Can 12" GSO Dob alt bearing be re-positioned?

casstony · #1 25-05-2006, 02:30 PM

Is it possible to re-position the alt bearings on the optical tube such that the focuser points out perpendicular to the base (i.e. rotate the tube so the focuser is horizontal)? How are the bearings attached to the tube and how much work would be involved?

I'm thinking of buying a 12" dob but I need to be seated when observing. I think it would be much more comfortable with the focuser sticking straight out sideways.

Thanks,
Tony

gbeal · #2 25-05-2006, 03:51 PM

Tony,
anything is possible.
I use a cradle around my tube, and this allows a form of tube rotation, as well as longitudinal adjustment for differing balance. It does however require a slightly wider base. Post a picture of the tube and bearing area, and I might be able to make a better suggestion. There are plenty of clever folk in this forum so I am sure you will get an answer.
For the record I prefer viewing seated as well.
Gary

astro_nutt · #3 25-05-2006, 04:33 PM

Hi Tony,
After looking at my 10" Dob, I guess if I had to do it..I would first remove the primary mirror, cover it and put in a safe place. I would then unscrew the hubs, being careful to remove the nuts from inside, and place the optical tube on a bed and support the tube with pillows. Using a length of wide cash register paper, wrap a length around the tube next to the holes from the hubs. Mark the position of the holes alongside the paper. Rotate the paper to where the holes for the hubs are to re-positioned being careful to keep the paper aligned next to the original holes as a guide. When you have re-positioned for the holes, mark the tube with a felt pen. Before drilling each side, place some newspaper inside the tube opposite the drill site to catch any debris. Drill a pilot hole using a cordless drill on the slowest speed, then drill the hole to the correct size. After drilling each side, stand the tube on it's end to allow the drill dust out. After removing the newspaper, give the inside of the tube a vacuum. Re-fit the hubs and plug the other holes with a rubber gromet, or a suitable nut-and-bolt, but be sure to give it a coat of matt black on the inside! Test the tube in the mount, then re-install the primary mirror, collimate, and away you go!
As for me, I have made a cradle for my tube plus a new base which is motorized on both axes. This allows me to rotate the tube in the cradle. And I prefer to be seated also!!

casstony · #4 26-05-2006, 12:25 AM

Thanks guys. I wouldn't go for the cradle as I want to keep things as simple as possible. Sounds like shifting the bearings is fairly simple. I like the idea of using paper wrapped around the tube to keep things square. Don't know about using the bed as a workbench - might have to wait until the wife's at work?

Tony

vespine · #5 30-05-2006, 07:54 AM

I find the "not quite horizontal" position of the EP quite comfortable and I also like to sit during observing sessions. More important then the EP in my opinion is what you are sitting on, I have a stool and a garden chair, depending on how high I'm observing determines which I'll sit on, mostly on the stool. Because of the angle of the EP, I can lean down, rest my elbows on my knees and still comfortably look into the EP, don't know if that would be as comfortable if the EP was horizontal.

janoskiss (Steve H) · #6 30-05-2006, 08:33 AM

I was also thinking that focuser along the horizontal would be more accessible and easier to use sitting down near zenith on the 12" Dob. But I'll probably just look for a different seat instead of modifying the scope. I'm using the Stellar seat from Bintel, which puts you further away from the eyepiece the higher it goes.

casstony · #7 31-05-2006, 12:26 AM

Leaning forward to observe for anything more than a quick look aggravates an upper back problem I have, and I can't stand to observe due to a nasty little arbovirus that's made its home in my hip joints, so I have very specific requirements for comfortable observing: seated with straight back and neck. I was in Melbourne two saturdays ago and tried to visit Bintel to check out scopes, but got stuck in a burke road traffic snarl and gave up; I'm in the city again in a weeks time so I'll give it another shot.

Tony

famajohn · #8 19-06-2006, 08:42 PM

Here's one from the "crazy ideas" department. Just went out to the shed to check it out..... a 1.25" 45 deg refractor diagonal fits perfectly into the GSO dob focuser AND brings the ep straight out from the tube (horizontal) as required.

The only thing I am not able to test (raining) is whether the thing would come to focus, probably not, I suspect insufficient in-travel on the focuser to get the ep close enough to the secondary. Low profile focuser? Magic wand?

Probably a dumb idea anyway but I will have a play next fine night. The bearing mod sounds like a heck of a lot of work and a good way to compromise the resale value of a great scope.

Told you it was a crazy idea!!!!