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Triangle shaped stars in star test...

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#1 pdfermat

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Posted 08 October 2012 - 11:31 AM

I purchased a used Orion xt12i over the weekend. I was very happy with what I got for the price - the focuser had been upgraded to a JMI 2-speed, Orion padded case, RACI finder and Telrad, and the first owner (not the guy I bought it from - he was the 2nd owner) had even installed Bob's Knobs on the secondary! However, when I attempted to view with it last night, I was disappointed to see stars could not focus to anything other than a 3-vaned "triangle". I'm fairly certain that this is due to the fact that the primary is being pinched by the Orion rubber retaining clips being screwed down way too tight. I've got a Catseye hotspot on the way, and when it arrives, I'm going to take off the existing center doughnut and center the hotspot with my Catseye template. I'll clean the mirror using the "Obsession method" that has served me well, and re-intall the mirror making sure not to over-tighten the clips.

The only thing that worries me is this: the guy I bought the scope from said he had only used it a handful of times and said he had never cleaned the mirror, so I assume he has never had the mirror out of his cell. However, tightening down the clips way to tight is most definitely a newbie mistake. I would not expect the first owner (the guy who installed the JMI focuser (correctly and square), as well as the Bob's Knobs) would know better than to tighten down the mirror clips too much. Furthermore, these triangle shaped images make the scope almost unusable at anything over a mag. of 100x. So, I can't imagine the original owner used the scope as-is.

Hopefully, my worries will be laid to rest once I re-install the re-spotted mirror in its cell, and I'm able to test the scope for its "2nd light". I'm almost certain that the clips are the culprit, since the scope had adequately cooled, I'm fairly certain the secondary isn't being stressed, I had collimated the scope well (using a Glatter laser and TuBlug), and I was using quality eyepieces. Here's hoping...

#2 MitchAlsup

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Posted 08 October 2012 - 11:44 AM

Triangular stars are almost always a fault of the primary mirror clips. What you want is for the primary to be held in place, but free to rattle around a couple thousandths of an inch. This brevends the mount from binding and allows the mirror to have the shape it is supposed to have.

So when you reassemble, take out a business card and place it between the mirror and the clip. Then tighten down the clips, and remove the business card.

#3 nirvanix

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Posted 08 October 2012 - 12:59 PM

Hi Pat, believe it or not, the overtightened mirror clips are often set that way by the manufacturer. It happened to me. Either they don't know any better at the factory or they worry about shipping accidents with loose clips?

As mitch says, the clips don't really need to press down onto the mirror surface - now mine just barely touch it - enough to keep the mirror from rattling round. It made a big difference in the view. That and remounting the hard glued secondary with silicone dabs.

I'm sure your issue will be readily fixed.

#4 Jon Isaacs

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Posted 08 October 2012 - 02:33 PM

Hopefully, my worries will be laid to rest once I re-install the re-spotted mirror in its cell, and I'm able to test the scope for its "2nd light". I'm almost certain that the clips are the culprit, since the scope had adequately cooled, I'm fairly certain the secondary isn't being stressed, I had collimated the scope well (using a Glatter laser and TuBlug), and I was using quality eyepieces. Here's hoping...



That used car I once bought.... it had only been driven a few miles... :)

Just think... that guy probably did all those nice upgrades because he couldn't figure out why the stars were not so sharp... Now, with the turn of a few screws, you will be looking at some nice sharp stars...

:jump:

Jon

#5 pdfermat

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Posted 08 October 2012 - 02:48 PM


That used car I once bought.... it had only been driven a few miles... :)

Just think... that guy probably did all those nice upgrades because he couldn't figure out why the stars were not so sharp... Now, with the turn of a few screws, you will be looking at some nice sharp stars...

:jump:

Jon


I very much hope so - I really want this scope to work out. I had been using a "loner" xx12 for the past 2 years, and it just found its permanent home away from me. My 8" is a great scope, but I have definitely been spoiled by the 12". When this deal came up, I just had to jump at the opportunity. The thing that will really take the wind out of my sails is if I get the mirror back in the cell properly, and those stars are still triangles. But I'll cross that bridge if and when I come to it.

