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Old 06-08-2014, 05:41 PM
Tony_ (Tony)
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Advice/opinions

Hello All,

This is an image I took a few nights ago. Don't worry about the overall quality - apart from the focus issue I'm actually fairly happy with the basics of it and I haven't processed it much.
What I am concerned with is the coma/distorted stars especially around the edges.
Its taken with an c9.25 f/10 and the first time I have used a new centralds cooled 60D. I don't think it's a camera issue - I have used a 40D and a 550D previously and I have never seen this before. Also it's not my initial focusing - also done the same way as many times previously.
I suspect it may be caused by my dew heater? It was a fairly cold night and I may have had the heater a bit high and may not have waited long enough for the temperature to stabilise? I took 3 other images that night - 1 of them showed the problem a little bit the other 2 were "normal". Maybe if the dew heater is too high it can cause a temperature gradient across the corrector plate and cause the image to distort? Or as it heated it change the focal length slightly (and changed the focal point)?

Does anyone have any other ideas or is my theory likely to be correct?

Regards,
Tony.
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  #2  
Old 06-08-2014, 06:03 PM
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Amaranthus (Barry)
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Are you guiding and properly polar aligned? It looks a bit like field rotation.
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Old 06-08-2014, 06:13 PM
Tony_ (Tony)
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It's unguided, 30 sec subs. The polar alignment was within 5' according to the hand controller. There was very little drift over the 1 hour total exposure time and there was no rotation from the first sub to the last.
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Old 06-08-2014, 06:31 PM
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Amaranthus (Barry)
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It looks a little like field curvature then. You said you are imaging at f/10, so you are not using a field flattener/focal reducer. That's probably the cause.

It could also be coma -- this does occur in SCTs despite the corrector plate, and can be made worse if you are not properly collimated. A f/6.3 FR/FF will help in managing this aberration too.
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Old 06-08-2014, 07:34 PM
Tony_ (Tony)
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I have never seen this before and the only thing I did different was the camera and possibly a higher dew heater temperature. I have never even seen stars with the slightest coma and I never use a flattener/reducer. Even when I have had poor focus in the past, I still don't get coma like this. Also like I said the next 3 images I took didn't have this problem. (possibly because the temperature had stabilized).

If you get a different result to usual then something has changed - I did the usual set-up and polar alignment etc. I checked collimation (good). I focused as usual, I had the dew temperature higher than usual and it was colder than usual.

I still suspect it has something to do with turning the dew heater on just before this image was taken. I suspect the change in temperature has affected the corrector plate.
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Old 06-08-2014, 08:00 PM
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cometcatcher (Kevin)
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Quote:
Originally Posted by Tony_ View Post
I still suspect it has something to do with turning the dew heater on just before this image was taken. I suspect the change in temperature has affected the corrector plate.
I suspect you are right.

Rapid changes in temperature can affect focus too. It may have been in focus when you started, but it didn't stay that way.
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Old 06-08-2014, 08:13 PM
Tony_ (Tony)
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I suspect that I used the word suspect too often?
I suspect hypothesize may have been a better word.
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Old 06-08-2014, 08:17 PM
Tony_ (Tony)
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I refocus before each image - which is probably why the next images were okay. The temperature had stabilized. (I suspect).

I appreciate any suggestions, whether they are right or wrong doesn't matter. I want to consider all possibilities even if I can eliminate based on previous experience and results.
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Old 07-08-2014, 10:55 AM
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Looks a bit like coma to me...but being out of focus can exacerbate this. How are you achieving fine focus? And are you focusing "up", by turning the focus knob anti-clockwise? The C9.25s I've seen seem to exhibit a little less field curvature than the bog-standard C8/C11 models, but visually it shows up as out of focus around the whole star, not streaking towards the edge. Of course it could be a combination of the two

Once I'd noticed the coma with my C8 visually, I couldn't "unsee" it and ended up replacing it with the Edge HD model
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Old 07-08-2014, 04:56 PM
Tony_ (Tony)
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Thanks.
I use a bahtinov mask and I always focus anti-clockwise to avoid mirror flop - which seems to work well. I take a fair time with the focus to get the diffraction spike centered and this always works well.
As I have said - I have never seen anything close to this before with my images. My stars are usually fairly round (apart from PE issues) across the whole field.
The only thing different to usual was a new camera (60d) - but I doubt whether that could cause this affect - it is similar enough to the 40d and 550d in overall construction.
It was cold (about 1-2 degrees) and I had the dew heater higher than usual and probably higher than it needed to be. I think this may have caused a temperature gradient from the outside of the corrector plate to the middle while I took this image. Since I focused at the centre this would be least affected - the outer part is probably way out of focus causing the coma. By the time I got to the second image the temperature had probably stabilized. Also, I think I turned the heater on after I focused - so the change in temperature may have affected the focal point. (I usually turn the heater on sooner).

