frame camera, you may actually achieve less aperture vignetting by placing the filter closer to the focuser than to the camera. Check the ray diagram of the RedCat; I don't remember it right offhand now; just doing this from memory. To minimize aperture vignetting, place the filter at a practical location where the ray diagram has the smallest deviation from the optical axis. With a simple optics, that would place the filter as close to the sensor as possible (since objectives tend to be larger than the sensor). But when you include reducers, flatteners and correctors (or the rear elements of the Petzval), it is sometimes better to place the filter closer to the objective side of the telescope.
If you check where William Optics had placed their 2" filter thread in the RedCat, it is as far from the camera plane as possible -- you should do the same with your filter wheel.
Just experiment with the filter arrangement if you cannot get a ray diagram from William Optics. You can do this in the daytime indoors with a diffusing element, like a T-shirt that is placed close to the objective (so the telescope does not focus on the T-shirt). Even better if you have a flat light panel really close to the objective lens.
This is one big advantage of Petzvals; not needing to worry about back focus (I use an FSQ-85, which is also a Petzval, although I do have to pay attention to back focus when I add reducers and flatteners). Especially, you don't need to account for the optical distance of a filter glass on back focus.
You see people trying to achieve some magical 59mm or 55mm back focus with a RadCat when it is totally unnecessary. In fact, 59mm is a terrible number to use since it gives you virtually no focuser travel to work with.
Chen
