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Ultralight Schmidt Cassegrain Project

ATM Cassegrain Celestron SCT
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#1 tturtle

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Posted 21 September 2021 - 09:52 AM

I will be embarking on a project in the next several months where I will take my Celestron 9.25 Schmidt Cassegrain OTA and disassemble it down to every nut and bolt and rebuild it from the ground up to reduce dead weight.  This is a continuation of a project that I started last year to convert this scope and mount into something that is much more suited to a quick, lightweight rig that can be set up with limited time and fuss and primarily visual.  The remounting is done (see image) and is a joy to use. Smooth, stable, lightweight, and vibration free.  Now I am ready to take a figurative chain saw to the OTA and, based on my background in structural engineering, I see a lot of opportunity for a lighter weight redesign . The table below shows the original OTA breakdown with total weight of 20.6 lbs, and my proposed weight of 10 lbs.  I have kicked around a lot of concepts up to now which include carbon fiber, aluminum, and wood, and have pretty much settled on wood for the bulk of the project.  This may come as a surprise to people so I will explain the design challenges of this particular project and why I selected this material.  Let me also add that I do not intend (at this point) to change any of the optics of this scope which is to say that the two main elements, primary mirror and corrector with secondary will be positioned in my redesign in precisely the same way they are positioned now. I expect the redesign to only require collimation to re-establish the original optical performance. This project will take many months to complete so I will follow up with periodic updates on the design and construction.

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#2 chanrobi

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Posted 21 September 2021 - 10:01 AM

Dude you can just do what the bike industry did.

 

Drill a bunch of holes everywhere



#3 Jon Isaacs

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Posted 21 September 2021 - 10:20 AM

Interesting.

 

One thing: Pyrex is one of the lightest glasses and has a density of 2.23gm/cm3 = 139 lb/ft3

 

Are you planning on using drumshell for the tube? 

 

Jon


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#4 RalphMeisterTigerMan

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Posted 21 September 2021 - 10:35 AM

Interesting project. Having been the owner of a Meade 2120 10" LX5 SCT telescope I can understand your reasoning for doing this project. The worst part of setting up the old 10" was having to lift the entire OTA with fork-equatorial onto the wedge and then installing the 3 hex bolts.

I don't care what anyone says but the SCT OTA's can be very heavy. I wonder if anyone ever lost control of holding onto one while trying to attach it to the wedge. I would hate to hear the sound of that.

 

Pl;ease keep us updated on your project. Thank-you.

 

Clear skies and keep looking up!

RalphMeisterTigerMan



#5 tturtle

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Posted 21 September 2021 - 10:46 AM

Interesting.

 

One thing: Pyrex is one of the lightest glasses and has a density of 2.23gm/cm3 = 139 lb/ft3

 

Are you planning on using drumshell for the tube? 

 

Jon

I am indeed looking at drum shell. This looks like an ideal material.


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#6 tturtle

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Posted 21 September 2021 - 04:13 PM

Interesting.

 

One thing: Pyrex is one of the lightest glasses and has a density of 2.23gm/cm3 = 139 lb/ft3

 

Are you planning on using drumshell for the tube? 

 

Jon

Yeah thanks for catching that. I guessed at the 100pcf number so I will revise the table. Are you saying that this mirror is the Pyrex at 139?



#7 PrestonE

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Posted 21 September 2021 - 07:09 PM

"I am indeed looking at drum shell. This looks like an ideal material."

 

I helped Remo Drums develop their Drum Shells and they are some

of the strongest and lightest ever made...

 

If you can get one of those, that would be the way to go...and so many sizes...

 

Best Regards,

 

Preston


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#8 MitchAlsup

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Posted 21 September 2021 - 07:55 PM

"I am indeed looking at drum shell. This looks like an ideal material."

 

I helped Remo Drums develop their Drum Shells and they are some

of the strongest and lightest ever made...

I went 5 pages into google "remo drum shells" and could find anyone selling raw unfinished shells.

Do you have a link ?

Do they make big shells (like 36") ?



#9 tturtle

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Posted 21 September 2021 - 08:08 PM

I think there are quite a few of these companies that make drum shells. This is one I found that looks like it can provide me with a 10” x 18” maple tube for a little over $100.
 

 

https://precisiondrum.com/


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#10 PrestonE

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Posted 22 September 2021 - 08:00 AM

"I went 5 pages into google "remo drum shells" and could find anyone selling raw unfinished shells.

Do you have a link ?

Do they make big shells (like 36") ?"

 

Mitch, I first developed the adhesive that they use to bond the Drum Head into the Aluminum Extrusion...

Sold them 40,000 pounds a month...hard to believe that anyone would make that many drum heads...as

it only took about an ounce per head.

 

Then worked on the Drum Shell process with Remo...that, was over 25 years ago and I am not sure

he is even alive.  Remo was one of the most interesting men that I ever met...he designed most all of 

the automated machines to make the drum parts himself...incredible experience working with him!!!

 

They ended up making their own Drum Shells as No ONE would make them the the specifications that

Remo Wanted...so he made his own...

 

Let me contact Customer Service and see if any of the old group still remembers me and I will ask about

Drum Shells unfinished for Telescope applications.

 

Best Regards,

 

Preston


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#11 tturtle

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Posted 22 September 2021 - 12:33 PM

A big part of why I started this project is that I expect to learn and discover a bunch of new things that can be useful outside of what I am doing with this particular scope.  Drumshells have been one of these discoveries.  I’ve gotten a lot of comments bringing up this project on other forums and a common response is that if it’s currently made of aluminum (as is the main mirror casting) then it must already be as lightweight as it can get. Compare the unit weights of drumshell plywood (~40pcf) vs aluminum (~170pcf) and the fact that there thicknesses are very similar. For a telescope tube of almost any reasonable diameter, I doubt there is a better material available than the drumshell.  Not Sonotube, not CFRP, not aluminum. And don’t forget you can easily glue sections together to get any length you want. Am I missing something?



