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Can a Celestron 80mm Onyx be collimated ?

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#1 astrodoc10

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Posted 12 February 2017 - 08:17 AM

My recently acquired used 80mm f/ 6.25 Onyx has wonderfully sharp star images using 15 to 2.5mm eyepieces, But when I use longer 20 and 30 mm high quality eyepieces the star images look like little flying geese. Switching between good 2" and 1 1/4" diagonals doesn't seem to help. There are no obvious collimating screws towards the lens end of the optical tube so I wonder if there is a way the scope can be collimated.



#2 Kent10

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Posted 12 February 2017 - 09:23 AM

This sounds like the astigmatism in your eye which is seen at larger exit pupils.  Poor collimation would be easier to see at higher magnifications.  I also just bought a used 80mm scope and checked collimation with an artificial star at extreme high power.  Thankfully it was fine.

 

Rotate your head when viewing your "flying geese"/ seagulls and if they rotate with you that is the astigmatism in your eye causing this.  If you wear glasses and can use them when viewing that might help eliminate the seagulls.

 

Congratulations on your new Onyx.  They are supposed to be great.


Edited by Kent10, 12 February 2017 - 09:24 AM.

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#3 Kent10

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Posted 12 February 2017 - 09:30 AM

You can check collimation at high power with a bright star.  Go inside and outside focus and you should see a perfect bullseye on both sides of focus. 

 

These instructions apply to a reflector but the pictures are good.

 

http://www.viewfroma...te-a-telescope/

 

Another good image

 

http://web.telia.com...li/comacomp.JPG


Edited by Kent10, 12 February 2017 - 09:32 AM.


#4 gene 4181

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Posted 12 February 2017 - 10:20 AM

   field curvature  ?



#5 astrodoc10

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Posted 12 February 2017 - 10:22 AM

Hi Kent 10,

      The problem may be astigmatism in my eye which I know I have. Star test inside and outside of focus is good and I can easily split stars down to 1.5 arc seconds which is the theoretical limit of the 80mm scope. With eyepieces less than 20mm I have pinpoint star images in the scope.

 

 Thanx......astrodoc10


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#6 Erik Bakker

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Posted 12 February 2017 - 10:58 AM

Astrodoc10,

 

You describe eyes that perform not so well and a scope that performs very well, as evidenced by it's high power views.

 

Views at lower magnification have very little to do with collimation, especially if high power views are good. They demonstrate to you the quality of your eyes at wider pupil diameters. Eyepieces with a focal length of 20mm and longer translate into bigger than 20/6.25 = 3.2 mm exit pupils. At night, our own pupils are generally wider opened than that and start to show their own deficiencies. If you would take a quality eyepiece like the 41mm TeleVue PanOptic, you would be shocked how bad the stars look in your eyes under a dark sky. But as soon as you help your eyes a little, by squeezing index fingers and thumbs of both hands to form a small diafragma with an opening of 1-2mm in front of your eyes, you will see the starts clean up immediately into tiny stars again. Try it and see for yourself. 


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#7 astrodoc10

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Posted 12 February 2017 - 11:15 AM

Sounds like my eyes are indeed at fault and I will "squeeze" my hand in front of my eye during the next viewing session.

Field curvature is not the problem.



#8 mwedel

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Posted 12 February 2017 - 11:45 AM

How timely! I was out doing some low-power sweeping with my C80ED the other night and noticed the same thing, for the first time. I have astigmatism and my glasses correct for it, but I don't typically wear glasses when observing. I did notice that the star images cleaned up nicely when I went to higher powers, which left me confused about what was going on optically. Thanks, Kent and Eric, for the clarification. I will try that thumb-and-finger diaphragm trick.


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#9 John O'Hara

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Posted 12 February 2017 - 11:59 AM

This may be coma rather than astigmatism (or a combo of both). At f/6.25, you would likely see it at the field edges at low power. How are images in the center?



#10 rmollise

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Posted 12 February 2017 - 12:08 PM

This may be coma rather than astigmatism (or a combo of both). At f/6.25, you would likely see it at the field edges at low power. How are images in the center?

Usually, astigmatism is more evident with longer f/l eyepieces, and manifests not as "seagulls," but ovals that change orientation on either side of focus.


