[Lvas] Telescope making

[Fred Rayworth]

Gary,

 

Just must've been the batch of plastic from the tubes you've seen. My 8" dob was saddled halfway along the tube, not at the bottom as you might expect. It was because of how tall the rocker box was, and the balance of the tube when I had the focuser on. In fact, the saddle was on it just a hair short of the middle of the tube's length.

 

It depends how much the resin deteriorates over time. You could be vacuuming out flock dust every month or none at all, at least for a few years.

 

The sealed tube systems only avoid the air current problems to some extent. Their lenses at the front end have their own temperature stability issues, though slight. Also, with the inside tube sealed, there is a temperature differential between the inside and outside air, that may or may not be a big issue. In humid environments, that could mean dew inside the tube or on the corrector lens. That is why Kendrick does such a big business with their dew shields.

 

It's not only breathing with a Newtonian. It's also cutting down on the surfaces the light has to either bounce off of or go through. The ad copy can tell you such and such transparency, like they do with Naglers, but that is not always what it's cracked up to be. The images will be brighter the fewer the mirrors or lenses that get into the light path.

 

Well, good luck with the scope!

 

Fred

 


From: voliton at gmail.com
To: lvas at lvlug.org
Date: Thu, 23 Jul 2009 19:42:24 -0700
Subject: Re: [Lvas] Telescope making




Thanks Fred.
 
The flocking is part of the manufacturingand not an option. I think it is embeded in the resin. I guess I can scrape it off if it becomes a problem. 
 
I gather your 8" was a Dob. It' s normal resting position is close to vertical and the weight is at the bottom. This may have contributed to your success. I saw alot of sway backed runs of 8" (horizontal) back in NYC. Out here I had a piece of 6" laying on the ground. It not only warped but charred and cracked in the sun! Anyway, weight was the main factor since this is for a GEM mount.
 
You have said several times that a Newt has to breath. I assume this is for mirror ambient temp.equilization? All of the Schmidt and other SCT variations have a closed tube How do they avoid this problem? Would having a tube with less thermal transmission help after the scope is at ambient?  What would happen if you sealed up the tube, ran it to a partial vacume and put a heater on the mirror to maintain constant temperature?
I'm heading out into the fringes now.
 
Regards,
Gary                 
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