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Old 10-02-2011, 08:23 PM
DJ N
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QHY8L : Issue with Internal Reflections

Hi everyone,

Before I start "stripping down" the imaging train to try and determine the root cause of the reflections I am getting with my QHY8L, I thought I would post a couple of examples of the issue I am having, in case someone may suggest where to start.

The imaging train consists of a Skywatcher Black Diamond ED120, with a Moonlite crayford refractor focuser, to which the QHY8L is connected, via a 2" male/camera adapter, together with the "centre and tilt adjustment ring", as supplied with the QHY8L.

Upon inspection of the 2" camera adapter, it is black but glossy. Not sure if this would have an impact, however, I did not seem to have issues when imaging with a DSLR.

As can be seen from the photos below (please disregard the processing ), the reflection seems to be of a significant diameter, so I am not sure if this means anything. In fact, there seems to be two almost perpendicular to one another. I am at a bit of a loss to where to start , so look forward to any tips!!

Thanks,

Daniel
Attached Thumbnails
Click for full-size image (M42 29Dec10 Reflection.jpg)
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Click for full-size image (Horse Head 01Jan11 Reflection Post.jpg)
134.8 KB221 views
Click for full-size image (Centaurus A 08Feb11 Reflection.jpg)
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Old 10-02-2011, 08:42 PM
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The other thing I did notice is that it is not on all images. Here is an image from the other night that does not seem to have a sign of the reflection. In this case the OTA was pointing towards the south-west, whereas he three images in the first post were all almost overhead.


Thanks again,

Daniel
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Click for full-size image (NGC2070 08Feb11 ED120 QHY8L 10x8min DSS PS3 Post.jpg)
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  #3  
Old 11-02-2011, 07:39 AM
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Have you got a dew shield for the scope? That would block any stray light from street lights - as long as you aren't pointed close to them.

Assuming that isn't the problem, and with a bit of plate solving:

Cen A - the most likely culprit is Iota Cen. Maybe SAO 204545
HH - the most likely culprit is Alnitak
M42 - the most likely culprit is Alnitak

A dew shield might block those too.

There is nothing particularly bright near NGC2070 to cause reflections.

Andrew
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Old 11-02-2011, 11:36 AM
DJ N
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Thanks for you reply Andrew.

I do not have a dew shield on the scope. In fact, this was going to be my first "trial". I was going to make up a dew shield to see if this has any impact. Thanks for the tip.


Cheers,

Daniel
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Old 11-02-2011, 11:39 AM
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multiweb (Marc)
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Could it be the reflection off the edge of the glass of a filter or nosepiece? What else is in your imaging train?
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Old 11-02-2011, 11:40 AM
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The first thing that struck me about these reflections is that they all appear to be very similar in terms of their radius and number of 'lines/elements' in them. If they were induced by out-of-image stars, I would expect more variability in both. Another possibility you may like to investigate is that the light is not coming from in front, but from behind the scope - most focusers' draw-tubes have a clearance of varying width between the tube and the body of the focuser. If light was shining towards the back of the scope from behind, some would find it's way up the drawtube and be subject to reflection back to the imaging plane off the back surfaces of - whatever.
Since you found the artifact did not appear in an image where the scope was pointing south and a fair way off vertical, it may be worth looking at where your laptop screen was pointing and any other light source behind the scope.
Just a thought.
Peter
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Old 11-02-2011, 11:58 AM
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Quote:
Originally Posted by multiweb View Post
Could it be the reflection off the edge of the glass of a filter or nosepiece? What else is in your imaging train?

Basically the image train consists of the following.....
  • Skywatcher Black Diamond ED120
  • Moonlite crayford refractor focuser
  • QHY8L is connected, via a 2" male/camera adapter, together with the "centre and tilt adjustment ring", as supplied with the QHY8L.
That is basically it, no other filters.

Cheers.
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  #8  
Old 11-02-2011, 12:04 PM
DJ N
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Quote:
Originally Posted by pmrid View Post
The first thing that struck me about these reflections is that they all appear to be very similar in terms of their radius and number of 'lines/elements' in them. If they were induced by out-of-image stars, I would expect more variability in both. Another possibility you may like to investigate is that the light is not coming from in front, but from behind the scope - most focusers' draw-tubes have a clearance of varying width between the tube and the body of the focuser. If light was shining towards the back of the scope from behind, some would find it's way up the drawtube and be subject to reflection back to the imaging plane off the back surfaces of - whatever.
Since you found the artifact did not appear in an image where the scope was pointing south and a fair way off vertical, it may be worth looking at where your laptop screen was pointing and any other light source behind the scope.
Just a thought.
Peter
Thanks for that Peter. In terms of the laptop, I have it sitting in the south-east corner of the observatory. So for M42 and Horsehead, the laptop would definitely be behind the camera. I do however, have a sheet of acrylic (red) covering the screen, but maybe there is some bleed off to the side. Definitely worth investigating further.

One thing that probably does not help, is that the interior of the observatory is cream colourbond. On my list of items to complete is to purchase some black cloth or something similar to line the walls.


Cheers,

Daniel
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Old 11-02-2011, 12:30 PM
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Quote:
Originally Posted by pmrid View Post
The first thing that struck me about these reflections is that they all appear to be very similar in terms of their radius and number of 'lines/elements' in them. If they were induced by out-of-image stars, I would expect more variability in both. Another possibility you may like to investigate is that the light is not coming from in front, but from behind the scope - most focusers' draw-tubes have a clearance of varying width between the tube and the body of the focuser. If light was shining towards the back of the scope from behind, some would find it's way up the drawtube and be subject to reflection back to the imaging plane off the back surfaces of - whatever.
Since you found the artifact did not appear in an image where the scope was pointing south and a fair way off vertical, it may be worth looking at where your laptop screen was pointing and any other light source behind the scope.
Just a thought.
Peter
Yes, I came across this problem a couple of months ago. While the reflections were not the same they were similar in nature to the images posted.
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Old 11-02-2011, 01:50 PM
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Quote:
Originally Posted by DJ N View Post
Thanks for that Peter. In terms of the laptop, I have it sitting in the south-east corner of the observatory. So for M42 and Horsehead, the laptop would definitely be behind the camera. I do however, have a sheet of acrylic (red) covering the screen, but maybe there is some bleed off to the side. Definitely worth investigating further.

