Further to my comment of 26 Jan I've now made & flocked a longer spacer tube between the ERF & etalon. The length of this tube has been calculated to work with scopes as fast as f6 and I don't think I'd be wanting to go faster than that. I tried it out this evening and it works OK. I did take an AVI, but the sun was close to the western horizon and was 'boiling'. I did think about including that proc. jpg, but I'll take another AVI in better seeing earlier in the day. The AVI I took today was with a scope of 1600mm FL, so I'm going to have to have good seeing conditions than I was able to get away with when using the f6 621mm FL scope that I've used previously.