paul mc c
member
Reged: 05/14/09
Loc: Belfast Ireland
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Secondary astigmatism
#5594048 - 12/29/12 10:09 AM
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I have noticed slightlt oval star images just inside and outside of focus,i have tried turning the primary to see if the ovals move and they dont.I hadnt noticed this before so i am wondering can flexture in the secondary cause this ie.when cold ect,maybe to do with the material that hold it,hope thet makes sence!
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dpwoos
Pooh-Bah
Reged: 10/18/06
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Re: Secondary astigmatism
[Re: paul mc c]
#5594063 - 12/29/12 10:16 AM
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Sure can. It is relatively easy to re-attach the seconday (properly, if it isn't) and if that doesn't fix the astigmatism then a new secondary is required.
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KerryR
Carpal Tunnel
Reged: 12/05/07
Loc: SW Michigan
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Re: Secondary astigmatism
[Re: dpwoos]
#5594381 - 12/29/12 01:12 PM
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I've seen this once. A new secondary fixed the issue.
If the secondary is the tube and batting version, ensure there's not too much batting pushing the mirror against the rim of the holder with too much force. Be sure the hook at the bottom of the tube, if that's what holds it in as opposed to a rim, is not pressing too hard against the mirror-- this is usually adjustable.
If the secondary is glued, you will have to cut it free and re-glue it-- 3 globs of silicone spaced off the holder a few mm with temporary nails or coins.
I've read that if the oval shape to the out of focus image is not detectable when there are 3-4 rings total in the out of focus star image, the astigmatism is unlikely to have visible impact on the in-focus image.
Still worth investigating a fix for the issue, though.
It's good that you already rotated the primary and know that it's not the culprit.
It's not astigmatism in your eye is it (rotate your head and see if it follows that rotation).
You're well collimated, right?
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pogobbler
sage
Reged: 09/30/08
Loc: Central Indiana, USA
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Re: Secondary astigmatism
[Re: KerryR]
#5595564 - 12/30/12 04:24 AM
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I had the exact same issue with an 8" Dob I purchased over the summer. I did the same as you, rotating the primary mirror to narrow the problem down to the secondary. I knew the problem wasn't in my own eyes since it was the only scope I've ever seen it on. In my case, I removed then remounted the secondary mirror in case that was inducing the astigmatism, but it remained, so I ended up replacing the secondary mirror and that did the trick. Turns out, judging by the detail I can see on Jupiter, that the primary mirror is pretty darn good. Definitely worth the trouble and expense of replacing the secondary.
Good luck with your scope. Hope you take care of the problem.
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Secondary astigmatism
[Re: paul mc c]
#5595654 - 12/30/12 07:54 AM
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Quote:
have noticed slightlt oval star images just inside and outside of focus,
did the oval shape turn 90° from inside to outside focus?
does it happen with short fl eyepiece?
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paul mc c
member
Reged: 05/14/09
Loc: Belfast Ireland
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Re: Secondary astigmatism
[Re: Pinbout]
#5595719 - 12/30/12 09:06 AM
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Quote:
Quote:
have noticed slightlt oval star images just inside and outside of focus,
did the oval shape turn 90° from inside to outside focus?
does it happen with short fl eyepiece?
Yes on both questions bud.
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paul mc c
member
Reged: 05/14/09
Loc: Belfast Ireland
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Re: Secondary astigmatism
[Re: paul mc c]
#5596545 - 12/30/12 04:50 PM
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Is there any test that i can do to tell if my secondary has astigmatism.
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dpwoos
Pooh-Bah
Reged: 10/18/06
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Re: Secondary astigmatism
[Re: paul mc c]
#5596598 - 12/30/12 05:22 PM
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If it isn't the primary and it isn't the eyepiece and it isn't your eye then it is the secondary. It should be simple to rule out these other possibilities. If it must be the secondary then re-mount (properly) and test again. If still there then I would buy a Protostar.
The folks at your local astro club can help you to resolve this lickety-split.
Edited by dpwoos (12/30/12 05:25 PM)
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Secondary astigmatism
[Re: paul mc c]
#5597249 - 12/31/12 12:44 AM
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you need a clear optical flat and a monochromatic light source.
this is what stig would look like, the s shape:
surplus shed has some inexpensive flat that are rated 1/10~.
but before you contemplate spending a little money. lay some artist or electrical tape across half of the focuser and focus it on a bright star, go thru inside to focus and outside to see if the knife edge [tape edge] clocks around. you can also see if your system nulls at focus.
you can also get a film can a drill out the cap and glue a ronchi grating to do a star test but again like the knife edge it only tests the system which would help verify your initial thoughts of stig.
these ronchi bands show a little stig cause one side starts to bow out more than the other side.