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#11 SteveG

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Posted 12 February 2017 - 05:27 PM

 

This may be coma rather than astigmatism (or a combo of both). At f/6.25, you would likely see it at the field edges at low power. How are images in the center?

Usually, astigmatism is more evident with longer f/l eyepieces, and manifests not as "seagulls," but ovals that change orientation on either side of focus.

 

Or little crosses. I was shocked when I finally figured out that it was my eyes, and not my low power eyepieces.


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#12 CounterWeight

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Posted 12 February 2017 - 07:39 PM

Congrat's on the new to you Onyx, I had one a few years back and it was a great little performer! 



#13 mwedel

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Posted 13 February 2017 - 03:44 AM

Or little crosses. I was shocked when I finally figured out that it was my eyes, and not my low power eyepieces.

Ahhhhh!!! I am SO glad I found this thread. I was seriously pondering monkeying with my C80ED to try and fix the problem. I knew it wasn't my eyepieces or my diagonals because I'd swapped them all around looking for the problem. I never considered that it might be me.

 

I just got in from a driveway session, where I spent a long time looking at the belt and sword of Orion, which served up enough bright stars over a wide enough area to be a good testing range. I tried the thumb-and-finger diaphram trick and it made a difference, but it was hard to hold steady enough to evaluate. Then I had the idea of using a collimation cap to stop down the exit pupil. The view got dim, but the star images cleaned right up. So I tried observing with my glasses on, which I never do, and whaddaya know, that cleaned up the star images, too. It wasn't the scope, it was me.

 

On one hand, that's a big relief, because that scope has always been a rock-solid performer and I was feeling a bit ill at the thought that it might have gotten out of whack.

 

On the other hand, I now need to prioritize eye relief in my eyepiece collection. I have a bunch that are too tight to show the whole field when I'm wearing glasses. So I have some decisions to make.


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#14 Jon Isaacs

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Posted 13 February 2017 - 07:11 AM

Sounds like my eyes are indeed at fault and I will "squeeze" my hand in front of my eye during the next viewing session.

Field curvature is not the problem.

What eyepieces are you using?  

 

F/6.25 is fast enough that quality eyepieces corrected for off-axis astigmatism are required if one wants sharp views.  And if one is using eyepieces that are well corrected for off-axis astigmatism, with longer focal lengths with wide fields of view, the curvature of the focal plane will be visible because it is there.  

 

Jon


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#15 SteveG

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Posted 13 February 2017 - 04:51 PM

 

Sounds like my eyes are indeed at fault and I will "squeeze" my hand in front of my eye during the next viewing session.

Field curvature is not the problem.

What eyepieces are you using?  

 

F/6.25 is fast enough that quality eyepieces corrected for off-axis astigmatism are required if one wants sharp views.  And if one is using eyepieces that are well corrected for off-axis astigmatism, with longer focal lengths with wide fields of view, the curvature of the focal plane will be visible because it is there.  

 

Jon

 

No doubt, at 500 mm I would see lot's of FC even with Naglers and Dioptrx.


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#16 John O'Hara

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Posted 13 February 2017 - 04:58 PM

 

This may be coma rather than astigmatism (or a combo of both). At f/6.25, you would likely see it at the field edges at low power. How are images in the center?

Usually, astigmatism is more evident with longer f/l eyepieces, and manifests not as "seagulls," but ovals that change orientation on either side of focus.

 

Hi Rod,

 

I was primarily trying to distinguish whether or not the problem was edge of field aberrations from those center field.  Astigmatism in the center of the field might be indicative of collimation, but of course, could have other causes.  What made me curious was the description of "seagulls", which sounded like off-axis coma, as opposed to astigmatism where stars become streaks that change orientation with focus (or eye position depending on the source).  I'm not telling you anything, as I know you're a seasoned observer and author.  I'm just explaining my previous post.

 

John



#17 Peter Besenbruch

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Posted 14 February 2017 - 10:31 PM

I now need to prioritize eye relief in my eyepiece collection. I have a bunch that are too tight to show the whole field when I'm wearing glasses. So I have some decisions to make.

Yay!! You get to spend money on eyepieces!!! It never ends. ;)




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