#6 stargazerfrank

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Posted 08 October 2012 - 03:01 PM

with the turn of a few screws, you will be looking at some nice sharp stars...

:jump:

Jon [/quote] I thought the same with a New Orion 8 f4.9 I took the mirror out to find the clips very lightly touching. The scope before and after was dead on collimation. Low power views of stars looked ok medium to high power stars looked like butterflies even in the not so sweet spot.

#7 Starman1

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Posted 08 October 2012 - 03:50 PM

[quote name="stargazerfrank"]with the turn of a few screws, you will be looking at some nice sharp stars...

:jump:

Jon [/quote] I thought the same with a New Orion 8 f4.9 I took the mirror out to find the clips very lightly touching. The scope before and after was dead on collimation. Low power views of stars looked ok medium to high power stars looked like butterflies even in the not so sweet spot. [/quote]

If the mirror is glued in, and the clips are tightened before the glue is dry, this can happen. The cure, then, is re-gluing the mirror and NOT tightening the clips on it.
The rest of your problems would have been due to the big 3 problems on most scopes:
--Coma (from the f/4.9 mirror)
--Astigmatism (from the eyepiece, likely)
--pinched secondary (usually yields astigmatism)

#8 johnnyha

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Posted 08 October 2012 - 04:25 PM

I wonder how many people have scratched their mirrors with business cards trying to get the "perfect" gap? In fact just back the clips completely off the mirror, say 1/8", and you will be fine. The clips are only there to keep the mirror from falling out.

#9 pdfermat

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Posted 08 October 2012 - 09:45 PM

Well, I pulled the cell after I got home tonight. The clips were screwed down TIGHT. So much so that the rubber was cracking on one of them. I loosened the clips and checked the center spot with my template (it was dead center). However, here's an interesting twist - I removed the mirror completely from the cell to install sturdier springs, and look what I saw in the middle of each of the 3 flotation cells:

Posted Image

That screw head in the middle ends up being the highest point of the flotation cell. So what should be 9 resting points for the mirror on the cork pads ends up being 3 resting points on the metal screw heads. That can't be good, can it??

Anyway, after a trip to Ace hardware with the mirror cell in tow (they said I won the prize for the most obscure piece brought in for the day), I came away with some flat-headed machine screws that sit well below the recess:

Posted Image

Now, all I need is the clouds to part for what will hopefully be a successful star test.

#10 PGW Steve

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Posted 09 October 2012 - 12:03 AM

I'm willing to bet the previous owners were passing along a sows ear until you got your hands on it. Congratulations looking forward to your updated first light with your new silk purse:-D

#11 Mirzam

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Posted 09 October 2012 - 08:43 AM

Nice work! I see a larger ATM project in your future.

JimC

#12 thetortoise

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Posted 09 October 2012 - 09:27 AM

I bought a used XT10G that had the same problem with triangle stars and planets. The clips were mashed down tight on the mirror. Loosening them improved the view greatly and I was getting the same contrast at 500X that I was previously getting at around 100X which was pretty amazing to say the least.

#13 GeneT

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Posted 09 October 2012 - 04:02 PM

You are on your way to a solution. Just stick with it and soon you will be enjoying your 'new' telescope.

#14 pdfermat

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Posted 09 October 2012 - 07:01 PM

Thanks all. Clouds and rain show no signs of letting up tonight. Fingers crossed for tomorrow!

#15 pdfermat

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Posted 10 October 2012 - 09:41 PM

A resounding success - I couldn't be happier!! I set up just before sunset, and after about an hour of cool down I took a peek. Sharp, tight stars! Defocused image gave nice concentric rings on both inside and outside focus. Clouds were rolling in so I only had a little time, but old friends looked as good as ever. M13 was resolved nicely to the core, the double double was split nicely with my 12t4, and I was even able to hunt down the comet in Pegasus (give it a look if you haven't yet - it's quite bright and shows a bit of a tail).

So, a huge sigh of relief. Thanks all for your advice and encouragement. Now I can start thinking about those no-pressure fun mods - hotspotting the mirror, milk jug washer on the secondary, flocking, firming up the base...it's all good!

#16 nirvanix

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Posted 11 October 2012 - 02:46 AM

A silk purse after all :lol:


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