Guidescope and Autoguiding Question

I've been searching for information on just what size guidescope is needed with modern CCD guiders. Everything I run across talks about double the focal length for visual guiding. Then I look at SBIGs website and see the e-finder for the STV. It certainly doesn't follow that rule. What I'm trying to do is this. I have a Vixen VC200L and a Celestron AS-GT, both "supercharged" by Dr Clay. I want to use my unmodified 300D for up to several minutes exposures. I can't fit much of a guidescope on that OTA mount combo, so I'm looking at options. Ideal would be the Atik line of B&W cameras or the new DSI Pro, but what about an LPI? What is the minimum focal length and aperture guidescope I could get away with? I'm guessing guide camera exposures should be kept under 5 seconds or so. Any suggestions?

Vixen VC200L: 200mm aperture, 1800mm fl f9

The rule that the guider must be at least the focal length or more is a hold over from the days of manual guiding and does not apply to modern CCDs. You need to compare the pixel scale of the imager and the guider and aim for a ratio of 1:10 or better. However, some mounts will provide accurate guiding at greater ratios, even 1:20 but that is probably pushing it. Most guiding software has accurate centroiding algorithms that provide subpixels accuracy. Check out the mini-Borgs.

Cal

Jason,
I have a similar set up to what you describe. You can see my approach here:

http://jthommes.com/Astro/CelestronC8.htm

Cal has pretty much said it all with regard to focal lengths. For me, I have used a webcam (similar to the ATIKs). I have .2 sec exposures with the webcam so I need about a mag 6.5 or brighter guide star. 5 sec would allow you to guide on a fairly dim star. I don't think my stock mount could do well with a 5 sec guide correction time though.