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Cutting secondary holder

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#1 david darksky

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Posted 14 August 2020 - 02:34 AM

I am sure this should be realy obvious. I have a 4 inch wide 6 inch long secondary mirror for a 20 inch primary. I want to make the secondary holder out of aluminium and then stick a carbon fibre sheet to it and the secondary to that. I am going to cut a round disc of alumium and then cut one end at 45 degrees. What diameter should I cut the disc to? 6 inches? Should I make the secondary holder a bit smaller than the secondary? I have a six inch hole saw I could use to cut the disc, so I will probably use this if it is the right size.

 

David



#2 TOMDEY

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Posted 14 August 2020 - 02:58 AM

Hi, David!

 

A sketch would help. Your description... I can't visualize what you intend to do. Why go from aluminum plate to carbon fiber to mirror, and how do you intend to attach them together?  All three materials have different coefficients of thermal expansion. If you bond them together, you have created a trimaterial thermometer that will pucker as the temperature changes. That will induce temperature-dependent power and astigmatism into the otherwise flat fold mirror.  Tom


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#3 david darksky

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Posted 14 August 2020 - 04:39 AM

It is a plate glass secondary. I read that it is a bad idea to attach this to aluminium as they have different thermal expansions, whereas carbon fibre has similar thermal expansion. Telscope Service in Gemrany sell rahter expensive secondary holders and spiders, I am trying to do something similar but with the mirror attached to carbon fibre rather than alumium.



#4 david darksky

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Posted 14 August 2020 - 05:12 AM

Here is a link to the secondary holder I want to base mine on.

 

https://www.teleskop...m-diameter.html



#5 Pinbout

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Posted 14 August 2020 - 05:37 AM

If your gluing yes you can make it smaller

 

no one has ever came out with a standard radius for support points on secondary mirrors.

 

for primaries with 3 supports - they lie on the 30% radius of the mirror. Can that translate to 2ndry’s? Don’t know.

 

i have copied some cellular 2ndry’s that have glued inserts and used that to locate supports. Never measured it though.

 

I’d just draw the ellipse for a template and jigsaw / bandsaw it


Edited by Pinbout, 14 August 2020 - 05:39 AM.


#6 david darksky

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Posted 14 August 2020 - 06:33 AM

I attach a picture of the secondary holder I am basing mine on. I am useless at maths/ geometry and picturing shapes in my head!

If the part of the holder at 45 degrees needs to be an elipse of width 4 inches and length 6 inches what diameter does the original round part need to be before I cut it?

 

I have an old secondary holder where the round part is 3 inches in diameter, the elipse is 3 inches wide and 4 inches long. So it seems that I should cut and aluminium tube of 4 inches in diameter and cut one end off at 45 degrees and this should give me the correct size?Attached File  secondary-picture.odt   16.57KB   11 downloads



#7 Oberon

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Posted 14 August 2020 - 07:11 AM

Yes, cut a 4” round tube at 45 degrees. You’ll need quite a serious saw to do it though.


Edited by Oberon, 14 August 2020 - 07:23 AM.

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#8 Pinbout

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Posted 14 August 2020 - 07:40 AM

here, I sketched something up on 8.5x11 sheet.

 

I put the support points on a .6 rad... similar to the cellular 2ndry's I've seen.

 

Attached File  6in 2ndry-Model.pdf   73.57KB   12 downloads


Edited by Pinbout, 14 August 2020 - 07:41 AM.



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