Simon has passed on a couple of slide with the key formulae he used as well as some links to further information for those who are interested.
Randy Culp’s original material is at www.rocketmime.com/astronomy/Telescope/telescope_eqn.html
A pretty comprehensive coverage of the subject is at www.telescope-optics.net - not for the faint hearted!
Simon's astrophotography adventures are constantly updated at www.simoninthelakes.co.uk
I'm sure we will all be interested to here from Simon at future CAS meetings.