Here's some from Friday night with a bunch of 30s exposures. I set it and forgot it. Didn't expect a ton with the moon like it was!
The general star shapes look good, but I notice that this was taken with an APS-C format camera and it appears to have been cropped or reduced in size (a little, the camera specs say 4928 x 3264 pixels, the image on Astrobin is 4109x2434, a change from 15.7 megapixels down to 9.8). However, the strong halos around the stars are a little disconcerting, and they appear even in the center of the field. Some of this could be from the processing. Did the OP use a star mask or a masked stretch during any of the histogram adjustments?
That said, there is also evidence of an off-axis component to the halos and flare, which is also commonly seen in camera lenses. This along with the helical focusing makes me think that the RedCat may just be a repurposed camera lens. A nicely redesigned (for astrophotography), fairly high-quality and sharp camera lens, but it MAY not be something that was formulated specifically as an astrograph (pure conjecture, as I don't really know).
Honestly, however, I have never owned a scope that produced flawless images over something as large as an APS-C format. I tried two different AT65EDQs (neither very good), a Tele Vue NP127is (not bad but certainly not pinpoint with perfectly round stars), and a Stellarvue SV80ST2 with a matched 0.8X Stellarvue reducer (again, not that bad, but certainly not flawless). I also have an EdgeHD that produces nice images at f/10, but less so with either of the reducers that I've tried (both an Optec Lepus and Celestron's reducer for the EdgeHD series).
What I've finally concluded is that you have to be lucky to find any scope that will cover an APS-C format and there may be very few if any that can do full-frame. Of course, you can always crop or resample the image to something smaller (meaning either a smaller field or a larger number of arc seconds per pixel). The sampling (arc seconds per pixel) is also critical, since you can combine a large format and large pixel CCD with a good number of high-end astrographs to produce good looking images (which is certainly a valid approach, although it makes it difficult to compare such results with what can be output using today's relatively small-pixel CMOS cameras).
Wei-Hao’s image of the Small Magellanic Cloud taken with the RedCat and a Nikon D800 (full-frame) does definitely show some off-axis aberration, but it’s still a very good image (it was selected as an "Image of the Day" on Astrobin). However, the D800’s native pixel size is 4.88 microns and the largest image on Astrobin is “just” 4800x3200 pixels when the camera’s native size is 7360x4912. So, it’s obviously either been cropped or reduced in size rather significantly. In fact, the Astrometry.net solve says that the image scale is 6.1 arc seconds per pixel which is eight times larger than what I get with my Tele Vue NP127is when using a camera like Sony’s IMX183 (or three times larger than I'd get with the IMX183 and the RedCat).
The RedCat/D800 combination should produce an image scale of 4.03 arc seconds per pixel, so taking that and the 6.1 arc seconds reported by Astrometry.net you get a reduction in size to 66%. Now applying that to the native pixel count on the D800 (7360x4912) and you get an estimated pixel dimension of 4865x3247, which isn't too far from the Astrobin size of 4800x3200. So, it looks like Wei-Hao’s image is resized but not cropped (much) from the full frame.
Now, if I wanted similar looking performance with my IMX183 and the RedCat I'd have to reduce the image to about 33% of its original size (to produce the same 6.1 arc second sampling). That would mean going from the sensor's native 19.7 megapixels down to approximately 2.15 megapixels (5496x3672 pixels down to 1814x1212). That's certainly usable (about HDTV) but probably not something that could be considered a "grand vista." However, the IMX183 is a smaller format sensor (less than APS-C), and it should be certain (hopefully) that the center-field performance on the RedCat would be notably better than at the edges of the full-frame D800. Thus, the needed reduction in size/scale would probably not be as great as the aforementioned 33% (meaning less reduction and a greater number of pixels in the final, "acceptable" image).
You can simulate the field coverage that you'd get with the smaller format IMX183 by cropping the center from Wei -Hao's image, that crop would just cover the Small Magellanic Cloud itself, without 47 Tuc or any of the surrounding field (from the top, bottom, left, or right). With that crop and at the same 6.1 arc second per pixel image scale the image quality looks nearly flawless (using Wei-Hao's sample image).
The compromise that I've finally settled on is to use a smaller format camera with a shorter focal length lens (to maintain a fairly large field). In the latter case, I had some hopes that the RedCat might be an ideal solution but I've yet to see much that would seem to confirm that possibility (at least not conclusively). Note that this approach (smaller format sensor with a shorter lens) is really not that much different than using a large format, large pixel camera with a longer focal length lens (except that you might sacrifice some resolution with the assumed reduction in aperture size and perhaps with a similar sacrifice in imaging speed if trying to match image scales against a potentially larger aperture).
Edited by james7ca, 17 July 2019 - 11:05 PM.