Re: RFT Adapter

If this is the RFT adapter that has a removable diagonal and telecompressor with a short t adapter, then it uses the original tele-compressor, which was listed as f5. This was the original style, later on there evidently was another style that was round end diagonal and I am not familiar with the f ratio of the lens used in this model.
Brian

That is the old (80's) Vixen/Celestron RF Diagonal - f/5 (although as you found out, the actual F ratio depends on 1. Scope used, and in the case of mirror focusing scopes, 2. the Eyepiece used)

The other RF Adapter mentioned was f7 I believe (higer for the c-14).

The problem with these units was the field curvature as they are simple Telecompressers - edge performance can be iffy. The Current 6.3 FR/corrector is much better for photo work.

That's the newer model. The older model used the standard diagonal with a special thread adapter from t thread to the diagonal thread. It had a t adapter that was about 1/2 the length of the normal t adapter and the standard f5 telecompressor. (I actually had someone make me the special T to diagonal thread adapter after I saw the unit in the early 80's. (I had extra t adapters, diagonals and a telecompressor, so it made sense to do it instead of buying a unit.
The vignetting was an issue with the telecompressors, especially with photography. It created a round image on the 35mm frame. Visually, unless using a real wide field eyepiece, it was no problem. (I used the Meade 20mm Erfle with mine all the time back in the 80s and liked it.)
Brian