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Catanonia

Strange shadows on my images and I can't get rid of them

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Hi all,

It has been a good 6 months since I have even opened up the observatory due to a new contract, but I am still encountering the same problem I had before. I have stripped everything down, checked and double checked.

As you can see from the Blue image attached, I am getting really strange shadows on my images. I have stretched this image so that you can see it more clearly. I get this in LRGB images.

Scope

Newtonian 200mm F2.8 (F4 with ASA corrector)

QHY9 Mono with QHY9 2inch LRGB Filters

Scenerio

  • This happens on all LRGB filters no matter what methods I use with flats
  • It does not happen on Ha
  • It has happened on 2 CCD's so is not camera related
  • It happens in the same place not matter on the camera or train orientation.
  • I have tried panel flats, sky flats, nothing seem to get rid of the problem.
  • The scope is Cats Eye collimated to an inch of it's live.
  • Dew shield in place and not.
  • Checked for light leaks, but that would not explain it and would cause issues on HA.
  • There is no LP filter in place.

It is really doing my head in.

The only things left that I can think of are LP filter is needed (but that is strange as it is the same pattern always)

Flat taking method. I have created sky flats with 23,000 ADU and averaged. I have done light panel flats and nothing gets rid of it.

On HA filters, no problems at all so I am stumped.

On Red images, no so apparent, green more so and blue the worst. Light pollution ?

Anyone ?

Does anyone have any clues as to what is causing this ?

post-3977-0-70158800-1369663733_thumb.jp

Edited by Catanonia

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Welcome back, Steve.

Yves sent me a detailed and slightly alarming paper from one of the SBIG engineers regarding flats and strange UV reflections that can be given off by matt black paints inside what you think are non reflective parts of your scope. It seems that blacks using dyes may not be as black as they look and that heat resistant barbecue paints use real pigments which don't reflect in UV. Of course, we don't see these UV reflections oursleves but the camera does.

Olly

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Welcome back, Steve.

Yves sent me a detailed and slightly alarming paper from one of the SBIG engineers regarding flats and strange UV reflections that can be given off by matt black paints inside what you think are non reflective parts of your scope. It seems that blacks using dyes may not be as black as they look and that heat resistant barbecue paints use real pigments which don't reflect in UV. Of course, we don't see these UV reflections oursleves but the camera does.

Olly

Thanks for the welcome back Olly, it has been sometime due to a new contract etc.

This scares me somewhat.

The scope is carbon and so the inside is the standard black from the carbon. I have however edged the primary and secondary mirrors in black permanent markers. Is this what you may be referring to ? I am not sure, but I think I had the problem before I did this. Can't be sure though.

Would this explain why it only happens on LRGB and not NarrowBand images.

Also it happens no matter what exposure I use for LRGB 1 - 5 mins.

It doesn't show on flats either.

Can you send this paper to me please mate : steve.allan @ gmail.com

Thanks

Edited by Catanonia

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Cat have you got any other subs of a different target?

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Cat have you got any other subs of a different target?

They come out with the same problem in roughly the same place.

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Its is odd Steve, It strikes me as some sort of internal reflection / light leak. Have you tried imaging without the ASA in place to eliminate the corrector, you say you have tried another camer wit same results ,was this with your DSLR. Have you tried imaging with no filters (camera cover slip issue) ? and when you image say a bright star is the issue more pronounced ?

Mark

Edited by Astroscot2

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Its is odd Steve, It strikes me as some sort of internal reflection / light leak. Have you tried imaging without the ASA in place to eliminate the corrector, you say you have tried another camer wit same results ,was this with your DSLR. Have you tried imaging with no filters (camera cover slip issue) ? and when you image say a bright star is the issue more pronounced ?

Mark

Been having a few conversations via PM and many thanks all especially Olly.

Some updates.

I agree that it does look like reflections from somewhere, especially since it is always in the same place and same shape.

I have always used a TS OAG and remember having these issue there too. Since then I have gone back to side by side with an ED 80 guide scope, BUT I still have the OAG attached for spacing. Next step is to remove this completely from the image train as it definately is a constant in the whole problem for the time I have had it.

Good call on the ASA reducer, I could try some subs without it in place and see if this is what is causing the problems.

The problem seems to exist whether I apply flats or not. They don't appear on the flats that I have experimented with. I have tried sky flats (series of them averaged) and single panel flats. This is the bit that really concerns me. If it was light leakage, the I would have thought that it would appear on the flats. However the flats are only 2 seconds compared to a light leakage of a 5 minute sub.

The experimentation continues.

Cat

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Doing some more thinking and after reading the posts in the discussion thread started by Olly, I am pretty sure it is reflections

If I DBE an image in PixInsight, the pattern appears whether it is calibrated with flats or not.

Edited by Catanonia

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Done a strip down of my image train and removed the TS OAG from it. Now precisely at 62mm from the corrector to the CCD chip.

Once I get another clear night, will give this a go and see if the reflections happen again. I hope it is as simple as this and the TS OAG was causing the problem.

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Fingers crossed for you Steve - let us know the outcome

Thanks, it has really put me off recently having this problem when time is precious to me for imaging and work. I am commuting 150 miles a day on a new contract and with this problem, I just couldn't be bothered to mess with the scopes for nearly 6 months until I gave myself a slap about having £1000's of kit sat in an observatory wasting away.

This weekend, I eventually replace my roll off roof so it stopped collecting water and sagging, so the scopes are next as I promised myself over the winter watching the lovely clear skies feeling work drunk.

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Also just ordered a T2 Astronomik CLS IR Blocked filter for the image train. Got fed up of massive gradients on my LRGB subs and have been meaning to get one for ages, but the backfocus of the ASA corrector and QHY9 combo with TS OAG made it impossible. Now I have switched back to Side by Side guiding, well, it was rude not to.

Might have a TS OAG for sale if anyone still uses them.

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SUCCESS

It is all working now :D

Added the Astronomik CLS filter in line and removed the TS OAG and now and uniform subs.

I don't know which one caused it and to be honest, I don't really care now :D

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As an addendum to Olly's comments earlier in this thread regarding barbeque paint, I painted the two aluminium parts I made for my PST stage 0.5 mod using a can of spray on barbeque paint yesterday and examining them this morning it appears to have done a very neat job. Just waiting to be certain everything is completely dry before I remove the masking tape now.

James

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