Your latest ascom driver requires ascom 6.5, but there is no official 6.5 yet. You provide a link to an ascom 6.5, but the official ascom web page is still on 6.4sp1, and their github is on RC2 of 6.5. Is your latest driver not compatible with 6.4sp1? If so, you may want to pull your release until t...

Ah, well, that explains it. You changed the size. :) The FOV will still calculate roughly the same, but the image scale will be off. So your spacing was less than half of what is suggested. That would explain the rather small reduction. :) The flattening part may not be a big issue yet. The 1600 mea...

Well, at least you're making progress. The FOV seems to be more in line with what that camera and scope should see. Astrometry indicates 123.6 x 93.6 arc-min. But it also shows the image scale to be 3.43 arc-sec/pixel, which is probably because it shows the image as being 2160 x 1633 pixels. Is that...

Hmm, the field of view reported by Astrometry seems to be far too small for that camera. My calculations, for a focal length of 480mm with 3.8 micron pixels and a resolution of 4656x3520, yield a field of view of 95.7 x 126.6 arc-minutes. (5,742 x 7596 arc-seconds) The reducer should make those numb...

"The connection setup I have is: telescope/ focuser-FR-50mm spacers-ZWO asi 1600 camera (no T ring) connected directly to the spacers. This gives me a total back spacing of 56.5mm, well within the tolerance of the recommended spacing." Are you saying that you have tried the spacing (56.6mm) that you...

I've read that the MI-250 can be difficult to balance, and does not like perfect balance but performs better with a little bias to keep the gears engaged and avoid "bouncing." Since you aren't guiding, and the backlash issues that can cause are not an issue, it is possible there's an issue with gear...

That is a tracking issue and not the camera. What mount are you using? Are you guiding the mount? 90 seconds is more than enough time for a Periodic Error issue to show. However, this type of image is common when guiding and encountering a backlash issue. So knowing the mount and whether you are gui...

The Sony IMX071 is listed under the heading of "Not Backlit + Not Stacked" in the Wikipedia listing for Exmor.

I think the question is about the little flange at the base. I'd guess it's only 2mm or so thick.

To NPB, why the concern? Are you trying to use a focal reducer or a flattener and have a specific spacing you are trying for?

With the ASI128MC in short supply or out of stock and seemingly discontinued, the new ASI6200MC-Pro, due late Jan. 2020, and similarly priced, would be the option. It's pixels are smaller, but with 60MP to play with, binning would somewhat address the pixel size issue.