Camera-focuser sag

[MishMich]

How do people deal with Camera sag? I have noticed that on some of my images the stars on the top of the frame look more egg-shaped than at the side or bottom, and am wondering whether this is due to the camera pulling on the focuser. When I used a field flattener, the effect became more noticeable (in fact that is when I first noticed it). When I use the camera with a 50mm lens at f1.8, the effect was there, but was not more pronounced at the top.

I use a 2" extension, and had read of somebody using two extenders so that the bulk of the focuser tube remains inside the focuser, giving the camera better support. I have the focuser adjusted fairly tight, but with enough slack to be able to focus without slipping.

Alternatively, has anybody got some way of supporting the camera that still allows you to focus?

M.

[samtheeagle]

Sounds similar to the problem disucssed here: http://stargazerslounge.com/imaging-discussion/90038-anyone-identify-imaging-issue.html

In the end I added some teflon tape to the runners of the focuser which removed a lot of the slop. Not perfect, but improved.

Oh yeah, and yes an extension tube definitely helps, as like you say most of the focuser tube stays inside the scope...

[MishMich]

Thanks. Was that an R&P focuser?

M.

[Merlin66]

I have similar problems with the spectroscopes... they weigh 1.5Kg and hang pretty far away from the focuser.

The connections are all threaded ( T threads) to reduce the problem of a 1.25" or 2" tube fitting an oversized hole!

I've also set up a Z shaped metal strap ( 25mm x 3mm) which does two things:

It supports the spectro and holds it in alignment and also acts as a "safety" strap....just in case.

It has a 1/4" hole at the spectro side to take a 1/4"W bolt and nut ( also works with the DSLR) and the other end is fixed to a reversed piggyback bracket on the OTA.

Obviously limits the focus travel if used on the 2" Zero shift focsuser, but I can leave it slightly loose until I have the "perfect" (!!) focus then tighten everything up.

You should be able to come up with something similar??

Ken

[samtheeagle]

Thanks. Was that an R&P focuser?

Yeah just the bog standard one that came with the C102.

[Glider]

I tried the Teflon tape approach mentioned ^^^ and it works for me. I've got 10m of the stuff spare.

[samtheeagle]

Heh heh, yeah I've got a fair bit left over too! Shout if you want a bit

[davew]

Some people use a third guide scope ring. This can hold the focus tube or camera / fittings. It also gives the ability to center the camera properly.

If it's doing it with a camera lens you can try putting a very small piece of masking tape on the camera body mount to straighten the lens ( opposite the sag ). Does that make sense ? If not I'll try and find a link to show it.

Dave

[MishMich]

Thanks, I have quite a long support bar that bolts on to the M3, I might be able to work out a way of bolting both camera and scope to the bar, if I can find a way of allowing some movement of the focuser. I'll have a look. I'm not sure how the teflon tape would work with a crayford, as it looks like it is adjusted pretty tight anyway.

M.