Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> ATM, Optics and DIY Forum

Pages: 1
oddog
sage


Reged: 11/11/08

Antares 10" DOB astigmatism problem
      #5626376 - 01/16/13 03:16 PM

Picked up a 10" Antares DOB off Craigslist the other day, cosmetically nearly new but seems to have an astigmatism issue. After center spotting the mirror it collimated fine. Stars are not sharp and their out-of-focus image is an oblong, changing directions by 90° on either side of focus. From what I can gather that sounds like classic astigmatism. A Ronchi test comes out OK but I don't think the Ronchi image will reveal astigmatism (is that correct?).

Where should I be looking for the problem? I checked the retention of the mirror, not squished anywhere I can see.
I thought about it being in the secondary mirror. I could remount it out of its current plastic holder (holds it pretty tight) but it would take some work. I could replace the secondary mirror but that would cost me about $80 (ScopeStuff has the 2.3" minor-axis size needed). Any way to isolate where the problem lies between the primary and secondary mirrors?

I only have $100 in the scope so I have some wiggle room to get it to work but maybe I should just sell it off or piece it out. I won't be keeping it in any event, have too many other scopes close in spec.

Thanks in advance for any input.

Tom Duncan/Oddog


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Antares 10" DOB astigmatism problem new [Re: oddog]
      #5626394 - 01/16/13 03:28 PM

Quote:

A Ronchi test comes out OK but I don't think the Ronchi image will reveal astigmatism (is that correct?).







here's my ronchi star test and you can see the one bar hook while the other is still straight



Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Antares 10" DOB astigmatism problem new [Re: oddog]
      #5626415 - 01/16/13 03:39 PM

Quote:


Where should I be looking for the problem? I checked the retention of the mirror, not squished anywhere I can see.
I thought about it being in the secondary mirror.

I could remount it out of its current plastic holder (holds it pretty tight) but it would take some work.




Tom:

This is a common problem with the 10 inch F/5 GSO dobsonians, the plastic secondary holder pinches the secondary and the result is serious astigmatism.

The solution is to relieve the plastic holder so it no longer pinches the secondary...

Then all should be well and you will have scored a real deal..

Jon

Edited by Jon Isaacs (01/16/13 03:44 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
oddog
sage


Reged: 11/11/08

Re: Antares 10" DOB astigmatism problem new [Re: Jon Isaacs]
      #5628130 - 01/17/13 01:49 PM

Jon, thanks for the info, I thought it might be a known problem, hence my posting. I had suspected the pinching secondary, I think I will cut off the sides of the holder and silicone the mirror back on.

Danny, are you suggesting the astigmatism is showing up in the Ronchi images?

Tom


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Antares 10" DOB astigmatism problem new [Re: oddog]
      #5628162 - 01/17/13 02:08 PM

in mine they are, I already new it had stig from a double pass test agaisnt a flat but since I made the screen I wanted to see how well it worked.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Achernar
Postmaster
*****

Reged: 02/25/06

Loc: Mobile, Alabama, USA
Re: Antares 10" DOB astigmatism problem new [Re: oddog]
      #5628275 - 01/17/13 03:11 PM

Point the telescope at Polaris, and note the orientation of the ellipses as you go from one side to the other of focus. Put a red dot sticker on the edge of the mirror and another on the tube in line with it, then remove the cell from the telescope, turn the mirror 45 degrees, then replace and re-collimate. Turn the telescope on Polaris again. If the orientation of the ellipses turned with the mirror, the primary mirror is defective. If it does not rotate with the mirror, it can only be the diagonal. Try loosening the holder, or replacing it with one that allows for properly mounting it without strain.

Taras


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Antares 10" DOB astigmatism problem new [Re: oddog]
      #5628324 - 01/17/13 03:35 PM

Quote:

Jon, thanks for the info, I thought it might be a known problem, hence my posting. I had suspected the pinching secondary, I think I will cut off the sides of the holder and silicone the mirror back on.

Tom




Tom:

How old is your scope? I got mine about 10 years ago, cheap because the owner said the optics were poor. I found the pinched secondary and just relieved it slightly and since then it is been a very good performer. It turned out later that this was quite a common problem. I would just scrape the holder until the mirror fit cleanly and try that.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
oddog
sage


Reged: 11/11/08

Re: Antares 10" DOB astigmatism problem new [Re: Jon Isaacs]
      #5637039 - 01/22/13 02:28 PM

Well I relieved the secondary holder quite a bit (just left 4 tabs) and the view improved (to less apparent astigmatism) but it's still not right. I may cut off all the tabs I left and glue it on to the secondary holder with silicone but I will first try rotating the primary to see if the astigmatism follows.

Tom

Edited by oddog (01/22/13 02:31 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Antares 10" DOB astigmatism problem new [Re: Achernar]
      #5638186 - 01/23/13 02:08 AM

Quote:

...remove the cell from the telescope, turn the mirror 45 degrees, then replace and re-collimate... If the orientation of the ellipses turned with the mirror, the primary mirror is defective. If it does not rotate with the mirror, it can only be the diagonal. Taras




In this situation it could also be the primary cell in which case, if possible, rotate the cell 45° and retest.

Dave


Post Extras: Print Post   Remind Me!   Notify Moderator  
oddog
sage


Reged: 11/11/08

Re: Antares 10" DOB astigmatism problem new [Re: dave brock]
      #5654431 - 01/31/13 02:32 PM

I rotated the cell 60° (one screw hole of the 6) and indeed the astigmatism followed so it looks like I have a bad mirror. I checked the mounting of the mirror in the cell and it is fine, not pinched or anything.

Looks like I have an "as-izer" for Craigslist!

Tom


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1


Extra information
4 registered and 19 anonymous users are browsing this forum.

Moderator:  ausastronomer, richard7, Starman81 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 846

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics