mounting guidescope rings to telescope

[glowingturnip]

Hi,

So I just picked up some guidescope rings on ebay that look like this...

My scope is a Celestron Omni XLT150 reflector (http://www.celestron.com/astronomy/celestron-omni-xlt-150.html), which has two rings round the scope attaching it to the mount, each has a threaded hole at the top, one of which has a screw for attaching piggyback DSLR's etc, i.e. normal camera tripod screw.

What else am I going to need to mount the guidescope rings on the telescope ?  Will I need to buy a dovetail bar, and if so, which kind, or will these rings just screw into the holes provided ?

(I don't have access to my scope at the moment, otherwise I could just try it and see of course)

Cheers,

[glowingturnip]

shameless bump - anyone using guidescope rings like these ?

Ta

[Alien 13]

Hi i dont know your scope but mounting the rings to the scope rings would not be a good idea because you would disturb the alignment every time you took the scope off the tripod.

You would need some sort of bar to mount the guidescope on so that it could be fitted to the original scope finder mount, sorry i cant help more.

[MattGoo]

I'm not that familiar with your scope, but I think it's similar to a Skywatcher 150P. I'd try and obtain another dovetail bar that is the same as the one on your scope, then use the camera attachments on the top of your existing tube rings to mount this new dovetail. Then you attach the guidescope rings to that - which may involve drilling holes in it.

Similar to my ED80 & ST80 mounting configuration:

[lensman57]

Hi,

You need to mount a dovetail bar simillar to the one on bottom of the scope, to the top of your scope's rings, holes are usually 1/4" UNC, the same as a camera tripod thread. you will then mount the guide scope rings   to the top bar and then your guide scope through the rings. It will work but it adds quite a bit of weight to the set up. You will also have to aware that  the screws need to be adjusted once the telescope has been outside for good few minutes to cool down, on one ocassion I found my guide scope to be quite loose because of the change in temp. Good luck

Regards,

A.G

[glowingturnip]

good info thanks guys, and makes sense - i'll have a look for a dovetail bar then

[IanL]

I have a similar setup to Matt above, and there are a few other things to be aware of:

- Guidescope rings are often a source of flexure in themselves.  I had a similar set of rings and stopped using them since they were causing more guiding problems than they solved.  You do not need the guidescope and the imaging scope to be perfectly aligned.  If the two are way off, then you can get field rotation on longer exposures/long focal lengths, but at 750mm I doubt you'd see major issues on (say) ten minute exposures.  By all means give them a try, but personally I switched to solid rings for the guidescope fairly quickly.

- Are the camera holes on top of the main rings flat?  Mine (and most I have seen pictures of) have a concave top profile which doesn't tend to mate too well with the flat bottom surface of the rings, or the flat or convex bottom surface of a dovetail (as you will likely be using here).  You may be able to use small washers in the convex camera hole surfaces to make them flatter, but everything I tried made it less stable rather than better.  Another option is to grind the top surfaces flat somehow - I didn't try that and I suppose it depends on what equipment you have access to.

- In my case I used a dovetail that was long enough to reach the front cell and the focuser and had a pair of cone error screw holes at each end.  I used the cone error screws to bear on the front cell (not enough to distort the optics) and the focuser body (used bits of fibre washer to protect the paintwork).  This along with reasonable tightening of the main screws prevented any flexure at all.  I don't know how practical this would be with a larger scope like yours - bear in mind the support screws were bearing on solid castings and not on the thin tube walls.  You might get some metal bar and cut/drill it to fit perhaps.  The advantage of a dovetail is that it is profiled so it doesn't twist/flex as much as a thinner flat bar might.

[glowingturnip]

food for thought, thanks.  I don't know if the camera holes are convex or not, will have to have a look (don't have access to the scope at the mo, which makes all this rather tortuous).  The rings haven't arrived yet, but judging from the pic, I guess they slide onto a dovetail and then tighten up with their own screws so should be easy enough.

As regards flexure - yep agreed, probably not a perfect solution, but i'm only using a cheapy guidecam at the moment (a Meade LPI) so it really does need you to find a nice bright star for it, and besides, anything's got to be better than the source of flexure I've got at the moment - the guidescope just screwed onto the one camera piggyback screw !