Hi
There are a couple of ways you can go about mounting a guidescope to a C8. You can go side by side or you can mount the guidescope on the OTA itself. You will need to take into account the rigidity of the system and how you actually mount the guidescope itself. The most common way of mounting the guidescope is by use of adjustable rings, these allow you to move the guidescope around to center a suitable star on which to guide.
Side by side setup like this will work although you will have to get the required dovetail plate:
http://www.starstuff.com.au/index.html or
http://www.astro.premcom.com/ADM/ADM...ducts/DSBS.htm
Piggy back style, you can either go 4" using Losmandy or similar plates, using the ADM mini system or the new Losmandy "V" Series.
http://www.astro.premcom.com/ADM/ADM...escription.htm
http://www.losmandy.com/v-series.html
http://www.losmandy.com/secondary.html
Or possibly make your own.....just remember that it has to be rigid.
Cheers