ATM 16" or 18" (first) DOBS

[Carl Woebcke <carlw@telis.org>]

Hello everybody out there,

For the last week I've been picking Mel Bartel's brain in the background
while listening to your conversations, and now I'd like to ask a few
questions of the group at large.  [BTW, re. the thread of deteriorating
coatings: QSP's (714-557-2299) "Endurobrite" is a full dielectric
coating with 98-99% reflectivity and selective bandpass filter, but ONLY
for flats ($51.50 for 2.6", $95 for 3.1").]  I'm on the fence between
building a 16" or 18" truss Dobs.  Money may decide it since I'm going
to BUY the mirror. (There. I said it.  Confessing that to ardent mirror
makers feels like taking my pants down in public.)  In partial defense I
built a 12.5" equatorial 30 years ago out of 6" sewer pipe for an 8th
grade science class I was teaching in Manhattan; and ground (but didn't
polish or figure) a 24"x1" porthole glass with John Dobson about 10
years ago.  Anyway, my questions are: 1) what are the virtues between
f4.5 and f5 (or even f5.5) for 16" & 18" apertures, besides scope height
and portability?  Is there a noticable difference in contrast, and is it
a big deal to get the diagonal smaller (2.6" or even smaller, instead of
3.1") by increasing the focal length, and why?
2)  What's a good book for building the LATEST generation, LOW PROFILE
DOB truss mounts, incl. measurements of truss blocks, rocker & mirror
box sizes, etc., for figuring out minimum diagonal size, where exactly
to position the focuser (horizontally and vertically) and all that good
stuff).
Thanks in advance.

Carl Woebcke