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What caused these relfections? And what to do?

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14 replies to this topic

#1 RJF-Astro

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Posted 23 November 2019 - 09:10 AM

Last night I had two unexpected hours of clear sky. I decided to go for the Gamma cas nebula in h-alpha. I knew Navi was going to be a pain, but I did not expect the big internal reflection I saw on the final image. And now I am wondering: what could have caused these reflections and what can be done about them?

 

The optical train: 130mm PDS newton | Baader MPCC m3 | ZWO OAG | Baader Ha-filter 7nm | ASI1600. I am guessing it is the MPCC, given the size of the reflection. Although there is also the small donut in the lower right corner. And there is microlensing from the ASI, but I expected that to occur. The image is a stack of 11x480s subs.

 

If I want to avoid these relfections in the future, would that require a corrector upgrade? A better filter? Or just shorter subs? Thanks for your input!

 

20191122 Y cas zonder flats stretched.jpg



#2 sg6

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Posted 23 November 2019 - 09:25 AM

Cannot really help but that big circular doughnut with the flattened base looks a lot like the shape of the ZWO OAG.

Could it be reflected light off the rear or internals of the assembly?


Edited by sg6, 23 November 2019 - 09:28 AM.


#3 pyrasanth

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Posted 23 November 2019 - 10:27 AM

I have experienced similar, but not as bad, issues with my C14 & components connected in the optical assemblies.

 

A good step is to flock the inside of the OTA with a good quality flocking paper. I don't think the issue is your corrector as it is likely to be very anti reflective both through its coatings and the internals being of a good design.

 

I would then spray the inside of all assemblies (if you can safely mask any optics) with Krylon mat black camouflage paint https://www.amazon.c...,aps,141&sr=8-1 The ring you are seeing is an internal reflection been emphasised by the very bright star. That light is bouncing around and producing the halo you see.

 

This should help a lot with dealing with this issue. 


Edited by pyrasanth, 23 November 2019 - 10:31 AM.


#4 Madratter

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Posted 23 November 2019 - 10:37 AM

From the size, I would suspect either part of the OAG, the focuser, or an extender if you are using one. I had something similar to this involving a focuser. I used self stick black velcro with the fuzzy side (not the hooks) to cure it.



#5 RJF-Astro

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Posted 24 November 2019 - 04:23 AM

Thanks all! Lots of input to work on.

I could try the OAG, rings, filter and camera on another scope and see what happens. Both the OAG and newton/mpcc are new, so I have not used the OAG on my other scopes yet.

If I do not see these reflections on others scopes, the next step will be the newton without mpcc. If the reflections remain then flocking the newt will be my challenge.

Please let me know if I am missing something!

#6 Der_Pit

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Posted 24 November 2019 - 05:01 AM

Hmm, hard to see how the OAG would produce such a 'reflection'.  What you see is a heavily defocused image of Navi, which means it has been reflected twice (at least) within your optics.

So like you, I'd first have suspected the corrector to be the cause.  I think the OAG is 'visible' as the small flattening of the pupil image on one side.  Such an obstruction would cause diffraction spikes on brighter stars, but with a newton they are likely buried in the main spider cross.

If that is the full frame of the ASI1600 then the size of the pupil image is 6.5mm, at F/5 that suggests an extra travel of 32mm, or two surfaces separated by 16mm.  So it could either be within the MPCC, or between it and the filter.  How/where is the filter mounted?  In the EFWmini?  IIRC there the filter distance from the sensor is around 10mm, too close for such a large halo....


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#7 sharkmelley

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Posted 24 November 2019 - 05:07 AM

Measure the diameter of the large reflection in camera pixels.

Use the dust donut calculator here https://www.ccdware....ources/dust.cfm

 

If the distance calculated by the donut calculator is twice the distance of your filter from the sensor then it's probably a filter reflection that is to blame.

 

If the filter is not at that position is there something else at that distance?  Or is there some other combination of reflections that could increase the total light path by the calculated amount?

 

Mark


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#8 RJF-Astro

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Posted 24 November 2019 - 02:34 PM

Thanks all, I think we are getting somewhere. According to the Dust Donut Calculator, the distance is about 30mm. Then I remembered a photo from earlier this year, from the Horsehead, which also has a bad reflection. This is with another scope (F7.2 refractor) and corrector, and no OAG. And suprise: the distance is about the same 30mm.

 

However, according to ZWO the distance from sensor to filter with the EFW-mini is 10mm. That means I am missing another 10mm. I could try a long exposure without filter and see how it goes?  

 

DSC_0233.jpg

 

20190322 horsehead ha stretched.jpg



#9 Cfreerksen

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Posted 24 November 2019 - 03:27 PM

Dust doughnut calculator? I have not seen this script.

 

Chris



#10 sharkmelley

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Posted 24 November 2019 - 03:33 PM

Thanks all, I think we are getting somewhere. According to the Dust Donut Calculator, the distance is about 30mm. Then I remembered a photo from earlier this year, from the Horsehead, which also has a bad reflection. This is with another scope (F7.2 refractor) and corrector, and no OAG. And suprise: the distance is about the same 30mm.

 

However, according to ZWO the distance from sensor to filter with the EFW-mini is 10mm. That means I am missing another 10mm. I could try a long exposure without filter and see how it goes?  

 

attachicon.gif DSC_0233.jpg

 

attachicon.gif 20190322 horsehead ha stretched.jpg

Are you sure the filter glass is only 10mm from the sensor?

 

I don't have this camera but I think this is its diagram:

zwo1600mmc.JPG

 

The sensor is 6.5 mm inside the camera and there is 4mm of thread on front of the camera.  This makes 10.5mm plus a gap between the filter glass and front of the camera thread.  So 15mm from filter glass to sensor seems a reasonable estimate to me and twice 15mm gives you your 30mm.

 

Mark


Edited by sharkmelley, 24 November 2019 - 03:34 PM.


#11 John Miele

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Posted 24 November 2019 - 04:39 PM

I think Mark is right on the distances...



#12 RJF-Astro

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Posted 24 November 2019 - 04:39 PM

The diagram looks correct, but the filter wheel actually screws directly onto the camera thread. So the filter is quite close to the sensor. 15mm from the sensor gets me halfway the filter wheel, just in front of the filter.

 

Maybe my calculations where a bit innacurate and the distance is smaller, say 27-28mm. Could it then be that the reflections come from the front of the filter? The front is the most reflective part, while the back is less shiny. I also know that filters change the focal point, but I do not know how this adds up here.

 

DSC_0235.jpg

 

DSC_0236.jpg



#13 sharkmelley

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Posted 24 November 2019 - 07:36 PM

Is it possible to reverse the filter so the shiny side faces the other way?  It may or may not make a difference.  Baader should know if it is important which side faces the sensor.

 

Mark



#14 RJF-Astro

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Posted 25 November 2019 - 03:22 AM

The filters are mounted, so that is difficult, although I could reverse the filterwheel. Acoording to the Baader website, the shiny side should be directed at the telescope. They advise only to reverse if you have a corrector behind the filter, but that is with unmounted filters.

I will contact them myself and ask if there is another solution.

#15 joshman

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Posted 26 November 2019 - 01:45 AM

HO! I'm having the EXACT same issue with my setup. ASI1600MM-Pro, ZWO EFW, Baader 7nm Ha filter and a giant obnoxious halo around Alnitak. tonight is looking clear enough to play, so i'm going to try out a few different configurations and I'll try and update here and my thread with what I find.




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