step by step instruction to discover root cause for bad calibration

Helge Kooistra

I'm struggling for more than a month now to discover the root cause of my calibration problems, with no effect at all. I searched but cound not find a good instruction how to determine step by step wich part of the process causes my problems...

The problem: I get a strange reflection in the center of my image after WBPP.

The target is the shark nebula, from a Bortle 9 backyard so quite challenging. LRGB, each channel has 5,5 hours of integration time

What i did so far:

Rule out the flats -> The flats are taken with a new dimmable flat panel. I thought that could be the problem, so took a new set of flats with a dimmable EL pabel i normally used. The EL panel has a very even light distribution. The rest (lights, darks, flat darks) the same. Result: same strange blob

Rule out the darks -> Took new darks and redid WBPP. No effect, same blob.

Rule darks and flats out -> Did wbpp with both of the above. No effect

1. So the only problem seems to hide in my Lights? But to me its seems strange that it happens on all filters.

2. The other strange thing i noticed is the very little difference in K-values betweeon the 'blob' part and the 'normal' sky background... 0.0123 vs 0.0124. 

3. The corners look wonky so definetely something goning wrong with calibration too

I would love to know a step by step approach to see where in the process things go wrong. I still hope its the calibration part because these subs took a long time to gather

WBPP: Flats, Flat darks, darks and Lights

Camera: Cmos sensor (ZWO ASI 294MM Pro)

The camera is known to be very sensitive for the flats, so these are 4 to 5 seconds

Lights gathered over 5 nights within about a weeks time. In Nina i use a sequence to shoot 2 subs per filter, do all filters and then dither and start over again. So all filters have experienced roughly the same circumstances

 

BlockHead

You have a corrector lens correct?

-the Blockhead

Helge Kooistra

It’s a 150cm Newtonian with a reducer/corrector that came with the scope. It brings it down from f/4 to f/3.45

BlockHead

The issue is the reducer. It is what is causing the circular flat field error and perhaps other issues.

This is a common issue. You are supposed to be impressed I looked at the image and identified you are using a particular piece of equipment.

Your next question will be- how do you fix this. The short answer is I do not know. For fast systems like yours, this is an issue. I do encourage people to try to take night sky flats and see if it helps. The strong illumination of a flat field panel can give rise to reflections that become more important. Night sky less off-axis rays and those that may exist will be below the limit of detectability...which improves the flat. 

--the Blockhead

Helge Kooistra

Don't get me wrong, i am impressed ;-) The thing is that i want to impress myself, be able to determine the issue by root-cause analysis...

What puzzles me, is that i stacked another image i took a few days before my Shark-debacle. With the exact same flats and darks. And with the exact same equipment. On that image there iseems no funny reflection visible

Only difference is that thats an SHO on and the shark one is LRGB

BlockHead

Yes, for the reasons I indicate above. When there is simply less light (especially from the sky), the error will also be less and may even be invisible. I think you will find broadband work will generally always produce the issue to some degree.

-the Blockhead