Flat Calibration Problem Generic equipment discussions · MichaelCR97 · ... · 4 · 158 · 5

Hi everyone,

I'm having an issue with flats/calibration in my setup.

My setup consists of a Carbon TS Newton 200/800 with Starizona Nexus (effectively 200/600), QHY ImageTrain (QHY 268M with QHY CFW), and AstroWanderer Rotator. A screenshot from MyNewton (if I measured/understood everything correctly) is attached. The OAZ is the Baader Steeltrack.

After calibration (whether using sky flats or a panel), I get the following master (the gradient from the top left to the bottom right could be explained by the moon - it matches the moon's position; however, I think LN should do a better job here):



In my opinion, my master flat looks good. Dark/FlatDark/etc. should also be fine - I've checked it multiple times:


(sky flat master)

Flats and lights are both taken with a dew cap (so everything is exactly the same) - same focus and exact same rotation (the rotator doesn't rotate anything within a night).

The best results I've had so far were when I took the flats with the panel directly at night - basically inserted between the lights. I suspect this is because the focus/temperature/etc. are most similar. However, this wasn't perfect either and is obviously not practical.

In my opinion, there are 2 issues with the calibrated master:

1. Gradient from top left to bottom right
2. Huge donut, quite central. The pixel diameter is about 6000px, which would correspond to a distance of about 63 mm from the sensor? That would be roughly at the first third of the Starizona Nexus => its sensor distance is quite exactly 54mm.

These problems also occur without flat calibration.

When I compare 2 sky master flats from different nights (by division), I get the following "difference":


When I perform an ABE with "Function Degree" 2 on a master flat, I get the following result:



What's going on here?

Theoretically, I have the following issues in mind, but I don't know how to address them further...

1. Light leaks from the outside (it's now very well sealed) and in my opinion, wouldn't explain the donut
2. Reflection through the Starizona Nexus? "Pupil Ghost"? Possibly wrong distance?
3. FS edge reflecting? I have a small center marking on the FS => maybe that's the cause? But due to the distance to the FS, it should only lead to a general reduction in contrast and not cause such a donut?
4. Filters not positioned exactly? Since they are 2" filters and the problem was less when the conditions like temperature/telescope position were as similar as possible, I don't think that's the issue either
5. Any other ideas?

Something in your imaging train is reflecting off-field sources of lights (e.g. street lamps), or letting unwanted light in (as in a light leak). There are multiple threads discussing this.
If the artifacts tend to disappear when you image from a dark site, that’d be the “smoking gun”.
Francesco

For a quick check before pulling everything apart, take an exposure with everything in imaging configuration but with the OTA capped. Do this during the day, just after sunset or just before sunrise would be best.  If the frame is anything but dark, then it's problem confirmed and the fun begins. If it is dark, you have just saved yourself a lot of work and at least, you'll know what it isn't.