Just to tidy up one point. Although the ALPY guide unit has just the single 23 micron slit, Shelyak offer a combined 23/200 micron photometric slit. In case anyone chooses to follow this route; I think I’ve solved my problems of inconsistency of continuum measurement by purchase of the latter – using the wide slit to determine the “instrument response”. It means taking 2 sets of measurements instead of 1 each time, but it seems worthwhile to overcome the variable effects of seeing, guiding errors, geometry & parallactic angle etc. I’ve not tested the new set-up extensively yet, but initial checks on some of Robin’s stars in Taurus get me to +/- 1.5 – 2% error w.r.t. references over range 4000A & 7000A. Also, had a quick look at HD35155 which is in the Miles library & used as ref. HD37507 [chose A5V in Pickles] – result in attached figure, still a bit of work to do, but seems promising.