Guiding, minimum focal length required, f ratio calculations.

[TheDecepticon]

Is there a minimum focal ratio needed to guide with in relation to your imaging scope? If you are using main scope at prime focus and both guider and imager are F5, does a 90degree diagonal increase focal length, or should you go to a 2x Barlow, does this make the field too small to find a guide star?
Can this be calculated, can /does someone know the formula/formulae and can they help me with the calculations or give me good examples to follow(I'm not very smart, but I can lift heavy things:lol:)?:doh:
Thanks!

[AlexN]

Well you're likely to hear mixed results on this.. I've seen setups using a 400/500mm focal length guide scope to guide 2400~3000mm imaging setups.. Even seen someone guide a 12" F/10 SCT with a 50mm F/4 finder scope (200mm F/L)

A lot of people recommend 1/2 the imaging focal length, however from results I've seen, and obtained myself.. You can guide with almost any focal length provided that it can find a guide star, and that the guide programs settings are tweaked to match the setup...

I've successfully guided my C11 @ F/6.3 with a 400mm focal length before... The biggest problem I had with that was the weight of the setup on my little mount was all a bit much...

Another option, is to use your imaging scope as your guide scope via an Off Axis Guider. I have a Lumicon Newtonian Easy Guider, and have used it once or twice (only got it recently) If your guide camera is sensitive enough, then these guiding setups are usually the best, as there is minimal chance of flexture, your guidescope sees what your imaging scope sees, hence, the guiding corrections should be spot on etc.. However they do offer a limited field of view in comparison to a smaller dedicated guide scope. I've found it difficult to find a guide star at times with the OAG, as the camera im using for guiding isn't the most sensitive unit around...

Theres a lot of people using 70mm F/10 refractors for guiding, 80mm F/5's are very common also... You cant go wrong when you buy something that a lot of other people use... The Orion ST80 guide scope is a great guide scope, lots of people use them to guide everything from 80mm APO's to 305mm SCT's to 250mm RC's... all with great results from a meger $200 scope with a 400mm focal length.

Alex

[TheDecepticon]

Cool, thanks! I was going to go with a 102 Achromatic that I can double as an imager with narrow band as Ive gone from a 10" Schmidt-Newt to an 8" standard Newt so my weight has changed heaps and the 80 ST isn't a good imaging scope. Both the 8" and the 4" are f5, so with a diagonal, the 4" should do okay.:thumbsup: