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Old 03-04-2016, 08:04 AM
VPAstro (Andrew and Cam)
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Chromatic aberration with SW BD ED80

Hi All,
We have noticed the over the last little while, we have started to get chromatic aberration on our stars in images. The scope is new, and was a 50th birthday present from my father in law. For some reason, our images seem to be getting worse and not sure if it is the camera, scope or processing that is the issue. The attached image was taken last night with a canon 600d and consists of 65 X 90sec lights at iso 800, (dithered), 30 dark frames taken at the same time, 30 bias and 30 flat frames. Stacked in DSS and processed in CS2. Light pollution is also a real problem here, but I was able to see Eta Carina neb with my naked eye. Noise has also become a pain now and is difficult to remove. Just trying to identify if the problem is the scope, camera or processing.
Thanks
Andrew & Campbell
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Old 03-04-2016, 08:14 AM
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Merlin66 (Ken)
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A & C,
The red and blue edges could be the atmospheric chromatic effect??
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Old 03-04-2016, 08:19 AM
VPAstro (Andrew and Cam)
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Thanks for that. Something I didn't think about. We have noticed this also in images taken with our 10" reflector recently. They also have put up some new very bright lights at the note printing works behind our house. I wonder if this could have anything to do with it as well.
Thanks
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Old 03-04-2016, 08:23 AM
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The attached image gives some idea of the atmospheric dispersion effect.
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Old 03-04-2016, 11:55 AM
raymo
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Ken has to be spot on; even a cheap achromat wouldn't exhibit CA
as intense as that.
raymo
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Old 03-04-2016, 12:55 PM
VPAstro (Andrew and Cam)
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Thanks Raymo.
It is strange that it only started, or seemed to start after the new lights at the mint. The fact that we are getting it on our reflector as well, was making me think that it could have been he camera. I had not thought of atmospheric dispersion.
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Old 03-04-2016, 01:30 PM
VPAstro (Andrew and Cam)
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Single sub

I thought I would include a highly cropped image of a single cr2 sub as well as the sub. The crop clearly shows the colour fringing and the sub heavily light polluted.
Andrew...
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Old 03-04-2016, 01:40 PM
raymo
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Reflectors don't suffer from CA when used for prime focus imaging, only
occasionally when using a poorly corrected eyepiece for visual or eyepiece
projection.
Atmospheric dispersion ranges from non existent to appalling, depending
on the climatic conditions at the time, and typically decreases as you go
higher in the sky. Are you getting this problem consistently every night?
A quick test would be to take a quick snap low to the horizon, one at around
45 Degrees, and one overhead. If it is atmos disp the effect should vary greatly between images.
If they are all the same, then you have a different problem. You could also
repeat the test facing in the direction of the new lights, and then 180 degrees from them, and see if they vary. The more data, the easier it will be to find a definite answer.
raymo
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Old 03-04-2016, 02:20 PM
VPAstro (Andrew and Cam)
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Thanks Raymo, will try next clear night. All our imaging is done prime focus, so camera is connected directly to the scopes. I have been looking through old subs, and this was not present 3 months ago. The lights from the note printing works are to the east, about 300m away and now very bright. The attached image was taken in the north east about 35' above horizon. The interesting thing is that the introduction of CA also corresponds to an increase in noise in our images.
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Old 03-04-2016, 04:06 PM
raymo
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Maybe it'll turn out in the end that there is a problem with the camera.
raymo
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Old 03-04-2016, 06:13 PM
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Quote:
Originally Posted by raymo View Post
Maybe it'll turn out in the end that there is a problem with the camera.
raymo
i can't imagine this is a camera issue...
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Old 03-04-2016, 06:22 PM
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I think it highly unlikely too, but I suppose you should never rule out
anything that isn't manifestly impossible. The faux CA will, I expect, be understood in time, but I have to say that the concurrent
arrival of extra noise is odd.
raymo

Last edited by raymo; 03-04-2016 at 06:25 PM. Reason: more text
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Old 04-04-2016, 08:30 AM
VPAstro (Andrew and Cam)
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Interesting

Hi all,
Thanks for your comments above. Last night, I decided to use the 40d instead of the 600d to see if it made a difference. The attached image is 25 X 90sec subs at iso 800, with 10 darks, 25 bias and 25 flats all taken during the same session. Looks like there is no CA, and the noise is a lot less. One thing that I did notice is that while the outside ambient temp was 11'c, the sensor temp on the 40d was +13'c according to byeos. The 600d's sensor temp the was at +25'c when the ambient was 13'c the night before. Thinking that it might be the camera..
Thanks
Andrew & Campbell
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Old 04-04-2016, 11:16 AM
raymo
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The sensor temp at 25 compared to 13 would account for the difference in noise. Had both cameras been in action for roughly the same amount of
time when the temps were recorded? With only 2 degs difference in ambient
temp I would have thought that the difference in sensor temp would have been similar. Still to discover the cause of the colour aberration with
the 600D.
raymo
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Old 04-04-2016, 04:20 PM
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I am following this thread with great interest. I too have occasionally seen some chromatic aberration from my ED80/Sony a6000 combination. It didn't occur to me the atmosphere could cause this effect. Always learning.
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Old 05-04-2016, 07:45 AM
VPAstro (Andrew and Cam)
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Hi Jim, I will continue to do more investigation when time allows. I decided to play with my old dsi ii pro last night capturing in nebulosity. It gave me better results than I thought. How do you find the a6000? My wife has just bought one for her main shooting camera.
Andrew
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Old 05-04-2016, 04:40 PM
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Hi Andrew, I am very pleased with the a6000 both for everyday use and astrophotography. Had it for about 2 years now. As you will find out it is very light sensitive but with very low noise. Quickly focus on-camera using the Focus Magnifier at maximum zoom and you'll nail that every time.
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