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Stars Shaped Like 0's

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#1 AstroEdge

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Posted 03 June 2020 - 10:09 PM

I have a TPO 6" F/4 newtonian and I have noticed that the stars on my newtonian when slightly out of focus are O's and not perfect stars. I was wondering if this could either be an imperfection in my mirror or could it just be my spacing?

 

The first image is when the focuser is slightly out of the critical focus point, and the second image is when the focuser is slightly inside of the critical focus point.

 

I am using a Sky-Watcher Quattro Coma Corrector.

 

Thank you,

Caleb M.

Attached Thumbnails

  • focuser out.png
  • focuser in.png


#2 Gary Riley

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Posted 03 June 2020 - 11:01 PM

Have you checked your collimation to make sure everything is properly aligned?

#3 TxStars

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Posted 03 June 2020 - 11:08 PM

Are you seeing this visually ? Or when imaging?

If when imagine the spacing between the sensor and the corrector is off..



#4 AstroEdge

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Posted 03 June 2020 - 11:20 PM

I did check the collimation, and with a laser, and autocollimator it seemed on point, but I was talking with someone else and they said that it is astigmatism in the secondary, so I bought a new secondary and hopefully it helps. I am using it for imaging.

 

The person who helped me has the same scope with the same issue, and he said that replacing the secondary fixed it.

 

Thank you,
Caleb M.


Edited by AstroEdge, 03 June 2020 - 11:22 PM.


#5 Cames

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Posted 03 June 2020 - 11:49 PM

A few diagnostic procedures.

 

Using the focuser knob, rack the eyepiece out toward you and note the axis of the '0'. Next rack the eyepiece away from you a similar distance past the point of best focus.  See if the long axis of 0 changes direction.  If yes, could imply astigmatism in the optical train somewhere.

 

Otherwise, could be a pinched mirror where mirror clips are cinched down too tightly.  If the mirror is glued in, adhesive could be tugging on the glass too much.

 

If test implies astigmatism, you can try rotating the primary mirror 45° and see if the axis of the 0 follows the turn of the mirror.

--------------

C



#6 AstroEdge

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Posted 03 June 2020 - 11:53 PM

I have had the clips where they were not even touching the mirror, and have rotated it and it has not changed the direction of the astigmatism.

 

Thank you,

Caleb M.



#7 AstroEdge

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Posted 03 June 2020 - 11:55 PM

This is what they look like in focus, and they look like this to the corners.

unknown.png



#8 Cames

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Posted 04 June 2020 - 12:19 AM

Examine out-of-focus images without coma corrector.  Then examine images while rotating coma corrector.

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C


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#9 Starman1

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Posted 04 June 2020 - 12:43 AM

It is astigmatism.
If it doesn't rotate withe the primary mirror, it's in the secondary.
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#10 Jon Isaacs

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Posted 04 June 2020 - 04:41 AM

It certainly looks like astigmatism. 

 

But it could be some combination of miscollimation and the coma corrector.

 

I would remove the coma corrector and inspect the images of an on-axis star both in and on both sides of focus.

 

What laser was used?

 

Jon


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#11 Asbytec

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Posted 04 June 2020 - 08:44 AM

The images in the OP seem to have both astigmatism and coma, especially the inside focus image on the right. Coma is probably due to the corrector or collimation. Astigmatism can be any number of things from the secondary, ground into or pressure on the primary, tilt of the corrector, to possibly field astigmatism. We presume those images are in the field center, yes? 

 

I might trouble shoot the scope itself for collimation and astigmatism sans corrector and imaging equipment, then add pieces and check for image quality. A good place to start is here: Lockwood's article, Why Aren't My Stars Round. More than a few tips helped me rid my scope of astigmatism, thankfully. 



#12 stargazer193857

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Posted 04 June 2020 - 10:33 AM

I would have rotated the primary before buying a new secondary.

Also, if the secondary is held unclosed with padding behind it, I would have checked the pressure of the padding.

Edited by stargazer193857, 04 June 2020 - 10:34 AM.


#13 AstroEdge

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Posted 04 June 2020 - 10:42 AM

I did rotate the primary and the stars stayed in the same direction.

 

Caleb



#14 Jon Isaacs

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Posted 04 June 2020 - 11:32 AM

I did rotate the primary and the stars stayed in the same direction.

 

Caleb

 

What laser did you use?

 

Jon



#15 AstroEdge

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Posted 04 June 2020 - 01:11 PM

I used an old Orion Lasermate, but that was to get them near where they should be. After the laser I used an autocollimator from Farpoint.

 

Caleb



#16 AstroEdge

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Posted 09 June 2020 - 11:31 PM

Yeah, it was definitely the secondary.

optimized-ycph.JPG?dwhu

And here is M13 in focus:

optimized-i0dc.png?wxsp

Before you could barely resolve the core stars, it all looked like a mush.

 

Thank you for everyone who helped with this,

Caleb M.

Attached Thumbnails

  • optimized-ycph.JPG

Edited by AstroEdge, 10 June 2020 - 01:27 PM.

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