GSO dob gravity collimation drift

[ericc]

Hi all-Just a quicky-- if anyone can help?
I have a new 12" GSO dob that showing a collimation drift(on altitude)
and is quite noticable using the Catseye teletube and Autocollimator.

After collimation(lets say at 60deg) you can watch the catseye triangle via the teletube, move untill zenith is reached and returning to 60deg, collimation moves back to perfect again.
Its not much--but its there all the same.



-I have fitted heavy primary springs.
-The mirror locking screws do not help.
-Mirror clips are just right?
-Spider vane is tight(with washers fitted underneath)
-Fitted sec bobs knobs(with milk jug washer mod)and are firm
-Focuser is tight

Am I missing something--or should I expect a little flexure in the 12"?
I have a 10" that is set up the same way --and its brilliant at holding collimation!



Any thoughts would be really appreciated people!:thanx:
Cheers-Eric

[barx1963]

Eric
Got your PM. I always collimate at the start of the night with the scope at roughly 45deg, that way I figure I minimise any movement that may occur as it moves through its range.
Take the locking knobs and chuck 'em away, completely useless if you have strong springs. I have used Bobs Knobs springs and seem to work OK.

As I only use my scope for visual work, I have not found geting too pernicky over collimation does at lot of good, quite frankly I could spend a lot more time, effort and money getting it just so, and probably couldn't tell the difference in the image anyways!
My time is much better spent looking at DSOs, drooling over scopes, or posting bits of largely useless info on IIS!:lol:

While a 12" needs to be well collimated and needs checking every night,
spending more than 1-2 minutes per session is a waste of time IMHO.

Malcolm

[Shiraz]

Hi Eric. The secondary support on the GSO is just adequate with the fairly heavy secondary. FWIW, the plastic secondary holder on mine was flexing, so I made up a much stiffer one using plastic water pipe fittings and it helped a lot - there is still some collimation drift as other parts move, but not as much. regards Ray

[ericc]

Thanks for the tips guys--
Yes Malcolm--I did ditch the lock screws(but used em for further diagnosis).
Ah ha--I will check out the sec holder (as I suspect the prob may not be on the primary end)?
But I now know that its normal for some drift to occur.
Will give me something to play with!
Cheers-Eric