#12 PrestonE

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Posted 22 September 2021 - 01:32 PM

Hola Tom, glued sections to make a longer piece will be weak at the joint....you would have to

do an over and under layment to achieve similar strengths to a solid single piece.

 

You could make a Wet Layup CFR part much stiffer and lighter than the Drum

Shell if you wanted to, though likely more expensive.

 

Regards,

 

Preston



#13 tturtle

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Posted 22 September 2021 - 02:00 PM

Hola Tom, glued sections to make a longer piece will be weak at the joint....you would have to

do an over and under layment to achieve similar strengths to a solid single piece.

 

You could make a Wet Layup CFR part much stiffer and lighter than the Drum

Shell if you wanted to, though likely more expensive.

 

Regards,

 

Preston

I respect your experience with this but I think your completely wrong. Modern wood glues are incredibly strong - as much as the actual fibers and this is well documented.  As far as weight savings it’s not really worth It that I can see. Another CNer has started a refractor build with a CFRP tube and it sounds like a nightmare. 



#14 PrestonE

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Posted 22 September 2021 - 02:43 PM

I respect your experience with this but I think your completely wrong. Modern wood glues are incredibly strong - as much as the actual fibers and this is well documented.  As far as weight savings it’s not really worth It that I can see. Another CNer has started a refractor build with a CFRP tube and it sounds like a nightmare. 

My background is a Polymer Research Chemist specializing in Epoxy and Polyurea bonding...

 

I think that I know what I am talking about.

 

There are Not Any Wood glues as strong as the fiber portion of the Drum Shells..

 

I designed the system that Remo Drums uses for their Drum Shells and it was 200% stronger than any

other Paper Wrapped system when it was designed...thus the reason that Remo started making their

own Drum Shells and not purchasing what was available on the open market.

 

I have documented making CFRP parts here before anyone else on CN...it is relatively easy if

you practice what your are doing or do it as I have shown.

 

Good luck with your project.

 

Preston


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#15 tturtle

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Posted 22 September 2021 - 02:45 PM

I called precision drum at https://precisiondrum.com/ very nice lady told me what they have in stock 0.12 thickness maple 5 ply 12” x 24” for $123 plus shipping. This will be so cool. When I finish the scope I will call it Ringo.

-tom



#16 tturtle

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Posted 22 September 2021 - 03:46 PM

My background is a Polymer Research Chemist specializing in Epoxy and Polyurea bonding...

 

I think that I know what I am talking about.

 

There are Not Any Wood glues as strong as the fiber portion of the Drum Shells..

 

I designed the system that Remo Drums uses for their Drum Shells and it was 200% stronger than any

other Paper Wrapped system when it was designed...thus the reason that Remo started making their

own Drum Shells and not purchasing what was available on the open market.

 

I have documented making CFRP parts here before anyone else on CN...it is relatively easy if

you practice what your are doing or do it as I have shown.

 

Good luck with your project.

 

Preston

I really appreciate your input on this project and I hope that you will continue to follow and contribute to the development of the design.  I also approach it with significant experience that is closely related which is to say that I am a practicing licensed structural engineer with 25 years of bridge design experience with timber, steel, concrete, and composite materials.  I currently have 2 active bridge repair projects in South Fla that are utilizing CFRP.  At any rate I don't need to join the shells as Precision Drum can provide a tube that is long enough.  


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#17 Don H

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Posted 22 September 2021 - 04:24 PM

AIT makes very nice drum shells, as well as longer pedestals and columns. I used a 10" Maple ply drum shell for the upper cage of my 8" travel scope. I really like the way it works. They even cut it into two pieces for me for free (6" and 3" tall). 

 

http://www.aitwood.com/default.asp


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#18 tturtle

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Posted 22 September 2021 - 04:37 PM

AIT makes very nice drum shells, as well as longer pedestals and columns. I used a 10" Maple ply drum shell for the upper cage of my 8" travel scope. I really like the way it works. They even cut it into two pieces for me for free (6" and 3" tall). 

 

http://www.aitwood.com/default.asp

Im like a kid in a candy store with these things. Probably order the one from precision drums today just because the lady was so nice but thanks for the recommendation. I have done a lot of woodworking so I don’t have a problem with cutting it on a table saw. What kind of finish did you wind up with?



#19 tturtle

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Posted 22 September 2021 - 05:01 PM

AIT makes very nice drum shells, as well as longer pedestals and columns. I used a 10" Maple ply drum shell for the upper cage of my 8" travel scope. I really like the way it works. They even cut it into two pieces for me for free (6" and 3" tall). 

 

http://www.aitwood.com/default.asp

I looked at their website and they list an 11” x 24” diameter birch which is great. The diameter is taken from the outside and is actually 1/4” smaller than listed so the 10” for me would be too tight. I thought the next step up was 12” so them having 11” is perfect.   



#20 Don H

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Posted 22 September 2021 - 05:08 PM

Im like a kid in a candy store with these things. Probably order the one from precision drums today just because the lady was so nice but thanks for the recommendation. I have done a lot of woodworking so I don’t have a problem with cutting it on a table saw. What kind of finish did you wind up with?

I forget exactly which finish I used, but I was very happy with the end result. It was 8 plies, but only about 3/16" thick. The shell is very strong, yet quite lightweight. Here is a close up and a link to the scope in my old website, with some other pictures and scopes...

 

Edit: I think the finish was General Finishes 450 Satin...

 

 https://donscopes.webs.com/travel8.htm

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  • 8cage.png

Edited by Don H, 22 September 2021 - 05:10 PM.

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