RE: ATM 12.5 mirror box/upper cage dimensions

["Matt Luttinen" <mluttinen@qwest.net>]

Shannon,

I am building a 16" f4 Dob using the Kriege book. You have a point
with the measurements, it looks like you should go 14.5" for the ID
of your mirror box.  This book is great, but it is not perfect.  Go with

Newt's measurement on the secondary opening. If you use Kriege's
recommendation, there *will* be vignetting at the secondary cage.
This is not a big deal, but why start starving your scope of light right
off the bat?

I made my secondary cage taller (12", 75%) for my scope. Sure its more
bulky, 
but the advantages include inherently better baffling and having enough
room
for the spider/secondary holder. I compensated for the increased weight
by using
1/2" plywood instead of 3/4". It weighs 13.5 pounds completed and
assembled.

My secondary ID and mirror box ID ended up being the same: 18". I do not
know
if there is any advantage to having the truss poles at a compound angle
(inward) as they
rise toward the cage, but mine are not. Perhaps you get a second "truss
effect" from the
inward angle but I am not sure if it matters with the light loads/short
distance for scopes
in our size range. No compound  angle also simplifies truss block
installation/adjustment.

I am using 1-1/4" tubing even though the chart says 1". I do not want
any flexure.

I have ended up looking at a number of manufacturers and how they
do things, and have adopted some design features not in the book.

For instance, my secondary cage is all wood--no aluminum struts used.
It is at least as strong if not stronger (stiffer), cheaper to build,
and looks better to me.

Regards,

Matt Luttinen
Saguaro Astronomy Club
Glendale Arizona
33:33:58 N, 112:09:54 W