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Antares 10" DOB astigmatism problem

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#1 oddog

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Posted 16 January 2013 - 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

#2 Pinbout

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Posted 16 January 2013 - 03:28 PM

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

Posted Image

#3 Jon Isaacs

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Posted 16 January 2013 - 03:39 PM

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

#4 oddog

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Posted 17 January 2013 - 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

#5 Pinbout

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Posted 17 January 2013 - 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. :grin:

#6 Achernar

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Posted 17 January 2013 - 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

#7 Jon Isaacs

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Posted 17 January 2013 - 03:35 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.

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

#8 oddog

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Posted 22 January 2013 - 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

#9 dave brock

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Posted 23 January 2013 - 02:08 AM

...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

#10 oddog

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Posted 31 January 2013 - 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






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