#1  
Old 17-11-2013, 06:15 PM
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coldlegs (Stephen)
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HA flat reflection problem

Began the process of setting up my QHY9M mono camera as a screw in Lum/Ha replacement for my QHY10C colour camera whenever the moon was up and ran into a serious reflection problem. The Ha flat looked like it had a major vignetting problem and when pre-processed through Nebulosity resulted in an image with bright corners and a dark inner area. Nebulosity couldn't handle the very sharp drop in intensity. Immediately googled high and low and could only find this link
http://saratogaskies.com/articles/ha-ff/index.html
but it seemed to be the exactly same problem. Sent an email off to Jim Solomon at Saratogaskies to see if he solved it but no he hadn't so plodded on. Should point out my system config was
10” F5 Serrurier truss
Onag
Mpcc II
spacer
Astronomik 6nm Ha filter
QHY9M with IR window

My first thought was the IR window on the camera so swapped it for a clear window with no effect. Then swapped the Ha filter and the spacer, putting the filter on the back of the coma corrector and noticed the circle got slightly larger. Thought the old MPCC II version might be a problem so borrowed a mark 3 but nothing changed. Began to think it was a combination of all three glass interfaces causing the problem then realised that I could place the filter in front of the coma corrector so tried it and it worked. There is still some kind of reflection going on but it is very faint and smooth enough to be handled during processing.
Pictures
1 Ha Flat reflection problem
2 over stretched image (tarantula) showing bright corners and darker center
3 Ha Flat with filter in front of coma corrector

Not sure why I got the reflection but maybe some optics engineers out there can.
Cheers
Stephen
Attached Thumbnails
Click for full-size image (1.jpg)
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Click for full-size image (2 overstretched.jpg)
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Click for full-size image (3 HA in front of mpcc.jpg)
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  #2  
Old 18-11-2013, 08:19 AM
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gregbradley
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What size filter is it and what nm?

QHY9 is the KAF8300 sensor which is quite small right? It requires 36mm filters unless the filter is really close to the sensor as in QSI 583 cameras and 1.23 inch filters work there.

What do the light exposures look like or is that Tarantula image an example of what they look like?

Greg.
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Old 18-11-2013, 08:58 AM
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coldlegs (Stephen)
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Quote:
Originally Posted by gregbradley View Post
What size filter is it and what nm?

QHY9 is the KAF8300 sensor which is quite small right? It requires 36mm filters unless the filter is really close to the sensor as in QSI 583 cameras and 1.23 inch filters work there.

What do the light exposures look like or is that Tarantula image an example of what they look like?

Greg.
Greg
I guess “small” is relative. The KAF8300 is 17.96mm(H) x 13.52mm(V) 5.4um/8.3MP.
Link here http://www.ccd-labs.com/Qseries/qhy9m.htm
It's not possible to get the filter closer than about 30mm from the sensor.
The Ha filter is a standard 2”/M42 size Astronomik 6nm.
The light image is severely stretched to show the problem but here's a final image. The effect is there but might be a little difficult to see given the jpeg download limitations.
Cheers
Stephen
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Old 18-11-2013, 09:56 AM
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Peter Ward
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I suspect there are two things going on here:

the field is being vignetted (while you can "get away" with a 1.25"...a 36mm avoids this ) and....

You are getting a radial drop-off in filter performance due the converging light cone ) again a larger filter helps... a thinner filter set may also help here.
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Old 19-11-2013, 08:56 AM
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coldlegs (Stephen)
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Quote:
Originally Posted by Peter Ward View Post
I suspect there are two things going on here:

the field is being vignetted (while you can "get away" with a 1.25"...a 36mm avoids this ) and....

You are getting a radial drop-off in filter performance due the converging light cone ) again a larger filter helps... a thinner filter set may also help here.
Peter
I don't believe it is vignetting as there are stars/nebula in the corners that would be cut of if vignetting occurred. Also moving the filter closer to the rear of the corrector resulted in the circle getting larger instead of smaller as would be the case if vignetting occurred. As for “filter drop off” , analysis of the master flat (attached image) with the Ha filter placed in front of the corrector instead of between it and the camera shows a normal gradient expected from a newt. I beginning to think others have encountered this problem and said “yup it's vignetting” and either completely changed their configuration to eliminate the problem or blamed something camera/filter/corrector for their problem.
Cheers
Stephen
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Old 19-11-2013, 04:45 PM
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Looks like vignetting to me. Vignetting does not mean no light in the corners. It means a reduction of light in the corners. You will still see stars etc in the corners of a vignetted image if its going to be correctable with flats. Just light drop off.

If you move the filter and it the circle gets larger I think you have your answer. F5 in a Newt is relatively fast and the steep light cone seems to be at least part of the issue. Your filter may be fine on some other scope that is not so fast.

Whether the idea that the circle should get smaller if you move closer to the rear of the corrector may be incorrect. You'd need a ray trace of your scope to see what is happening with the light when you move. I am not 100% sure but I think a corrector is bending the light back to more straight lines rather than a cone.

They usually only slighty increase the F ratio if at all, so that is something to consider. So moving it forward to backwards should not make too much difference in terms of vignetting unless the filter is too small for your light path.

If you can get your filter even closer and get that circle bigger you may solve the problem.

Greg.
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Old 19-11-2013, 06:42 PM
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coldlegs (Stephen)
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Quote:
Originally Posted by gregbradley View Post
Looks like vignetting to me. Vignetting does not mean no light in the corners. It means a reduction of light in the corners. You will still see stars etc in the corners of a vignetted image if its going to be correctable with flats. Just light drop off.

If you move the filter and it the circle gets larger I think you have your answer. F5 in a Newt is relatively fast and the steep light cone seems to be at least part of the issue. Your filter may be fine on some other scope that is not so fast.

Whether the idea that the circle should get smaller if you move closer to the rear of the corrector may be incorrect. You'd need a ray trace of your scope to see what is happening with the light when you move. I am not 100% sure but I think a corrector is bending the light back to more straight lines rather than a cone.

They usually only slighty increase the F ratio if at all, so that is something to consider. So moving it forward to backwards should not make too much difference in terms of vignetting unless the filter is too small for your light path.

If you can get your filter even closer and get that circle bigger you may solve the problem.

Greg.
Greg
You might be right but I was originally thinking it was a mechanical vignetting problem as the massive drop off in intensity was so sharp. Not sure if optics can cause such a sharp edge when vignetting but it might be possible. The closest I could get the filter to the back of the corrector was about 7mm and it only slightly increased the circle. The thing that really bugs me is how the heck does putting the filter in front of the corrector cure everything? Still it's all working now so I'm a happy little astro-photographer. Just thought I should warn people if they get this Ha problem then “stick the filter on the front of the corrector!”
Cheers
Stephen
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