Thanks guys,
Four followers is more than enough for me :-)
Ok, but let me first explain what the Korean War has to do with it.
Back in the early 50s the Australian government purchased a stockpile of optical glass for military purposes. Some 20 odd years later they concluded that WW3 was not going to happen for a while, or if it did, it would not last long enough to make any optics out of it and so they sent the glass to land fill. Fortunately some people managed to save some (unknown proportion) of it.
Some of this glass started circulating through various markets and by various individuals and a large porion of it was stored in ammo boxes on a farm near Warrnambool. I started collecting optical glass back in the 80s and came to poses about 40kg of various size slabs and discs, mostly purchased for around $5 per Kg.
One of my ambitions is to turn as much of my glass into optics as I can and this is where the Korean War connects with my BRH astrograph project.
As a first step I selected the largest slabs of suitable glass from my collection and that is what determined the size of the clear apperture of the instrument, which will be 150mm.
Busach's prescrption calls for all elements to be made from the same type of borosilicate crown, but my largest suitable slabs were of a Schott glass called PSK3 and after much raytracing I decided to use BK7 for the small corrector lenses - mainly because I had a couple of old laser mirrors with Ohara BK7 substrates.
There is a neat raytracer on Busack's website called PointSpread,
http://www.busack-medial.de/download.htm that allows you to play with your own version, although I used Zemax to optimize mine.