ideal guidescope

Hey
Just trying to work out what guidescope i should use.
I have read around a bit and kinda come to the conclusion that
a long focal length (low cost) scope should be enough. My main imaging
scope will be a 10" f4.8 with a canon attached.

I currently have a soligor RT1000/3.7" Refractor. This is a cheap
scope with reasonably ok optics and i would say ideal as a guidescope.
My main problem with this scope is that it is long and i think i would
suffer with flexure. After looking at other scopes though it appears flexure
will be a problem with any long focal length scope. I have read that some people
have used Orion ShortTube 80-T Refractor as a guidescope, this seems to go against
the general rule of a long focal length as a guidescope.

Given that the main imaging scope is an f4.8 will the ST80 at f5 be enough?

Guidecam will be a philips webcam.


Thanks
Mike

While I have not gotten into guiding yet myself, I found this talk by Craig Stark very informative.

While it is intuitive that the focal length of the guidescope should be as long as the main scope it isn't as important as some might think due to sub-pixel tracking. It's been a while since I watched this but I think I recall him saying that.

If true then the advantage of going somewhat short is possibly wider field of view and better of choice of guide star. A webcam is itself a limitation if FOV is too small and only faint stars are available.

Also a shorter scope will help minimise flexure which is - from what I read - the biggest hurdle.

www.stark-labs.com/movies/NEAIC08_Guiding.html

I haven't (as stated) gotten into this yet - but I recycled my ETX-70 (350mm FL) and have mounted it side-by-side with my 1200mm Newt on a dual bar for eventual use in this role. Time will tell if tis is actually a good configuration. At the moment I've been using it as a finder scope.

Hope the video helps - it certainly informed me.


Mark

Hey Mark
Thanks for the reply! Yea i have being browsing through
starks lab website, i missed that presentation but it certainly
makes the ST80 a good guidescope while having a wide field scope
also, which was something i planning on getting also. I haven't gone
through the entire presentation but will.
If it came down to it, is it possible to just barlow the ST80 to get
more focal length?
I am wondering now how good my philips webcam is for actual guiding, as it is a color cam?
Will post back when i have tried it out.

Thanks
Mike

Mike

Please do post how you get on.

I only tried guiding once - with a Meade LPI (a long-exposure capable webcam). I had absolutely no success!

I cranked up the exposure to a second or above. I had one barely perceptable spot in my guiding image to track on. I still can't work out how but I have a sneaking suspicion that I was attempting to track an internal reflection or source of light leakage due to me forgetting to flip the mirror (on the ETX) between eyepeice and LPI! Its either that or webcams or useless for this task - but Craig Stark says you can get by with them and Craig Stark is a very pragmatic fellow.

I haven't gotten round to a second attempt.

Another little hurdle that I had was balancing the scope. A side-by side system adds an extra dimension to the balancing effort. Maybe I bought too short a side-bar for my dual mount but I had great difficulty in balancing the system - I couldn't slide the main OTA inwards enough. I eventually got round the problem by attaching an old 35mm SLR under the ETX to obtain sufficient counterweight and rotated my 8" Newt such that my Atik 16IC CCD and focuser were orientated to the side where the ETX-70 was.

How long an exposure can you get with the Phillips webcam?

I suspect that unless you have a bright star you're gonna need to take around 1s frames with a webcam. If you add a barlow then your f-ratio goes up and you'll push further into the limitations of the webcam's light insensitivity (if I understand this at all correctly).

Your 80T refractor (if I have got my numbers right) is 400mm FL compared to your main scope's 1200mm that's comparable to my 350mm vs 1200mm - okay I haven't had success yet but clearly I'm of the mind that its worth trying.

I'm just referencing a book "Digital SLR Astrophotography" by Michael A Covington and in it is a picture of his Meade LX200 and for a guiding system he has an SBIG ST-V imager with improvised yoke constructed from a recycled lens from a pair of old binoculars. Its a piddly little thing! It is my expectation that the rule of thumb in having similar focal lengths for guide scope and main imager is over stated.

Another sanity check here is that when guiding was first conducted it was done manually with a fellow at the eyepiece of an auxillary scope and the medium was a photgraphic plate. While tedius it worked and I doubt if the auxilary scope was close to the FL of the main scope. We now have sub-pixel tracking in software.

However, not having any direct/successful experience I would fear that I steer you adrift and I'd appreciate it if some of our experienced guiders would submit a comment or two as you have a submitted a good question and I also would benefit from any further response.

Very best of luck.

Mark