Colimation shifting dramaticaly as I move the scope on mount (TS-Photon F4)

[pipnina]

Hello

I upgraded from a 130P-DS to a TS-PHOTON 200 F4 very recently and while mostly the build quality is an improvement, I have noticed that even though the locking screws are quite tight, the scope still shifts collimation quite a bit when i move it around the sky on the mount.

I have the baader laser colli in the focuser (relatively collimated itself, I rotated it in the focuser and saw only minimal movement, within the helping circle on the primary), and where the telescope might be almost perfectly collimated in one position, if I swing it around (notably DEC) it shifts somehow and the collimation becomes very dodgy indeed! About half way to the edge of the laser collimator's guide mesh.

This is a heavy scope, is it possible that it's simply overwhelming my HEQ5 somehow (dunno how that'd affect collimation but...), or flexing under its own weight? I do have a finderscope in the finder shoe, but only for a lack of other ways to mount it currently as the skywatcher dovetail used an unusual imperial thread while the photon's dovetails and tube rings are presumably metric (certainly they don't fit together).

Is it also possible that the locking screws are *too* tight on the primary adjustment? Secondary alignment doesn't seem to shift much (again stays within, although sometimes at the edge of, the center guide circle on the primary)

Any ideas, is this collimation error deadly for my images? It'd be very sad if I had to collimate for the specific patch of sky I want to image every night : (

Any help appreciated! Thanks!

[Starwatcher2001]

Does it move back into collimation if you move it back to the original position?  I'm wondering if something is shifting, main or secondary mirror?

 

[pipnina]

8 minutes ago, Starwatcher2001 said:

Does it move back into collimation if you move it back to the original position?  I'm wondering if something is shifting, main or secondary mirror?

 

Yes it moves in and out of colimation in the same pattern as I turn the DEC and RA axis.

So if I flip DEC 180 degrees, it moves out of collimation, then I move it back to the staring position and it's back to being collimated again.

[markse68]

maybe the focuser flexing? if you gently push the top of the focuser where the laser is attached does the collimation shift easily?

Mark

[Peter-uk]

I have the TS-PHOTON 10" f/4 yet to use it as i'm waiting on a Losmandy bar and looking for springs

this thread maybe of some help to you

 

Pete

Edited April 26, 2022 by Peter-uk

[bosun21]

It could be the primary mirror collimation springs are too weak for the mirror. I had the same problem with a 8” Dobsonian and once i upgraded the springs all was well. Good luck 👍 

[pipnina]

I could swear I saw a post only recently talking about GSO/TS F4 newts, where the guys at FLO said spring issues were common but have been revised, and changing springs should no longer be necessary. 😕

Maybe it was true, maybe not. All i know is that at the moment I can't seem to keep it collimated as it slews around the sky

[pipnina]

3 hours ago, markse68 said:

maybe the focuser flexing? if you gently push the top of the focuser where the laser is attached does the collimation shift easily?

Mark

The focuser does flex slightly when I push on the laser, but if I turn the laser in a loose focuser it seems to stay put near enough.

[Ricochet]

If the locking screws are tightened the strength of the springs doesn't matter as they are not being used to hold collimation. If the cell is firmly locked then it is likely that the movement is of the mirror in the cell. There may be some play in the position of the uprights that hold the mirror clips to allow you to move them slightly nearer or further from the mirror edge. It is a very fine balance between too loose with the mirror moving, and too tight with pinched optics. 

[alacant]

Bottom of the range telescopes need a few modifications to bring them up to astrograph standard.

The GSO mirror cell and floppy tube in particular need to be fixed before the thing will hold collimation. Here's hands on with a GSO 203 f5:

• seal the mirror to the cell using neutral silicone sealant to coincide with the GSO cork; lose the mirror clips. Leave the mirror to settle under gravity for 24 h. You are not aiming to glue the mirror to the cell. It must have a mm or so of silicone to allow for thermal.
• fit six 1.6mm wire springs. Three active and three passive, the latter over the locking screws. leave the latter loose; tightening them distorts the flimsy casting.
• spread the rings on a 500mm losmandy width dovetail plate. this prevents most of the tube flexure
• fit a light plastic secondary support.

As if by magic, collimation is held at all tube angles. Between sessions even.

Cheers and HTH

 

Edited April 26, 2022 by alacant

[Louis D]

10 hours ago, alacant said:

Bottom of the range telescopes need a few modifications to bring them up to astrograph standard.

The GSO mirror cell and floppy tube in particular need to be fixed before the thing will hold collimation. Here's hands on with a GSO 203 f5:

• seal the mirror to the cell using neutral silicone sealant to coincide with the GSO cork; lose the mirror clips. Leave the mirror to settle under gravity for 24 h. You are not aiming to glue the mirror to the cell. It must have a mm or so of silicone to allow for thermal.
• fit six 1.6mm wire springs. Three active and three passive, the latter over the locking screws. leave the latter loose; tightening them distorts the flimsy casting.
• spread the rings on a 500mm losmandy width dovetail plate. this prevents most of the tube flexure
• fit a light plastic secondary support.

As if by magic, collimation is held at all tube angles. Between sessions even.

Cheers and HTH

 

Here's a webpage describing the gluing process in detail with images.

Just remember, if you ever need to clean the mirror, either the cell and mirror get cleaned together as a unit, or you have to cut the glue, clean the mirror (including glue residue), and re-glue the mirror to the cell.

Edited April 26, 2022 by Louis D

[alacant]

1 hour ago, Louis D said:

cut the glue

Thanks for the link. I used to think consistent collimation was something only achievable on high end telescopes.

Don't be alarmed however. Silicone sealant is easy to cut with a craft knife blade and lifts cleanly from glass surfaces. Once every 10 or so years.

Cheers