tube rotation methods?

[garymck]

Hi,
I recently purchased a GSO 8" f5 Newt for ccd imaging purposes with a QHY8 from Bintel. The unit has nice optics, a 10:1 focuser, rear fan, and 63mm (2.5") secondary giving a very nice fully illuminated field but, and a very big BUT, the tube sucks.

Specifically any rotation of the tube in it's rings causes collimation to go out substantially (eg a 100 deg rotation moves a laser spot up to 15 mm from the center - it was worse than this but I was able to partially fix the tube. It seems that the tube should have been 132 mm in diameter, but is actually 135mm. This causes warping of the tube when the end rings/cell are screwed down tight. In addition the tube rings are made for the 132mm diameter, so tightening them down squeezes and distorts the tube slightly.

I'm therefore going to make a new tube. I have a Sono tube from a previous 8" f6 dob that I made that is black velvet lined that I will cut down for the optics/hardware.

What I would like is suggestions for tube mounting/rotation systems, something more sophisticated than Wilcox rings, that provides smooth rotation and minimal image shift. I have woodworking tools available (router etc) and may be able to get a friend to do some metal lathe work.

Ideas, links,photos's needed..

cheers
Gary

[Ian Robinson]

You pay for what you get I'm affraid , maybe you should invest in a better mirror cell , better diagonal holder and better spider.
Maybe ditch the tube and get something more rigid. (what's it made off ?)

[pelu]

Your tube is this?

http://www.telescope-service.com/newtonianspics/start/GSN2001Astro5.jpg

Perhaps you can use a rings from SkyWatcher or rings for PVC pipe, you can find it in your local hardware. If the interior of the rings are 5 to 10mm bigger that the tube will be great, You can glue some cork bands.

Luck!

[OneOfOne]

Have a look at the Antares rings and see if you can get any ideas:

http://www.antaresoptical.com/new.htm

I was looking at a pair of these a couple of years ago, but they had not made it to production at that time. Now I have an SCT. Scroll down a bit on this page to see the rings. Oh, and check out the F10/F15 refractor...looks like a bazooka!

I believe Paramount also make rings, but I wasn't able to find them on the web. The look very similar to the Antares.

Maybe these will give you some ideas, especially if you have access to a lathe and some other tools.

[Garyh]

Hi Gary,
I have similar parts floating around here from a old 8" f/6 OTA which I will use for my f/4.7 project and I hope I don`t get similar problems! Though the mirror cell and tube end ring seems to fit on without pinching the tube.
If the tube is that easy to distort maybe you could get some baffles spot welded inside there where the rings would fit on the outside. Should strengthen the OTA up heaps. Also maybe make up a cradle and strap the OTA down?
I never rotate my OTA at all as being mainly a imaging scope I find no need to. Just get it in the best position for balance and leave it there.

[garymck]

Hi Guys,
thanks for the suggestions. I've decided to initially try wrapping some 1mm aluminium strip around the mirror cell and end ring to increase their diameter to match the id of the tube. This should hopefully make the tube rounder.

Seems also that this problem is a known issue with GS scopes, and to a lesser extent with Skywatcher scopes. Have come across similar reports of tube problems from quite a few sources.....

cheers
Gary