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Defraction rings aren't perfectly round

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#1 Guest_**DONOTDELETE**_*

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Posted 05 September 2005 - 01:10 PM

I have a C9.25 and noticed the other night that my defraction rings when out of focuse aren't quite round. There's a little "bump". Below is some images I captured of this. I also included a focused star. Is this normal or do I have a problem here?

Jeff

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  • 587552-Small-Collimation-Issues.jpg


#2 trainsktg

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Posted 05 September 2005 - 01:26 PM

Jeff,

I have a small classic refractor that shows a similar losenge-shaped pattern...is it possible that there is somthing in the optical train that isn't quite parallel with the light path (like the front lens/ meniscus) that is causing this pinching effect? Mine gives really good images though, so I haven't thought much of it, but since your scope's value is so much more, I can see where this could be a concern. I hope someone out there can help better'n me...good luck!

Keith

#3 Don W

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Posted 05 September 2005 - 02:45 PM

Looks like the system had not completely cooled down.

#4 Don W

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Posted 05 September 2005 - 04:37 PM

Herman, I sent you a private message about your oversized images. Please read it.

#5 Starman1

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Posted 05 September 2005 - 06:08 PM

Other than cooling down, I've seen this sort of image in an SCT on which the screws holding the corrector plate compression ring (the thin ring around the front that holds the corrector in place) were way too tight.
Though I agree with Don W. that this is likley inadequate cooling (one should let the scope cool for several hours before doing a star test), it doesn't hurt to loosen all the screws on the corrector plate ring (point the telescope up first) and re-tighten them *just* snug. Very little pressure is required to hold the corrector in place.
As for cooling, consider a Lymax SCT Cooler if you want perfectly cooled optics just after it gets dark. Otherwise, wait until 2 am or so before doing this test.

#6 Guest_**DONOTDELETE**_*

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Posted 06 September 2005 - 01:08 AM

Thanks for the replies. Sorry about that huge image. I goofed on that one. I assume a moderator will take it off soon. Below is a more manageable image... "for the record". :-)

I'm pretty certain this is not due to thermal issues. I see this exact irregular pattern all night no matter what the conditions, including at 4am several nights ago after imaging all night! :-)

So the corrector-plate being too tight might be a good theory here.

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  • 588221-C9.25-Optics-Problem-small.jpg


#7 Jeff Young

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Posted 06 September 2005 - 06:29 AM

Jeff --

Keep us posted. I'm curious to know what the conclusion is....

-- (another) Jeff.

#8 Psa19one

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Posted 06 September 2005 - 08:40 PM

Don,

Wouldn't the corrector screws really have to be super tight to cause that kind of warp? It just seems like glass that thick would require quite the pinch to distort its function. I've also heard the corrector does relatively little "correction" anyway. I'm not saying all this to say you're wrong. Rather, I'm looking to be educated on this stuff! :-) Please let me know if these things I've run across in other posts is simply erroneous. Thanks!

#9 Don W

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Posted 06 September 2005 - 08:45 PM

Huh? I never said anything about corrector screws.

#10 Psa19one

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Posted 06 September 2005 - 09:09 PM

No...the other Don (Starman) :foreheadslap: Sorry...

#11 Starman1

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Posted 06 September 2005 - 09:24 PM

Well, the aberration isn't extreme, either. A too-tight screw on one side of the corrector (or all of them) could warp the corrector, which is quite thin. The reason I mentioned it is that I've seen aberrations like this that were solved by loosening the screws.
After my post, I thought of a couple more likely candidates to produce this:
--astigmatism in the primary mirror
--an out-of-round secondary baffle
--a bad star diagonal mirror
--a distorted secondary caused by too-tight collimation screws.
--astigmatism in the secondary mirror
All can be easily checked except the primary mirror.
A slight rotation of the corrector can find out whether the problem is in the secondary/baffle/corrector. Ditto for the star diagonal. Just be sure to return the corrector to the proper orientation after your test. The is likely to be a mark along the edge of the corrector to do this. If not, use a little white-out to mark the corrector and cell so you can return them to the same orientation later.

#12 Psa19one

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Posted 06 September 2005 - 11:06 PM

I see. Thanks for the info, Don P. I've learned alot from things you've posted!


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