Tube flex in GSO 12 inch Dob

[koputai]

Well about two years ago I got sick of not being able to use my GSO 12" effectively because it would NEVER stay in
colimation, so I took it apart with the intent of fixing it, but never did. So it got put away in pieces and it's been
there ever since.

Now a few weeks ago a neighbour asked which scope they should buy as they want to have a go at astronomy, so I
took my 8" Dob and a few eyepieces over for them to have a loan of for a while. Unfortunately this has left me with
no easy to use scope, so the 12" has to come back out.

I've put the bloody thing back together and done a colimation (Howie Glatter Laser) and all looked good until I tilted
the scope. I could see the reflected spot back at the eyepiece moving around as the scope tilted!

The spider seemed tight, but any touch would send the spot wandering, so I whacked some washers behind the screws
so I could do the spider up tighter. Better, but not perfect. However, the spot still moves a long way (5 to 10mm) when
the altitude of the tube changes. everything is tight, it seems there is flex in the tube itself.

Can anyone suggest how I might get rid of this flex? It basically makes the scope unusable.

Thanks,
Jason.

[Wavytone]

a. Secondary spider movement,
b. Tube could be flexing,
c. The primary moving in its cell,
d. Focusser sags as the orientation shifts.

or... all of the above, to some extent.

[Shiraz]

as above +1

also check the secondary http://www.iceinspace.com.au/forum/showthread.php?t=64387

and it might help to tighten up the focuser
http://www.iceinspace.com.au/forum/showthread.php?t=105392

good luck

[Satchmo]

You need to get the heavy duty springs from Bintel. Are the lock nuts done up on the cell ? 5-10mm is a lot of sag..

[asimov]

You can rule out #2 if it's a solid tube. Good luck, but it'll be something simple.

[mental4astro]

What focuser does it have? Some GSO focusers have located under the drawtube two thumb screws placed in line: one is a tensioner ( the one closest to the EP) the other is the stop (closest to the tube). If the tension screw is too loose it will introduce slack into the drawtube. The stop screw will also act to tighten the system. My 8" GSO Newt has this dickie focuser too, and it still catches me out. Bloody nuisance, really.

A little jiggle of the drawtube with an EP in place will quickly show any slack.

Does this movement of the laser show up with the drawtube rucked in all the way & with both thumbscrews tightened? If so, & you've firmed up the spider & secondary, then the mirror cell might be loose, or the collimation screws wound in too far (ie, the mirror is up too high). The latter just needs the primary to be lowered closer to the housing. Might be that the mirror cell needs a bit of attention. I'm assuming the focuser proper is firmly secured to the tube, :rolleyes:

I doubt it is tube flex. You wouldn't get so much movement in the laser if it was, as John says.

[asimov]

If it moves out of collimation just be moving the scope (without racking the focuser in/out) it can't be the focuser can it? I'd doubt it. Reckon Mark has nailed it with the primary wandering around on it's bolts/springs without the locking screws done up. I could be wrong of course.

[gary]

Based on the experience of analyzing pointing test data from Argo Navis customers
over the years, you will not see discernible tube flexure on this model of scope.

So as others have noted, you can eliminate it from the list of possibilities.
The good news is that whatever is causing the shift will be correctable.

As the scope rises in elevation, in which direction does the spot on the primary move?
Or is it simply wandering around every which way and settling down at some random point?

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[barx1963]

Having used a GSO 12" for a few years, I would suggest the most likely culprit is the mirror springs. They are pretty average. I replace the spring, primary and sec knobs with Bobs Knobs and it was great!
Much easier to collimate and with the netter springs I could chuck the locking knobs altogether.
Most nights it needed minor tweaking at the start and only check it if it was a long session.

Malcolm

[koputai]

Thanks for all the suggestions guys, much appreciated.

I'm pretty sure I can rule out the focuser and drawtube. I had the
locking screw done up, so no slop in that department, and a heavy
hand on the focuser didn't exacerbate the issue.

I've already replaced the springs and knobs with Bob's from Bintel, but
there could still be something going on down there I suspect.

There is definitely tube flex, when I have the scope vertical and give it
a gentle hug the spot moves.

Next step is closer inspection of the springs and how far they are
compressed (or not).

Cheers,
Jason.

[Tandum]

I removed the primary on mine and taped some paper over the end instead. I found the dot still moved around on the paper. In my case the spider was moving in and out. I replaced the spider and holder and that removed almost all the movement.

[White Rabbit]

I have a 12 inch light bridge that has the same problem. My primary moves quite a bit even with bobs knobs springs and the original springs on it at the same time. I've come to live with it...

[koputai]

OK, I had a look down below, and yes the primary was sitting quite high in the cell. I've wound it quite a way down, recollimated, snugged up the locking screws, and given it a test. A lot less error now, only about 1.5mm end to end.

I think the remaining flex is in the spider, it does seem quite weak.

Thanks for all your help guys.

Cheers,
Jason.