chdunkel What filters are supposed to be installed into the 2" EFW to get no vignetting? Is it 50mm unmounted filters?
There are actually two diferent things related to sensor size and OTA... one is the aperture vignetting, and the other is the spot size (check the Takahashi site for the spot diagrams for the FSQ-106).
You will get vignetting with full frame, even if you don't use filters. However, the spot size of the FSQ-106 should be pretty good (but not perfect) up to 22mm from the optical center (the distance of the corner of a full frame).
The light falloff (aperture vignetting) of the FSQ-106 is spec'ed at 10% at 27mm from the center (this is larger than SLR full frame). So you can expect a non-zero falloff even at 44mm diameter, but smaller falloff than 10%. And possibly worse with even a 2" filter.
On my FSQ-85, I measured a 7% falloff at the corners of even an APS-C frame, and much worse at full frame, so I do not use my ASI6200 on the Baby-Q. The stars of the Baby-Q are pretty good to the corners of an APS-C frame, but again, I personally do not use the Baby-Q on a full frame.
(By the way, I use 50mm unmounted filters even with my ASI2600.)
The 106 Q is quite a bit better than the Baby-Q, though. If not for the weight, I would have bought a 106, but as it is, I can barely handle the Baby-Q with a filter wheel and camera attached.
If you can afford it, you can simply get the ASI6200 and you can always choose to capture a smaller ROI (to save downloading time), or crop it during post-processing. This is what I do on my Sigma 135mm ART lens. I have a ASI6200 attached to it, but choose to use the camera as an APS-C frame size most of the time when I want tack sharp images to the corners. But I have the option to get a larger FOV for certain images (albeit will less sharp stars at the corners).
You can easily correct for light falloff (especially with just a 10% type falloff) that is caused by aperture vignetting by calibrating with "flat frames."
However, you cannot correct for the diffraction effect that is caused by vignetting.
Basically, the symmtrical Airy disk is only applicable for a circular aperture. Vignetting causes the aperture to look more like a cat's eye (almond shaped, kinda like an American football shape) and what that does is to cause two dark notches within the disk that surrounds very bright stars
(If you are have Fourier Transform tools, do a Fourier Transform of said almond shaped "disk," and you will see the dark notches.)
A bright star right at the optical axis would look perfectly symmetrical because it sees a circular aperture, while the vignette affected stars get worse as you get closer to the edge of the frame (see, especially the last post on this thread:
https://www.cloudynights.com/topic/547028-anyone-know-what-causes-this-artifact-on-bright-stars/ ).
Chen
