Since we are near Halloween, have a look at these pages concerning focal reducers on Christian Buil’s site.
http://www.astrosurf.com/buil/isis/lisa_ri/index_en.htm
http://www.astrosurf.com/buil/dispersion/atmo.htm
The first one describes precautions to be taken with focusing using a focal reducer with a LISA but the second one, looking at the effects of the interraction between chromatic aberrations and atmospheric dispersion is particularly frightening !
Robin