Guidescope Recommendation for RCOS 20" ?

Image scale is what counts here, not focal length per se. If your current guide scope can be coupled with a camera (and modern one too) to give 1/5 the image scale of the main imaging camera you're all set and don't need much else. If you can find an old achromatic refractor such as the 4" Vixen f/8 - f/10 or some of the Russian ones that would be sweet too, as at 4" that won't really be that sensitive to seeing which would be felt mostly as scintillation. I have guided with this setup at 2800mm and it was sweet although not light.

Many thanks for the quick replies, Andrea and John!

Andrea, I like the idea of an older F8-F10 and this would likely be an inexpensive (or free)  experiment for us.

We use this telescope for:
- Public Star Parties (we capture single subframes or we do live stacking and display the results on large screen TV monitors for the public to see
- Astrophotography
- Research

We have three cameras on the scope :  CMOS Mono, CMOS Color and a CCD.  Our image scale changes depending on which camera we are using and to what degree we are binning.  That said, we can calculate the most commonly used image scale and base the guidescope on that - thanks for that suggestion.

John, we discussed an OAG and rejected it for the following reasons:

- We are pretty certain we can't install an OAG between the telescope and the Perseus 4-port mirror.  That means installing three OAG's - one in front of each camera.  We're not sure we can handle the complexity of that
- We have two sets of filters in front of the mono camera:  UBVRI for the research team and LRGBHaOiiiSii for the Astrophotography team (and obviously we don't stack any filters).  We're nervous about how an OAG would work behind the filters.
- Often when we are imaging at that focal length there seems to be very few suitable stars for guiding in the FOV.  We thought that a separate guidescope would give us more flexibility for selecting suitable guide stars.

Is our thinking incorrect here?

And one more question if you don't mind…  If we do try with a longer focal length guidescope, is there an easy way (other than poor guiding) to determine if we are indeed seeing significant mechanical flexure?

Thank you!
Neil

And one more question if you don't mind…  If we do try with a longer focal length guidescope, is there an easy way (other than poor guiding) to determine if we are indeed seeing significant mechanical flexure?

That's quite simple; if your guiding graphs shows no anomaly and it is consistent around the zero correction line but the imaging results show consistent (same direction with respect to the horizon) trailing then you have mechanical differential flexure. But a well designed attachment can be made to avoid it.