Hi all. Question re FITS viewer in EKOS. I recently bought a 1.25inch Optolong Ha 7nm filter which was on offer and thought I would first check
it out by taking some flats with my colour camera so see if there were any blemishes or colour abnormalities using FITS Viewer When I examined a flat (example attached) it looked awful - just all blotchy and not uniform. The filter is in an ZWO EFW and I took some flats with other filters at the same time and these look perfectly good - so not a problem with condensation or rest of optical train or the camera sensor. I had a close examination of the filter surfaces and they look pristine and clean.
I was going to send the filter back, but I then examined the same flats using both Astropixelprocessor and Astap and they look perfectly reasonable.,
so my guess is that it is just an artifact in FITS viewer. If so then obviously this is not a problem. FITS viewer is useful but not critical.
Advice welcome.

In this case FitsViewer is stretching an image that doesn't need to be stretched (you have a ton of pixels over 40K ADU). See the highlights slider. If you moved that slider back to 1.0, or turned off stretching altogether (by clicking the toggle button the left of "Shadows") I'll bet it would look a lot better.

My wild guess is that those blotches are real, but something you should ignore (unless they show up without stretching) as they're being strongly emphasized--i.e. possibly they're caused by your optical system and part of every image you take at some tiny level.

Using narrow band filters with a color camera is a bad idea. Basically, 3 of 4 pixels will be black if you use and Hα filter. And when you then try to de-bayer it will do that based on the (black) green pixels. If you want to get a reasonable image you'd have to display only the red pixels, without de-bayering before. So you likely need a real image processing program for that...

Thanks Hy, that makes sense. I was thinking it was some sort of processing error as other software didn't show the effect.
Yes if I toggle shadows off I get same thing as Astropixelprocessor and Astap. It just took me by surprise as I hadn't seen that before with Tri-Band and Light pollution filters.
Still waiting for my mono camera so I can use it properly.........
Cheers.
Nick