Tightening side bolts on GSO dob

pneuman (Leigh) · #1 14-07-2008, 11:37 AM

Hi guys,

I've had my Bintel/GSO dob for a little over a month now, and I'm finding that the bolts that hold the circular side bits (ie: the altitude bearing bits that sit in the dob base) to the OTA are coming loose. I tried tightening them with a screwdriver, but on closer inspection it looks like there are nuts inside that I need to keep still while tightening it.

What's the best way to reach in to the telescope to hold the nuts still? The secondary mirror stops me reaching in from the top, but it'd be quite a stretch anyway, so it might be best to take the primary mirror cell off and reach up from there. Is that easy enough to do? Will I have to recollimate the scope afterwards?

peter_4059 (Peter) · #2 14-07-2008, 12:00 PM

I had the same problem and took the primary cell off. Taking the primary mirror off is easy but you will probably need to re-colimate after you put it back on. It is worth noting the orientation before you take it off so it goes back together in the same position.

Good Luck

Peter

erick (Eric) · #3 14-07-2008, 12:09 PM

So you cannot get your arm down past the secondary in the 8"? I'm sure I tightened them this way early on with my 8" and my arms arenn't particularly slender. Or maybe I did it when I had the spider and secondary out, for another reason.

If you cannot get your arm past, holding a suitable ring spanner, I suggest you remove the spider rather than removing the primary. Unless you think your primary needs a wash - then remove it instead and do two jobs.

If removing either, have a good look at the screws and spider/mirror cell positions so you can reinstall as close to the same orientation as you can. Note the "depth" of each of the four screws folding the secondary to the OTA wall and try to reproduce these on re-installation. If they are set correctly, they centre your secondary mirror accurately on the mechanical, and hopefully optical axis (if the primary is well centred in the tube).

Obviously work with the OTA horizontal to reduce any risk of dropping a tool onto the primary.

Yes, you would need to recollimate after removing either end. If it's a long way out, and it's not because the secondary mirror came loose with the handling, check all the installation and screws that nothing has gone wrong - If you put the mechanicals back together as they were, the collimation shouldn't be too far out.

pneuman (Leigh) · #4 14-07-2008, 12:47 PM

Quote:

Originally Posted by erick

So you cannot get your arm down past the secondary in the 8"? I'm sure I tightened them this way early on with my 8" and my arms arenn't particularly slender. Or maybe I did it when I had the spider and secondary out, for another reason.

If you cannot get your arm past, holding a suitable ring spanner, I suggest you remove the spider rather than removing the primary. Unless you think your primary needs a wash - then remove it instead and do two jobs.

Nah, I couldn't quite reach through with the secondary still in place, at least not without knocking it around a bit, which I figured would be just as bad as taking it out. I'll give that a go tonight -- hopefully it doesn't put my collimation out too far. Might be a good excuse to finally pick up a collimation tool and learn how it's all done!

Quote:

Originally Posted by erick

Obviously work with the OTA horizontal to reduce any risk of dropping a tool onto the primary.

Ah, that's excellent advice! I hadn't thought of that, and knowing me, that's exactly the kind of thing I'd do. You probably just saved me the cost of a new primary

erick (Eric) · #5 14-07-2008, 01:45 PM

My recollection, Leigh, is that when I took the spider out and replaced it, the collimation was hardly affected. Make sure your three secondary adjustment screws are in firm against the holder - that should keep it well in place. I cannot remember how I got it out. Of course, the ends of the screws are wider than the OTA. Must have undone them all then was able to push the spider further through two holes to release the other two ends so it could be lifted out.

ps. here is where I make brief mention of removing and re-installing the spider in my 8".

http://www.iceinspace.com.au/forum/s...ad.php?t=20612

stringscope (Ian) · #6 14-07-2008, 02:36 PM

G'day Leigh,

This is a not uncomon problem with the the GSO solid tube dobs. I have carried out this maintenance task on 4 of them over the last couple of years. My "20 cents worth" for your consideration

:

1. Yes, regardless of how you tackle this task, you should get a collimation tool and become familiar with Newtonian collimation. This will help you get the best performance out of your scope. With a bit of practice, collimation is easy and fast.

2. I suggest the safest way to approach this task is to remove the primary mirror.

3. To avoid having to repeat this task, you should consider applying some "Loctite" to the bolt threads prior to retightening. if you decide to do this, you will need to completely undo all 4 altitude trunion bolts.

4. To remove the mirror (in its cell), you need to:

Prepare a padded flat surface of sufficient size for the telescope tube and the mirror cell laid flat behind it.
Have some rolled up towels or padding to place along side the tube to stop it rolling.
Have somewhere safe to place the mirror and cell while you are working on the tube. This should have a cover of some sort to avoid accidents - a very large cake tin or carboard box etc. This shold be placed well away from the tube while you are working on it.
Have a small container for the screws that hold in the mirror cell.
If you haven't done this sort of job before, have at least 90 min free - don't rush.
Lay the tube horizontal on the padded surface and place the extra padding along both sides to stop it rolling.
Using a couple of pieces of stick tape, or similar, mark the tube and rear of mirror cell so you can replace it in the same orientation.
Remove the 6 primary mirror cell attachment screws and put them in the small container.
Have an assistant hold the telescope tube and carefully pull the mirror cell out. This may require a bit of force as it may be quite a tight fit - make sure you are not working at the edge of the table for this in case the cell comes out in a "hurry" .
Place the mirror and cell in a safe place and you should now be able to easily reach the trunion attachment bolts for maintenance as required.
Once you have tightened the bolts up, reverse the above procedure, collimate the scope and you are back in business.

Regards,

peter_4059 (Peter) · #7 14-07-2008, 04:10 PM

I agree with Ian - taking the secondary out introduces a lot more variables than the primary.

I ended up replacing the nuts with a plate attached to the inside of the tube with threads tapped in it. This way I can remove the altitude saddles when I mount the tube on my EQ6 and put them back on when I want to use it as a dob and the nuts are no longer an issue.

Peter