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What causes this strange pattern?

Collimation Maksutov
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#1 Paradoxdb3

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Posted 28 February 2021 - 05:17 AM

This is Sirius, taken through my SkyWatcher SkyMax 180. Single exposure, 30s. When in the center of the frame, I get this weird pattern to the right of the star. When I move the star over to the right, the pattern is gone. The telescope looks near perfectly collimated when I defocus on a dim star (or lower the exposure on Sirius) when in the center of the frame. The scope had cooled for about 3 hours outside before this image. Does anyone have any insight into what causes this? I appreciate any help.

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#2 Tapio

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Posted 28 February 2021 - 05:56 AM

Bright stars can create all kind of weird effects.
Common are reflections and microlenses effects.
Some are due to poor anti reflection coatings and not much you can do about it.
Were you trying to image Sirius B or why the long exposure?
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#3 Paradoxdb3

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Posted 28 February 2021 - 06:19 AM

Okay, thank you for the reply. I may have messed up the coatings myself in the past by wiping dew away from the corrector plate. I am always gentle and use a microfiber cloth, like the ones I use for my glasses. I still see the green and purple coatings in the light, but maybe that doesn't mean much. Anyway, I was trying to image the stars around Sirius and show how bright it is in relation to the others. I imaged with my C5 as well, and that turned out really well. Again, thank you. Dan

#4 jrcrilly

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Posted 28 February 2021 - 06:55 AM

You can't judge critical collimation with an out-of-focus star, though severe miscollimation can be seen that way. Fine collimation requires an in focus star bright enough to display the Airy disk.


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#5 Migwan

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Posted 28 February 2021 - 07:52 AM

I agree with John.  Final collimation needs to be done in focus at a high power on a night of very good seeing.   In my C11, a mag 3 to 4 star like Pherkad when it is near the meridian at 467 to 667x shows a diffraction ring the best.   Your 180 will likely prefer a mag 2-3 star.  

 

jd


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#6 Paradoxdb3

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Posted 28 February 2021 - 01:02 PM

You can't judge critical collimation with an out-of-focus star, though severe miscollimation can be seen that way. Fine collimation requires an in focus star bright enough to display the Airy disk.


So are you saying the articaft above is due to collimation? I mean, that's quite a way off center to be finely tuned, isn't it? I mean, if I moved the star from where it looks good to the center... But what do you think?

#7 Paradoxdb3

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Posted 28 February 2021 - 01:14 PM

So are you saying the articaft above is due to collimation? I mean, that's quite a way off center to be finely tuned, isn't it? I mean, if I moved the star from where it looks good to the center... But what do you think?


Oh, and can collimation be done indoors in a controlled environment with a homemade artificial star? I almost never have good seeing where I'm at. Even when I choose a star as close to Zenith as possible, the image is tricky to see. I've been able to get a good collimation indoors, though.

#8 Migwan

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Posted 28 February 2021 - 03:19 PM

Seems to me that it is best to collimate at something over 45°.  For fine collimation, you also need to be able to see an in focus diffraction ring.  I'm not sure you can see one with an artificial star.  You might try it, kicking up the power to find out.

 

 Your images look like pretty fair seeing to me, but I'm no image expert.  Maybe you could try collimating using your camera on a mag 2 or 3 star at something around 500x.  Sirius isn't likely to let you see the diffraction ring.

 

You might also want to post your question in Beginning Deep Sky Imaging, just in case the effect isn't due to collimation.

 

jd



#9 Paradoxdb3

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Posted 28 February 2021 - 03:23 PM

Awesome, thank you!

#10 ngc7319_20

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Posted 28 February 2021 - 04:38 PM

Theres lots going on in this image -- all sorts of reflections.  Probably reflections from sensor window to secondary back to sensor -- large donuts.  Probably from sensor micro-lenses to sensor window back to sensor.  Scattering rays from micro-lenses on sensor.  I don't think much of it has to do with collimation.

 

post-301375-0-18575000-1614507317 x.jpg


Edited by ngc7319_20, 28 February 2021 - 08:26 PM.

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#11 Paradoxdb3

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Posted 28 February 2021 - 07:12 PM

Theres lots going on in this image -- all sorts of reflections.  Probably reflections from sensor window to secondary back to sensor -- large donuts.  Probably from sensor micro-lenses to sensor window back to sensor.  Scattering rays from micro-lenses on sensor.  I don't much of it has to do with collimation.

 

attachicon.gifpost-301375-0-18575000-1614507317 x.jpg

Thank you kindly for your feedback. I appreciate it. 



#12 Jon Isaacs

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Posted 28 February 2021 - 07:20 PM

Theres lots going on in this image -- all sorts of reflections.  Probably reflections from sensor window to secondary back to sensor -- large donuts.  Probably from sensor micro-lenses to sensor window back to sensor.  Scattering rays from micro-lenses on sensor.  I don't much of it has to do with collimation.

 

attachicon.gifpost-301375-0-18575000-1614507317 x.jpg

 

:waytogo:

 

Agreeing with ngc7319_20 is probably the easiest way there is of guaranteeing one is right. 

 

Jon




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