(ATM) large tube AND short diagonal

[Klaus Peter Schroder <schroder@ast.cam.ac.uk>]

Hi there,
this just comes to my mind, while reading the atm-mailes:
Bob and Chuck raised (for very good reason!) the point of
a sufficiently oversized tube-diameter to minimize tube-currents 
in the light-path of the newtonian. Since at the same time, you   
want a short distance of the focal plane from the diagonal (to
get away with a diagonal as small as possible without loosing field),
there seems to be a 'you can only have either this or that' situation.

However, there is a way round it (I used it for a 6" and a 14" newton with
closed tubes myself): lower the base of the focuser towards just being
outside the primary's limb. For doing so, cut out a squarred section
of the tube around your focuser position, just large enough so that a
filled-in sheet of plywood nearly touches the radius of the primary -
cutting short the rounded tube-wall. For reinforcement, put in fitting
plywood in the sides between the tube und the flat plywood (I really 
would like to draw this, this description sounds terrible, I know). The 
focuser then rests on this flat new ground, very close to the primary's 
limb (in projection). The only problem that I can see, is (with a very short 
focuser, as you want it) interference of the tube-walls with an off-axis
guider, if the plywood-inset is not wide enough. BTW, most Newtonian
focusers do fit to a flat plate, because they are made to fit a whole
variety of tube-diameters, i.e. flat = infinite radius .... :-)

Cheers, Klaus-Peter