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Well, nothing was gained from another full day's online research and practical trials with four different telescopes.
Both the ZWO and Neximage5 appear against a ground glass screen background in white light in SharpCap on both the laptop and my indoor PC with a large [HD] monitor. In H-alpha [deep red light] the background becomes a much coarser, rippled glass effect. The sun's real surface was obscured behind this ripple and the rippled screen was then carried over to the recorded video. Making it impossible to use Registax to produce a sharper image. The ripple can be seen in the first preview image in Registax.
I normally use RGB24 for a larger fields of view for prom and sunspot searches but have tried all the different options of RAW and MONO in 8, 12 and 16, in all frame sizes/resolutions down to 320x320, or whatever.
I have uploaded a 1m video to YT to show the obscuring effect as I drive the telescope around in slow motion. Full Screen makes it easier to see:
https://youtu.be/hHIlU7H2RA0
Note that this is not a moving screen but a static overlay [effect filter] overlaid in front of the real image on the screen. The Sun's surface can be seen moving behind the obscuring "rippled glass" when the telescope is moved or deliberately shaken. There is a definite depth or perspective effect on the laptop screen in SharpCap. The rippled mask does not rotate with the camera. Which suggests that it is undamaged or innocent.
It has suddenly occurred to me that I bought an external SSD to cope with the high speed and large file sizes of the ZWO120MC-S camera. There may be a clue here regarding gain versus exposure times. Debayering or untangling the colour camera's sensor matrix as it reaches the computer software may be the problem. When handled wrongly the effect is similar to seeing a cloth pattern. I see a tartan effect when I really over-sharpen my images.
Imagine a uniform "cloth" pattern but out of focus. It may even take on the form of my rippled mask. In fact the mask lies in front [on top] of the active sensors and tells the computer which colour is represented by the underlying light [grey] sensors. Colour is produced by the camera software. Colour sensor cells are really just shades of grey until the overlying mask confirms the intended colour of each individual cell. Don't take my word for it. I just scanned a few websites to try and get a grip on debayering. Here's a useful guide just to get you started.
What is Debayering, and why should I do it before stacking my images? | Altair Astro Cameras
Click on any image for an enlargement.
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