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Cracked casting on primary

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#1 swizzlr

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Posted 18 April 2021 - 09:11 PM

Cracked is an understatement. I was installing an Ez Focus in a vintage LX200 12” and saw the damage through the hole for the focuser knob. I’ll try to get the back off sometime to get some pictures, but I can see some definite chipping on one edge where something nasty happened. I’m at least the third owner, and apart from one moderately gentle excursion from my hands into soil lens cap first, I can’t think of a cause. Could well predate me, or even the prior owner. Because...

 

the strange thing is, it works just fine! The mirror looks perfect, with no distortions, and stars are basically perfect.

 

I suppose it’s only a matter of time before the cracks get worse (I keep it outside, which prior owners didn’t). Perhaps there’s a way to mitigate it, windshield repair glue or something.

 

anyway, just thought I’d share this delightful stroke of luck/oddity, and ask if anybody else has encountered something similar.?



#2 maroubra_boy

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Posted 19 April 2021 - 06:36 AM

Hello swizzlr,

 

I've read and re-read your post, and I am struggling to figure out exactly what it is you are talking about.  Those pictures will certainly help.

 

For a crack to continue propagating there must be a force that keeps pushing through.

 

Are you talking about the corrector plate?  I just cannot think about anywhere else a crack could appear.

 

If this is the case, then the solution is a very easy one - remove the corrector plate retention ring!

 

If the corrector is cracking this is because the screws that hold the retention ring screws are wound in WAY TOO TIGHT.  These screws need to be ONLY finger tight, not wrist crushing tight.  Remember, these screws are pushing on to glass.

 

Another thing about these screws being wound in too tight is this will be inducing a strain into the glass which will be distorting the corrector plate.  So while you may think that the image the scope is producing is good, with the corrector plate being distorted you really have no idea just how much better the image CAN be!

 

This exact same problem occurred to a Meade SCT I bought new.  I struggled and struggled to get the image to play nice.  Collimation "appeared" to be good, but the image was still poor, refusing to go over 175X before the image went to crap.  Out of desperation one day I decided to check the secondary mirror.  To my surprise I had to really lean into the screwdriver to remove those retention ring screws! shocked.gif  .  And when I lifted the retention ring itself, BLOODY HELL!  There were two clamshell chips around where two of the screws pushed on the glass!!! These chips were hidden by the retention ring.  I was really cross about this especially as I struggled with this scope for two blooming years, only to find the blasted thing was defective from the blooming factory, and at this stage could not make a claim on warranty.  That this was a factory fault was evident to me because the chipped glass had been removed, and the retention ring replaced.  When I reset the retention ring with the correct pressure on these screws, guess what?  The image finally cleared up because the corrector plate was now no longer under strain.

 

This problem of over tight retention ring screws can affect not only SCT's but Maks as well.  There will be literally thousands of instruments out there with overtightened screws distorting corrector plates, and the scopes owners are unware of this through no fault of their own, only through ignorance about not knowing not to over tighten these screws, or like me, the factory itself is to blame.  I recently also fixed a mate's brand new C8 who had sought to do the right thing by cleaning the corrector plate but over tightened these screws when he replaced the retention ring.  We were looking to improve the collimation that night when I noticed something odd about the image of the defocused star.  So I asked him if he had done anything to the corrector plate when he told me he had and what he had done.  So we backed off these screws, the corrector was freed from the strain, and the image improve straight away without tweaking the collimation screws.  Now his C8 is humming nicely.

 

As for keeping your scope outside, how exactly are you doing this?  I would rather know first how you are looking after the scope with it being "outside" before saying anything more here.  There are just too many variables.

 

Alex.


Edited by maroubra_boy, 19 April 2021 - 06:40 AM.


#3 ETX2112

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Posted 19 April 2021 - 09:19 AM

You say it's the primary mirror? You see the problem through the focus hole...perhaps it's just appears to be damaged but is really just the aluminized coating on the back of the mirror playing tricks on your eyes?

#4 Nippon

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Posted 19 April 2021 - 01:35 PM

The back side of an SCT primary can be a rough looking thing. They certainly do nothing to make them nice in appearance since you can't see it in an assembled telescope.  



#5 Joe Bergeron

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Posted 19 April 2021 - 05:27 PM

The word “casting” leads me to suspect this is not related to the optics. 



#6 Nippon

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Posted 19 April 2021 - 05:39 PM

I would agree that "casting" is not a term I associate with an optical element. Hopefully the OP will clarify.



#7 alphatripleplus

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Posted 20 April 2021 - 11:22 AM

As mentioned in an earlier post, a picture or two would help understand what happened.



#8 RalphMeisterTigerMan

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Posted 20 April 2021 - 12:42 PM

This is a problem of glass manufacturers and has happened numerous times. From what I understand, fine annealing is the process of giving the poured glass a lot of time to cool. I believe the 200-Hale telescope was given something like 6 - 8 months to cool, despite being ingenously honeycombed on the back.

 

If you want to talk about a cracked blank ask the guys who cast the 236-inch 5-foot thick mirror for the "biggest Telescope" built on a high peak in the Caucasus mountains in Russia. From what I understand the first 5 castings cracked before a viable blank was produced. The telescope was finished in the mid-1960's and so far has never actually been usuable. You do the math, a 236-inch 5-foot thick monolithic piece of glass, how long would it take to cool enough so it could be used! The answer - never!

 

It doesn't sound like your mirror is cracked, that could be easily seen! Good luck!

 

Clear skies and keep looking up!

RalphMeisterTigerMan



#9 maroubra_boy

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Posted 20 April 2021 - 06:09 PM

Yeah, I did read "casting" in the OP.  But the rest of the information provided is vague.  I took it as the glass.  I honestly cannot figure out what metal casting is being referred to.

 

Where are you swizzlr?




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