[Lvas] Telescope making

Wed Jul 22 19:45:00 PDT 2009

Gary,

Just a couple of things. Sounds like you already have the answers you were looking for. Hope it all works out.

Second, DON'T flock your tube! Trust me on this. I worked at a rubber plant that flocked extrusion and I know what is involved and what will eventually happen to it. Stay away from that deal. Flat black paint works just fine.

As for PVC, I had an 8" f/9.44 planetary scope with a 10" PVC pipe, untreated except for flat black on the interior. Kept it outside in southwest Oklahoma for nine years and never had a problem with it. Didn't warp, didn't get brittle, just the outside got a bit of a rough texture after about seven years. I ended up taking the optics out of it and throwing the tube away because it wouldn't fit in the rental van. By then, I was using my 16" anyway.

Like I say, cheap and easy.

I've only vaguely heard of Hubble optics. If they have a good rep, you should end up with an excellent mirror. A star test once you get it mounted will tell.

Last of all, as I said, a Newtonian needs to breathe, so keep that in mind.

Good luck with your new scope!

Fred

From: voliton at gmail.com
To: lvas at lvlug.org
Date: Tue, 21 Jul 2009 22:13:35 -0700
Subject: Re: [Lvas] Telescope making

Hi Fred,

Thank's for the answer.

Eureka! Just located the right size mirror cell at Astro Parts. to fit the Protostar tube. Damn things are hard to find. Looks like the fully setup OTA will weigh in at just over 23 lbs so the mount shouldn't be a problem.

I thought about different tubes. Sonotube - degrades fairly quickly, is vulnerable to moisture and is relatively heavy, PVC - very heavy, dries out and becomes brittle, will sag and warp under its own weight within weeks Aluminum - cheap but heavier, painting the exterior changes emmisivity characteristics - it would look like a big tin (aluminum) can - needs alot of work finishing ie. painting flocking etc. The protostar, acording to the manufacturer, is very rigid, has a 99+% light absorbing to 700nm flocking, extremely low emmisivity, is easy to cut , drill etc. and a 48 " tube is only 7 lbs! 

I plan on using caps of the same material on both ends ( removeable ) One cut out for the Baader Turbo film. It will be interesting to see if there is any light absorption by the film, they say not. Worst that can happen is I wind up with a see-through dust cap.

I've done enough gemsone polishing over the years to know that it is more of an art - the science is in the testing. I made a 1/4 wave many years ago, but a 1/20 wave here in the land of wind and dust ... I don't think so. Besides the kit costs more that a finished primary and secondary put together! The mirrors are from Hubble Optics f/4.72  I've seen the test report and found all good reviews on several ATM forums. Hopefully I won't get burned.

I mentioned a design program NEWT 2.5. Very easy to use It was written in the early 90s for Windows 3.1 . Plug in the appropriate numbers and it calculated all the proper stats and measurements by ray tracing. Found another newer program last night and came up with the same results, happy days! Anyway, it will give me a starting point for cutting the focuser holes and spider. Then I can move the mirror in to optomize for ccd camera operation with out running out of back travel. I can always add extensions for eyepiece use. It also showed that I can reduce the secondary from 2.5 MA to 2" with full illumination giving me only a 19% light obstruction.

That's the theory anyway. Anyone see any holes feel free to poke.

Regards,
Gary   
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