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What thickness blank will you choose for a 20" blank?

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#1 cnchillstar

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Posted 14 October 2021 - 03:13 AM

Hi,I have opportunity to buy a 20" mirror blank.One is 45mm thick and the other is 50mm thick.Matieral and anneal is same.  

Which one you will take and why?



#2 Stathis_Firstlight

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Posted 14 October 2021 - 05:08 AM

I would say, approx. 35 mm thickness be an optimum compromise between stiffness, weight and cool down time for a portable Alt/Az mounted 20" mirror. Of course the backside should be ground flat and the rules of large thin mirror making should be applied. ...And of course the mirror cell should be adequate.

 

Only for a stationally use on a really strong parallactic mount I would preffer 50 mm thickness.

 

What kind of telescope do you plan to make?


Edited by Stathis_Firstlight, 14 October 2021 - 05:10 AM.


#3 sixela

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Posted 14 October 2021 - 08:34 AM

I find even the 45mm a bit on the thick side (and tricky to get into thermal equilibrium with its surroundings.) Unless the prospect of an 18 point cell seems daunting you, I'd always pick the thinner one in this case.


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#4 MitchAlsup

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Posted 14 October 2021 - 12:56 PM

35mm was my first guess.



#5 danmdak

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Posted 14 October 2021 - 01:46 PM

Why is 35mm suggested when he only has access to 45 or 50mm?  And to the OP, where did you find this blank? I am thinking of grinding my very last telescope, probably an 18" f/4 but may decide upon a 20".  I cannot seem to find the blanks anywhere.



#6 MitchAlsup

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Posted 15 October 2021 - 02:46 PM

Why is 35mm suggested when he only has access to 45 or 50mm?  

35mm has been shown to be "thick" enough to prevent issues of figuring that thinner blanks have;

While being thin enough to cool rapidly and perform better "all night long" than thicker mirrors.

 

20 years ago the right answer would have been 45-50mm, the state of the ATM-art has advanced over that time period.



#7 Stu Todd

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Posted 15 October 2021 - 06:11 PM

Why is 35mm suggested when he only has access to 45 or 50mm?  And to the OP, where did you find this blank? I am thinking of grinding my very last telescope, probably an 18" f/4 but may decide upon a 20".  I cannot seem to find the blanks anywhere.

http://www.bvctek.ca...ror-blanks.html



#8 danmdak

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Posted Yesterday, 07:15 PM

35mm has been shown to be "thick" enough to prevent issues of figuring that thinner blanks have;

While being thin enough to cool rapidly and perform better "all night long" than thicker mirrors.

 

20 years ago the right answer would have been 45-50mm, the state of the ATM-art has advanced over that time period.

Thank you but you missed the point. He ONLY has access to 45mm or 50mm, so the fact that 35mm is optimal is a moot point as it is not one of his options.


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