Focus Tube Slop

[johnrt]

I've been struggling with star shapes again - I thought the collimation was dropping out very easily on my Altair 6" RC, until I discovered the amount of movement in the focus draw tube.

The issue here is you have to unlock the focuser after you have slewed to a star to focus, the draw tube then drops under gravity and you tighten up a misaligned focuser tube.

Clearly the cause of the odd stars with the light point offset to the side I posted a picture of earlier this month.

I've made a quick video of the movement in the draw tube. All the movement in the video is the focus tube wobbling from side to side - the OTA was wedged in place against a wall.

Does anyone have any thoughts on where to go from here?

Focus tube slop | Flickr - Photo Sharing!

[coulthamst]

Is there a plate between the focus knobs on the other side of the focuser to that depicted, containing:

1) The focus lock (knurled manual screw)

2) Four screws in an oblong formation (Phillips or Allen type)

3) One Allen screw central to the above four?

If so, loosen the focus lock, firmly tighten the set of four, and finally adjust the central allen screw until it just begins to affect how easily the focus knobs operate the focuser. Then slightly back off the tightness of the central Allen screw until the focus kobs operate freely again, and the focuser should be set up correctly.

If the focuser is used in a more vertical than horizontal configuration and carrying a heavy load (eg cameras and filter wheels) it may be necessary to set the adjustment tighter to prevent slippage. Careful adjustment to achieve the happy medium will be required.

Hope this helps

Jim.

[johnrt]

Hi Jim.

Thanks for the advice.

The 4 small allen screws on the underside of the focuser have been tightened as much as possible and the knurled screw on the underneath has been adjusted for the right amount of tension to engage the focus knobs.

None of this has any effect on the movement in the tube. It rocks back and forth even when the screw on the underside is done up very tightly. Only the screw on the top which locks down the focus stops the movement (there is still a very small amount even when it is tight). This of course is useless as you are locking down a crooked focus tube when the scope is in use.

Is there a plate between the focus knobs on the other side of the focuser to that depicted, containing:

1) The focus lock (knurled manual screw)

2) Four screws in an oblong formation (Phillips or Allen type)

3) One Allen screw central to the above four?

If so, loosen the focus lock, firmly tighten the set of four, and finally adjust the central allen screw until it just begins to affect how easily the focus knobs operate the focuser. Then slightly back off the tightness of the central Allen screw until the focus kobs operate freely again, and the focuser should be set up correctly.

If the focuser is used in a more vertical than horizontal configuration and carrying a heavy load (eg cameras and filter wheels) it may be necessary to set the adjustment tighter to prevent slippage. Careful adjustment to achieve the happy medium will be required.

Hope this helps

Jim.

[coulthamst]

Sorry, I don't think my message has been clear. If the focuser is the type I think it is, then the knurled screw should NOT be used for adjustment of the focuser. The adjustment of the "fit" of the drawtube is by the central Allen screw (ie a grubscrew with no head, only a hexagonal hole in its centre into which a hexagonal section right angled key is inserted to tighten it).

I am assuming that your focuser is of the Crayford type. If, however, I am mistaken and you have a rack and pinion focuser, please ignore my wafflings, as I have misunderstood the situation!

Perhaps you could post a picture of the other side of your focuser to clear things up.

[johnrt]

Here is the underside of the plate. The large screw in the middle engages the focus knobs - you adjust depending on the weight of camera / filter wheel etc. The 4 silver screws are all done up very tight.

[coulthamst]

Thanks for the pic. The arrangement of your Crayford is not like others I have come across, which have separate locking and adjusting screws. I can only suggest that when refocusing from "locked" you slacken the knob by the merest fraction - sufficient only to allow movement - and then retighten to lock.

Your only alternative would seem to be to replace your focuser - unless there is a further lock screw not seen in your pics.

Jim.

[johnrt]

There might be a light at the end of the tunnel yet.

I thought I would try a quick poke around inside the Altair focuser before I condemned the whole scope. Removing the four allen screws and the housing shown in the top picture exposed 4 small phillips head screws.

These were so loose that as I waggled the draw tube they almost popped out of the holes. A quick tighten and the draw tube is now very sturdy without any lateral movement.

The focuser and draw tube now feel like a solid unit, no slop or play at all.

[coulthamst]

Great stuff John. Good on you for not giving up. Not a pattern of focuser I've come across before, always something new to learn!

Best wishes

Jim.

[johnrt]

Thanks Jim, I'm not tempting fate until I see some nice round stars!

Great stuff John. Good on you for not giving up. Not a pattern of focuser I've come across before, always something new to learn!

Best wishes

Jim.

[johnrt]

Well after a lengthy test run last night where conditions were awful for real imaging but good enough for some testing I'm disappointed to report the problem still remains. Elongated stars with their central points offset to the left. It seems all I have done is locked down a crooked focuser.

Not sure there is anywhere left to go with this other that to either return the scope or pay out a further £250 for a moonlight unit.