Regards,
Tony.
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Old 07-08-2014, 05:56 PM
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Hi Tony,

What was the old camera you used? Same sized chip?

The distortion also doesn't look symmetric so you could also have some tilt in image train
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Old 07-08-2014, 07:18 PM
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Out of focus doesn't cause the coma, it just exacerbates it, makes it easier to see. The coma would be there with perfect focus, only less noticeable and/or sharper. And it's not the corrector that affects the focus during cool down, it's the whole scope...since it's a metallic object.
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Old 07-08-2014, 07:30 PM
Tony_ (Tony)
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I've used a 550d and a 40d before - this is a 60d. I think it has the same size chip and overall configuration (distance from adapter screw to sensor)?
M17 was almost overhead so not likely to be camera tilt. The other images were lower and did not have this coma. The overall fit of the 60d in the adapter is probably a bit better than the other cameras.
When I focused for the second image and used the bahtinov mask I noticed that the center spike had moved a fair bit off center - that's why I think the heating of the corrector plate has affected the focus.
I have had mirror flop once a few years ago and it didn't cause any coma, it just caused the image to shift and only affected the focus slightly. I have never seen coma in my f/10 images ever - so it has to be something a lot different to what I usually do. The next 3 images (same night with the 60d) were okay - so it's not the set up etc. I will test this next image I take by deliberately taking some subs out of focus.
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Old 07-08-2014, 08:12 PM
raymo
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Hi Tony, I notice that all the stars are elongated horizontally, so I suspect
that your P.A. is not good enough for 30 sec subs. Even with my 8" f/5
Newt at prime focus I need to be down to a max of 1' to get round stars,
and down to just a few seconds for 45-60 sec subs. As far as the coma like problem is concerned, along with the others, I think you are probably right.
raymo
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Old 07-08-2014, 08:28 PM
Tony_ (Tony)
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I didn't have much drift - and the drift was actually in the vertical direction not horizontal.
The PE is in the horizontal direction - this is usually the main cause of my oval stars. I have a guider but I didn't use it that night.
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Old 08-08-2014, 07:00 AM
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This is mostly a focus problem. The image is a considerable way out of focus. You must refocus regularly when imaging. The image is slightly out of focus and this has highlighted the slight optical aberrations in your system.
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Old 08-08-2014, 04:45 PM
Tony_ (Tony)
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I think there may be a combination of issues - but the main problem does seem to be focus. I don't usually have this problem. I focus on a star and then go to the object - but I don't usually check the first image too carefully. The first sub was already out of focus. Either the dew heater theory is the cause or maybe the focus (primary mirror) shifted during the final goto.
I've had a closer look at some of my earlier images and there actually is some coma but it is sharp and nothing like this. It usually doesn't bother me too much (but it might now I've noticed it in more detail)
When I checked the focus after about 1 hour the bahtinov spike had moved quite a lot - more than I have ever seen before so there was definitely a change in focus.
I will take more care next time. I usually focus in the center of the image - is it better to focus off center?
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Old 10-08-2014, 10:11 PM
Tony_ (Tony)
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It still looks a bit crappy - but I managed to process it a bit that I think looks better?
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Old 10-08-2014, 10:38 PM
DJT (David)
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Am currently having an adventure with an RC 8 but I expect my stars to be pointing into the centre of the FOV. Yours don't. they are all over the place.Do you have a collimation issue?
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Old 11-08-2014, 05:40 AM
Tony_ (Tony)
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The original image was out of focus also affected by PE, in general the coma is pointing away from the center. I will check the collimation again next time - but it was fairly good.
The second image has had a lot of processing - lens correction, star reduction, deconvolution etc.
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