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Old 28-08-2020, 11:01 AM
Startrek (Martin)
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Large Dust Donuts

Last night imaged M30 globular cluster with my new 2600MC due to a 60% illuminated moon , captured about an hour of data plus darks ( no filter )
Processed last night and found 2 large dust donuts close together about half way out in the field
So removed the coma corrector thinking it had dust and replaced it with a plain adapter
Adjusted focus again then captured another hour on M30
Processed image and found the original 2 large dust donuts had disappeared but now 2 new dust donuts in a different location in the image but not in the same rotation plane ( camera installed in focuser same orientation as first capture)

Any ideas ?
Surely my brand new camera hasn’t got dust on the sensor ?
Or in the process or cooling and warming the sensor had formed micro evaporation spots ??

Very frustrating

Never had any issues with dust donuts after using my DSLR for 3 years

PS I’ve never needed to take flats

and yes I use a hurricane blower on all my optics before use

Welcome any advice on the weird problem above
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Old 28-08-2020, 11:09 AM
Startrek (Martin)
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Sorry forgot
A photo of the original dust donuts would help
See attached
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  #3  
Old 28-08-2020, 12:43 PM
jahnpahwa (JP)
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Thats a bummer, Martin. I'm not sure What you mean by "not in the same rotation plane"? Did they mover relarive to eachother, or do they seem to be moving together?
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Old 28-08-2020, 01:13 PM
Startrek (Martin)
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With coma corrector, 2 original distinct dust donuts on right hand side of field about half out
Without coma corrector 2 faint dust donuts on left hand side of field about quarter of the way out , original 2 distinct dust donuts have disappeared
So I’m assuming their not the same dust donuts due to position and rotation plane from centre

So my question is why have the original 2 disappeared ? coma corrector May have had dust or scratches

But why now 2 new faint ones appear that were on the original

Doesn’t make logical sense ??
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Old 28-08-2020, 01:45 PM
jahnpahwa (JP)
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I wish I knew more about the optical path so I could offer some better thoughts, mate.

(Here's some pure conjecture, in case you're interested) I am going to go out on a limb and say/guess the source of the donuts is the same for with and without coma corrector and not on your sensor. I reckon the movement of the donuts is due to the correction effect of the coma corrector. Hows the secondary lookin?
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Old 28-08-2020, 01:54 PM
Startrek (Martin)
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Secondary looked ok but I gave it a clean anyway
Next chance when I’m free to do more testing is Saturday night
I’ll get to the bottom of this issue eventually

I will have to take some flats in any case , but some folk advised that flats may not totally remove your dust bunnies

It’s incredible I had no issue at all with my DSLR over 3 years of use
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Old 30-08-2020, 11:09 AM
Startrek (Martin)
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Cleaned the secondary
Cleaned the coma corrector
Cleaned the protective window on the camera sensor
Took some crude flats last night using a portable led flood light mounted on a bracket about 2m from the end of the OTA. Doubled a white T shirt stretched over OTA

Took an hour of test data on the Star Peacock with moon overhead
Took darks
Took flats using APT flat aid tool at 20000 ADU, 25000 ADU and 30000 ADU
Processed today
No dust donuts
No vignetting

Problem solved and 30000 ADU achieved a cleaner result in the stretched image

Buying an illuminated A3 led drawing pad to achieve a more even distribution of light when taking flats
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Old 30-08-2020, 12:14 PM
jahnpahwa (JP)
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Winner!!


Great news mate
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