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Strange rings in Images


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#1 lukepower

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Posted 24 February 2014 - 08:23 PM

Hi there,

 

during testing I found in all of my images some strange rings appearing. Look here:

Attached File  CCD-Image-243b.jpg   132.65KB   2 downloads

 

The imaging train is as follows (from front to camera):

  1. Tube opening (tube is covered with a black cloth)
  2. Secondary mirror f/3
  3. Primary mirror, f/3
  4. Microtouch focuser, 4"
  5. ASA Cassegrain Reducer-corrector (x0,72), 4"
  6. Distance adaptor
  7. FLI Filterwheel
  8. FLI PL 16803

 

I don't really know, maybe  it's something with the baffles, or the reducer lens. If any of you have an idea, I'm open for everything :)

 

Thanks


Lukas Demetz
------------
www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL


#2 nakbrooks

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Posted 24 February 2014 - 11:14 PM

Has this just appeared or has it always been like this? If it has just appeared, what changes have you made recently to the optical train?
Nigel Brooks
Stratis Observatory, Hautes Pyrénées, France.
http://www.facebook.com/stratisobservatory

#3 lukepower

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Posted 25 February 2014 - 07:45 AM

This has always been so :) Well, the scope is new, so "always" means since it is running, a few days ago...


Lukas Demetz
------------
www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL


#4 lukepower

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Posted 25 February 2014 - 08:20 AM

Attached File  Image16_DBE.jpg   70.86KB   1 downloads

Here again... IMHO it's the imaging train, either the corrector or the camera, as the position stays always the same. I still have to try to rotate the camera and corrector to see if the rings follow them...


Lukas Demetz
------------
www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL


#5 nakbrooks

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Posted 25 February 2014 - 08:27 AM

What make and model is the OTA?

As you say, you should be able to narrow down which component is responsible by rotating them individually.
Nigel Brooks
Stratis Observatory, Hautes Pyrénées, France.
http://www.facebook.com/stratisobservatory

#6 RamaSpaceShip

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Posted 25 February 2014 - 08:41 AM

Hi Lukas,

 

You can remove pieces from the imaging train to keep only the minimum to make a picture, and check if the circles still appear. If you have easy access to another camera, it would be helful to do the switch. This will allow you to identify where the issue comes from, and where it doesn't.

 

You can also check if you can see the circles with an eyepiece only.

 

I got once a single circle on a picture. It was due to a drop on the edge of the corrector, created by accumulated dew,

 

Regards

 

Bernard



#7 lukepower

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Posted 25 February 2014 - 09:05 AM

Hi Bernard,

 

drop of dew... Hmm, actually in the morning there was heavy dew on both the main mirror and the carbon structure. Could be a possible cause...

I will try to rotate the individual components first, as the tube is heavy and very (!) prone to fine balancing. If I remove the corrector I can forget to use the mount as it would go crazy :P

Nigel: The OTA is custom-made by a renowned german telescope builder, with a 20" mirror from Zen (Italy) and correctors from ASA. 

 

As I was so concentrated on getting decent collimation and the mount to work, I dedicated no time to this problem, at first I even considered it small :P I thought covering the open truss with a black cloth would be enough, but nada :P


Lukas Demetz
------------
www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL


#8 lukepower

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Posted 25 February 2014 - 01:37 PM

ALright, small update: I inspected the light path from the front of the scope to the camera, and saw three stainless steel screws (with a respectable head), which keep the last bit of the main mirror baffle in place. They definitively reach into the light path, as I am seeing them in the defocused star images. They are made of shiny steel, so reflections should be there. I removed one (as I have no exchange part for them) to see if it makes any difference from yesterdays image.

 

Let's hope for clear weather and I'll try. Damnit, yesterday we had 21.5 SQM over here, would have been a great night :)


Lukas Demetz
------------
www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL





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