Thanks for the help guys, it is definitely making more sense now. I now understand the idea of the field size and it will be easier to calculate everything now. But it will make the design a bit more complicated.
As I mentioned previously, I want to segment the tube for 2 reasons. To make it easier to store and transport AND have a second back end with the etalon of a PST in it for solar viewing. I was planning to have a nice big illuminated field for using the scope at night, but a much smaller field is better for solar work as merlin suggested in an earlier post. There may have to be a compromise made in baffle arrangement t get the best of both worlds. I'll need to spend some more time with the pen & paper, and see what I can work out.
Some pictures of the scope as it came to me.
http://i305.photobucket.com/albums/n...7.jpg~original
http://i305.photobucket.com/albums/n...3.jpg~original
Tube is made of very thin wall steel, and is very flimsy. The focuser is a cheap 1 1/4 R&P job that you find on many older scopes. The history of it was that the owner put it together after sourcing the objective from the surplus shed some 30 odd years ago. He put it together himself, with a minimum of cost. However, the optics (aside from being filthy and very dusty) are still in first rate nick.
http://i305.photobucket.com/albums/n...4.jpg~original
http://i305.photobucket.com/albums/n...2.jpg~original
The finder is a little gem, a 26mm x6 with circle T optics.
http://i305.photobucket.com/albums/n...3.jpg~original
And despite the general 'Jerry Built' appearance, it is very functional, and gives a wonderful image. The colomation system for the cell is very effective.
http://i305.photobucket.com/albums/n...6.jpg~original
I'm looking forward to getting it up and looking much better. I'm uming and ahing about investigating AR coatings, but I'm not sure if it will be worth the cost or effort - especially if it can't be done in Australia.
Regards
David