Many thanks for lead…
I looked at the yahoo group posts and spent the afternoon getting to grips with Francois instructions… I could see that, in my current configuration, the slit didn’t apper central in the image of the guide scope aperture; it was way off…
However, to correct it, I had to tilt the whole mirror assembly forward slightly which was effected using a washer on the top screw. I set up earlier but its clouded out here now so, unfortunately, I will have to wait for a proper field test.
On the matter of the position of the main mirror: I carried out several experiments last night and hope to get back with proper analysis later…But preliminary thought are that the best throughput (total flux) appears to be with the spectrum in the lower half (as you suggested) but it does increase the height of the image. However I think this might not matter – some careful examination of results required.
In any case, having explored the construction of the ‘business end’ more carefully, I have a better understanding of the light path. I think perhaps they should make you assemble them after all!