Advice on attaching a guidescope

[John K]

Hey guys,

I am looking at the prospect of attaching a small guidescope to my 12.5" NGT telescope which has a truss type tube.

The first questions is:

For a 1600mm focal length (12.5" f/5), will a 90mm Cassegrain with a focal length of 1250 be good enough to guide with? I have a 12.5" Plossl guiding eyepiece and 2 x Barlow. The guidescope will be something like this:

http://www.telescope.com/shopping/pr...iProductID=367

Secondly, does anyone have any experience attaching a guidescope on a truss tube telescope? This is what my scope looks like:

http://www.flickr.com/photos/johnkaz...7594183210993/

One option is to simply mount the guidescope on a flat piece of good with bolted on guide rings and then "wrap" this around the truss tube and attach or remove as required using a belt system.

The other is to mount the guidescope on the rotating nose assembly on the top of the telecope but as the guidescope will weigh 2 kgs JMI have recommended against this.

Interesting stuff hey. Any advice or thoughts you may have would be welcomed.

John.

[citivolus (Ric)]

Hi John.

Mounting will definitely be a problem. You don't want any flex, and I can't see eliminating that with any mount to the truss tubes, short of actually bolting/clamping something on with three points of contact. Have you considered an off axis guider?

I have the Apex 90 and picked it up for playing with guiding, but have yet to really try anything serious with it (I had 2 kids instead). It does seem to be fairly solid, and I have not noted any significant mirror slop, but guiding with it will definitely put it to the test.

I'll let others address the focal length question, as my expertise does not yet lie in that area I was going to spout something about a 2:1 guide to imager focal ratio, but it would seem that modern guiding software can overcome that limitation.

Eric

[doug_swg]

I've got the same JMI NGTscope and tried attaching a variety of guidescopes.

Heavier ones, like this stellarview:
http://www.softwaregod.com/jmi/guidescope1.jpg

...mounted with an adjustable 3-point connection to the struts required such heavy counterbalancing it ended up interfering too much with normal use of the scope. What's worked best so far is a relatively lightweight cheap 500mm camera lens hooked to a eyepiece adapter and then either to an eyepiece or an ST-4 with a flip-box and an illuminated eyepiece.

http://www.softwaregod.com/jmi/guidescope2.jpg

A Canon DSLR on the opposite side of the scope, or a heavy televue makes for almost a perfect balance without any extra counterweights. The 500mm is sufficient magnification for ST-4 tracking, but is slow enough to require somewhat brighter guide stars.