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Comet Tails on Stars

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#1 mikenoname

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Posted 14 May 2022 - 09:10 AM

Have a weird optical abberation showing up on my images the last couple nights. See pic.

 

My 6" imaging Newt is well collimated according to star testing and the MPCC seems to be mounted correctly to the 183, but all of the stars have short comet tails/smudges.

 

Scratching my head as to what it could be. Any guesses?

 

Capture_00001s.jpg


Edited by mikenoname, 14 May 2022 - 09:11 AM.


#2 emflocater

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Posted 14 May 2022 - 10:04 AM

Are your clips too tight on the mirror causing pinched optics? Camera tilt?

 

Cheers

Don



#3 AtlantaAstro

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Posted 14 May 2022 - 10:11 AM

Looks like pinched optics to me. Clips may be too tight. Note the double diffraction spikes on your main star


Edited by AtlantaAstro, 14 May 2022 - 10:12 AM.

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#4 mikenoname

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Posted 14 May 2022 - 10:59 AM

Hmmm. Pinched optics.

 

I've owned this scope for a couple of years now and never messed with the clips and this just started after I collimated it a couple days ago. Could a collimation pinch them?

 

Here is a higher res image to show the issue better.

 

Capture_00001.jpg



#5 AtlantaAstro

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Posted 14 May 2022 - 11:11 AM

Pinched optics typically create triangular or lopsided stars depending on whether it’s refractor or reflector from what I understand. At first I thought it was coma but since it’s in every star, I think the mirror may either be pinched or the collimation may have caused this if you’re adjusting mirror screws, one end could be lopsided or too tight potentially


Edited by AtlantaAstro, 14 May 2022 - 11:13 AM.

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#6 c131frdave

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Posted 14 May 2022 - 11:45 AM

I had that same thing start happening a week ago.  I wasn't sure what it was.  It started happening midway through an imaging session and I thought it was simply the focuser getting cold and loosening up and the camera tilting a little bit (there was a drop in temperature of about 30 degrees that evening.  But I went out again two days ago and the same thing happened during the entire session from the start.  This is my first newtonian, and I had collimated the scope just prior to last week's imaging session.  Could this "pinching" happen when one of the collimation screws is too loose?  Like a mirror flop or something?  Like I said, for me it didn't happen until 2 hours into the run as the scope moved from pointing pretty much straight up to about 45 degrees (the entire session was 5 hours, and the change occurred a little under half way).  Had the screws been too tight, I would think you would have seen it from the start.


Edited by c131frdave, 14 May 2022 - 11:46 AM.


#7 AtlantaAstro

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Posted 14 May 2022 - 02:25 PM

That’s a good point it could be mirror flop



#8 mikenoname

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Posted 14 May 2022 - 08:57 PM

Thanks for the input. I will try and play around with it tonight.



#9 Alex McConahay

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Posted 14 May 2022 - 10:26 PM

You may have pinched optics, but you most certainly have spider vanes that are not properly aligned. YOu notice how you have the two spikes in each of the four quadrants. You should only have one in each quadrant. The spider vane from the top to the middle and from the bottom to the middle should overlap so that you see only one in the top to bottom of the picture. And the same thing for the east-west quadrants. The simplest way to say it is that your secondary is not centered (or, your vanes are not at 90 degrees away from each other!). 

 

Deal with that first.....it is pretty important, easy to fix, and may be causing other problems. 

 

Alex


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#10 mikenoname

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Posted 15 May 2022 - 01:42 AM

Thanks for that Alex.

 

You say it's easy to fix but I'm afraid I don't know how, so please enlighten me as to the best way to align them and center the secondary.



#11 Alex McConahay

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Posted 15 May 2022 - 05:38 AM

First, take a good look at the secondary spider vanes.  They are attached to a ring that holds the secondary and the focuser. This is the secondary cage.  The far ends of the spider vanes go through the walls of the secondary cage and are fastened with a nut. the vane heads from that nut to the secondary holder and that is attached to the the vane running to the far side. (It may be one piece of metal.) That vane then runs through the wall and is attached by a nut.  By loosening the nut on one side while tightening the nut on the other, you can move the secondary back and forth across the front of the scope.

 

Pull a string or ruler across the front of the scope and you will find that instead of a straight line across your vanes, there is a bend at the secondary holder. That bend is caused by the OTHER set of vanes being off centered.  So adjust the nuts on the ends of that one to eliminate the bend. 

 

You our may have to go back and forth on the two axes to get both sets of vanes straight. 

 

Alex


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#12 mikenoname

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Posted 15 May 2022 - 09:21 AM

Thanks, Alex! I'll give that a shot.

 

Hopefully that is all that is causing all of this issue because I really don't want to have to pull the primary cell out of the scope to check for pinching. :)



#13 Alex McConahay

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Posted 15 May 2022 - 09:35 AM

I think there may be more.....the pattern I see in other stars is not what I am seeing in that big star in the middle. My assesment of non-linear spider vanes was based on the big star in the middle.

 

But, it is easy to check. 

 

Alex


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#14 mikenoname

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Posted 15 May 2022 - 06:52 PM

I used your string trick (actually used a piece of floss as I couldn't find any string) and got the spider vanes nice and perpendicular again. Then recollimated secondary and primary with cheshire eyepiece. Will go out and star test to fine tune the collimation and then we'll see what results.

 

Thanks for the help!



#15 Alex McConahay

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Posted 15 May 2022 - 07:22 PM

I hope that does the trick overall.....It should get you back to one diffraction spike per quadrant. That may be all you needed, although I doubt it. But with that out of the way, you can concentrate on the other problem whatever it may be. 

 

It was relatively easy once you figured out what I was talking about---huh?

 

I spent a long time as an amateur telescope maker. Have not finished one for a long time all the way from grinding the mirror to observing with it under dark skies. But, I still incorporate the tricks I learned from that discipline in my other astro adventures. 

Alex




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