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What would cause flats to not correct properly?

Planewave CDK14 Processing Calibration Deep Sky
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Rouz Astro avatar
Rouz Astro avatar
  • Pix Insight WBPP
  • QHY600m – CDK14 + Reducer
  • Filters are 50mm with filter masks, the Astonomiks even have a built-in mask (plus additional plastic masks).
  • Chroma filters have the same pattern
  • Spike-a-flat panel
  • There are no light leaks  -   daytime darks = nighttime darks
  • Darks – Flats – Darkflats all matching gain, offset
  • Focuser and rotator at exact same position as lights

 Any ideas would be great.
Pablo Gazmuri avatar
Pablo Gazmuri avatar
Things I've learned:

1. Make sure your offset is high enough. The left side of your histogram should be zeroed out.
2. Make sure you are taking flats at the same focus point where stars are most in focus. Even if you haven't touched the focuser, if the temperature has changed, you may be out of focus.
3. Make sure your average ADU reading is high enough on your flats. If you have a non-linear sensor then a lower average ADU will likely work better.
4. Take separate flats for each filter, as they alter the light path.
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andrea tasselli avatar
andrea tasselli avatar
Spike-a-flat panel


That's your answer
Rouz Astro avatar
Rouz Astro avatar
andrea tasselli:
Spike-a-flat panel


That's your answer

You mean its not even?

I tried sky flats, was similar.

My old smaller Spike a flat and smaller cdk and reducer calibrated perfectly.
Rouz Astro avatar
Rouz Astro avatar
Pablo Gazmuri:
Things I've learned:

1. Make sure your offset is high enough. The left side of your histogram should be zeroed out.
2. Make sure you are taking flats at the same focus point where stars are most in focus. Even if you haven't touched the focuser, if the temperature has changed, you may be out of focus.
3. Make sure your average ADU reading is high enough on your flats. If you have a non-linear sensor then a lower average ADU will likely work better.
4. Take separate flats for each filter, as they alter the light path.

Thanks Pablo,

Offsets seems ok, lowest pixel value is 231ADU


Focus point is accurate, I use offsets per filter, very accurate focuser, carbon truss, fused silica optics. I doubt its focus shift.


Tried flats from 30% to 80%, didnt solve the problem

Each filters has its own flats, its own darkflat and filter offset.


Im suspecting internal reflections maybe?
andrea tasselli avatar
andrea tasselli avatar
I tried sky flats, was similar.


How do you take them? And how do you reduce them (this in general, not specific to what type of flats you're having). Open truss scopes are notoriously finicky in these matters and sky or dome flats are the (professional) way to go.
kuechlew avatar
kuechlew avatar
Does it look the same for all your filters?

Trying to compile a list of "what are all the things that changed in my setup since I had it working last time" is a good start to nail down the potential issues.  

Looking at the image I can only come up with the question: What in your setup is different in the lower right corner compared to the other corners?
Admittedly the upper right corner is not perfect either but maybe address the main issue first.

If this was my image I would bet on the OAG obstructing the edge of my sensor. I would rule this out for an experienced user though …

Good luck and clear skies
Wolfgang
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Rouz Astro avatar
Rouz Astro avatar
I may have just found something.I went to the dome, the telescope and baffle had a dark cover on. With a bright LED flash light I checked each one of the 15 connections and taped them all up. One connection near the camera had a light leak, that could be it. With the flash light it was a severe light streak on the image.The flat panel may have illuminated that side externally, causing that issue. Its clear tonight so Ill try lights and flats from scratch. Will report.
Jim Matzger avatar
Jim Matzger avatar
Try the Sequence Generator flats calibration wizard to check the proper exposure time.  It worked for me.
Well Written
Rouz Astro avatar
Rouz Astro avatar
Jim Matzger:
Try the Sequence Generator flats calibration wizard to check the proper exposure time.  It worked for me.

Thanks, Im using NINA exposure calculators.
Ordered some flat black paint and will paint the entire imaging train.