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: Secondary astigmatism
[Re: Pinbout]
#5597463 - 12/31/12 07:09 AM
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Keep in mind.... a flat secondary only has to have a bit of power to show astigmatism in a 45 degree orientation. It does -NOT- have to show the error as in Danny's pics above. I have a printout from this subject from Mike Jones. A secondary will show this 'stig fairly plainly with anything worse than 1/8-1/10th wave surface error of power. (meaning, it's a smooth long spherical surface- otherwise accurate with no trace of 'hook' at the edge)
Power (curvature in the bands) can be either positive or negative to cause this.
About 90% of secondaries sent to me which are suspected of being the cause of the problem have had power; but are otherwise smooth.
M.
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Secondary astigmatism
[Re: Mark Harry]
#5601386 - 01/02/13 02:51 PM
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a flat secondary only has to have a bit of power to show astigmatism in a 45 degree orientation.
ah...from vlad's site
Quote:
It is the difference in depth along the two radii that induces astigmatism, not the surface P-V error itself
http://www.telescope-optics.net/newtonian.htm
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paul mc c
member
Reged: 05/14/09
Loc: Belfast Ireland
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Re: Secondary astigmatism
[Re: Pinbout]
#5612472 - 01/08/13 06:33 PM
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After trying almost everything to sort this out,convinced it was my secondary that wasnt good,i took the mirror off the holder and remounted it with new silicon,collomated and waited on a clear sky, and would you believe it, sky clear as a bell tonight.Set the scope out to cool,then turned the scope on Jupiter and couldnt believe what i was seeing,perfect focus amazing detail even inside the red GRS!
The moons actually showed as tiny discs,so to celebrate just ordered my self a 11mm 82 E.S eyepiece.
Thanks for all the help and advice people.
Paul
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Jon Isaacs
Postmaster
Reged: 06/16/04
Loc: San Diego and Boulevard, CA
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Re: Secondary astigmatism
[Re: paul mc c]
#5613181 - 01/09/13 06:15 AM
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Quote:
After trying almost everything to sort this out,convinced it was my secondary that wasnt good,i took the mirror off the holder and remounted it with new silicon,collomated and waited on a clear sky, and would you believe it, sky clear as a bell tonight.Set the scope out to cool,then turned the scope on Jupiter and couldnt believe what i was seeing,perfect focus amazing detail even inside the red GRS!
The moons actually showed as tiny discs,so to celebrate just ordered my self a 11mm 82 E.S eyepiece.
Thanks for all the help and advice people.
Paul
 
Jon
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Vic Menard
Post Laureate
Reged: 07/21/04
Loc: Bradenton, FL
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Re: Secondary astigmatism
[Re: paul mc c]
#5613297 - 01/09/13 08:41 AM
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...i took the mirror off the holder and remounted it with new silicon...
Did you also attach a safety line between the secondary mirror and the spider?
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dpwoos
Pooh-Bah
Reged: 10/18/06
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Re: Secondary astigmatism
[Re: Vic Menard]
#5613408 - 01/09/13 09:42 AM
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You know, I have mounted a lot of secondaries (aquarium silicone) and have never had one detach. Also, so far as I know no one in my club has, either. However, I have sometimes wondered it it could happen. Is this a real possibility?
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Vic Menard
Post Laureate
Reged: 07/21/04
Loc: Bradenton, FL
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Re: Secondary astigmatism
[Re: dpwoos]
#5613861 - 01/09/13 02:54 PM
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It's never happened to me personally, but I've read, in these forums, where it has happened to others...
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KerryR
Carpal Tunnel
Reged: 12/05/07
Loc: SW Michigan
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Re: Secondary astigmatism
[Re: Vic Menard]
#5613903 - 01/09/13 03:11 PM
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Parks had some issues with AZ6 primaries, which were glued, coming loose in shipment. I was able to push mine out of it's cell with thumb pressure when I needed to re-glue it to cure some astigmatism.
I think this had more to do with the composite cells than the silione-- it stayed on the glass but pulled off the cells. Maybe due to mold releasing agents on the injection molded cell? Point is, it can happen.
But, I'd venture to say that if you glue on a secondary or primary, and attempt fairly hard to pull it off, and it stays put, it's probably fine. I was able to pull off the Parks primary, but I sure couldn't get my Meade secondary off it's holder. 'Course, I don't know what it's glues with for certain...
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ccaissie
sage
Reged: 09/13/10
Loc: Whitefield, Maine
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Re: Secondary astigmatism
[Re: paul mc c]
#5618591 - 01/12/13 08:30 AM
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Is there any test that i can do to tell if my secondary has astigmatism.
You can star test by using a simple refractor, and aiming it at 45 to the ground, lay the secondary face up on a flat board and observe a star image in it.
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