One thing that probably does not help, is that the interior of the observatory is cream colourbond. On my list of items to complete is to purchase some black cloth or something similar to line the walls.


Cheers,

Daniel
Do you use a headlamp? I often have to smack myself across the knuckles when I turn away from something and glance across at the scope while it's imaging because 9 times out of 10, I have the headlamp switched on and the rear end of the scope cops a blast. The colour of these artifacts in your images does not really reek of "red" such as red from a screen protector. it does seem to be more 'white' light doesn't it? So I'd be looking for a light source that was more white than coloured.

The other thing I was going to mention is that most of the artifacts from out-of-image brioght stars that I have experienced have tended to be strongly ovaloid whereas these are quite distinctly round/circular. The other type of artifact is the ray type that you can get from Alnitak and/or Alnilam when doing a horsehead - these take the form of pronounced rays
Peter.
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Old 11-02-2011, 02:49 PM
DJ N
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Quote:
Originally Posted by pmrid View Post
Do you use a headlamp? I often have to smack myself across the knuckles when I turn away from something and glance across at the scope while it's imaging because 9 times out of 10, I have the headlamp switched on and the rear end of the scope cops a blast. The colour of these artifacts in your images does not really reek of "red" such as red from a screen protector. it does seem to be more 'white' light doesn't it? So I'd be looking for a light source that was more white than coloured.

The other thing I was going to mention is that most of the artifacts from out-of-image brioght stars that I have experienced have tended to be strongly ovaloid whereas these are quite distinctly round/circular. The other type of artifact is the ray type that you can get from Alnitak and/or Alnilam when doing a horsehead - these take the form of pronounced rays
Peter.
Hi Peter,

I do wear a red LED headlamp, however I do not use it whilst the camera is imaging. I will further investigate to see if there is a possible light source. Could be reflections off neighbours lights inside the obs. Will keep investigating!!
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Old 11-02-2011, 07:21 PM
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Try looking thru the scope with the camera removed. You may catch the streaking light thats causing the issue. Look for shiny surfaces, like the inner of the focus tube, proper baffling, etc.

Theo
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Old 12-02-2011, 09:50 AM
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Try looking thru the scope with the camera removed. You may catch the streaking light thats causing the issue. Look for shiny surfaces, like the inner of the focus tube, proper baffling, etc.

Theo
Thanks for that Theo. I will do that for sure!!
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Old 15-02-2011, 10:31 PM
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Try looking thru the scope with the camera removed. You may catch the streaking light thats causing the issue. Look for shiny surfaces, like the inner of the focus tube, proper baffling, etc.

Theo
Well, I ended up having a look through the scope with the camera removed. With the QHY8L, the Moonlite focuser is extended approximately 90% of its total travel. I did this during the day and there was definitely a streaking ring of reflection off the inside of the focuser. When I retracted the focuser a bit, this shiny section was covered by the draw tube. I really think this could be the source of the problem. So in the meantime, I have added a 2 inch extension tube to the imaging train and now just need to give it a go during an imaging run. Fingers crossed this is the root cause.

I will report back when I have a 'clear' night. Very much appreciate everybodys input.

Cheers,

Daniel
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Old 16-02-2011, 05:41 AM
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Well, if you feel it is the inside of the focuser, why not slip a bit of flocking paper inside.
Gary
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  #16  
Old 16-02-2011, 10:11 AM
DJ N
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Well, if you feel it is the inside of the focuser, why not slip a bit of flocking paper inside.
Gary
Hi Gary,

I just want to confirm that the reflection is in fact being instigated by the focuser. Adding the extension is a "quick fix". Once I have confirmation, I will remove the focuser and have a better look at a permanent solution.

Thanks

Daniel
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Old 16-02-2011, 01:32 PM
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Quote:
Originally Posted by DJ N View Post
With the QHY8L, the Moonlite focuser is extended approximately 90% of its total travel.
Well it's best if you use an extension tube and rack the focuser back all the way in. You'll get better result overall and less flex.
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Old 16-02-2011, 10:54 PM
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Agreed, best to have the focuser screwed in. But it's odd to hear a moonlight has internal reflective surfaces, I wouldn't expect that.
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Old 02-03-2011, 11:36 PM
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Well I finally managed to get out there and test the new arrangement utilising an extension tube to eliminate the almost full extension of the focuser draw tube. I am happy to say, at this stage it looks as if it has sorted the issue.

I have attached a quick and dirty process, and I am now feeling a lot happier (I did not get a chance to redo my flats so there is a bit of vignetting going on). Hopefully will get a chance to do some further imaging later in the week.

The first image attached was the issue I was having, and the second is from tonights effort (3x15sec...3x30sec...3x120sec and 3x480sec).

Really appreciate the help from everyone.

Cheers,

Daniel
Attached Thumbnails
Click for full-size image (M42 29Dec10 Reflection.jpg)
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Click for full-size image (M42 02Mar11 ED120 QHY8L 3x480sec DSS PS3 Post.jpg)
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Old 03-03-2011, 01:38 AM
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Hi Daniel,
This is exactly what i thought it would be.
The shape and size of the arc shows that its off centre and most likely from the focuser tube or similar.
Anyhoot, good to see your back out again.

Theo
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