[ATM] Vignetting at focuser/Crayford protrusion

[Jay Kirkland <jaykirk@compusmart.ab.ca>]

Having finished re-building my 20" f5.7 just last year, rather then
actually using it, I thought I'd just re-build it again. Actually, it's
still a hernia model and I've been inspired by Mel Bartels tri-lateral (see
his web site or the upcoming August S&T). As on my other re-builds this
year, this time I drew out the optical path full size, and was startled to
discover that the scope vignettes at the 2" focuser. I double checked on
NEWT and it confirms the vignetting. I have to move the focuser an
inordinate distance from the diagonal to fix this. 

Out of curiosity I played with some numbers on NEWT and discovered that
virtually every scope 20" or larger with a 2" focuser placed reasonably
close to the tube vignettes the 75% ray. 

NEWT seems to say that either you have to move the focuser farther away
from the secondary (and then the need for a significantly larger secondary)
or that you have to use a larger focuser. I've opted for the latter and
yesterday morning routered out all the parts for a 2.75" Crayford  (this
time I'm making the focuser drawtube itself out of wood, too -  a 3" long
tube with a 2.75 ID and walls 3/8" thick made from rings of 1/4" baltic all
glued together has cured hard enough this am that I can stand on its side
(really not recommended) and not break it. I'm going to gently router down
the bearing races and line them with  $2 of 028 stainless strips from the
model shop.)

But tho I'm committed (the focuser will probably be mostly finished by the
time you read this), I'm still wondering: am I interpreting my drawing and
NEWT correctly? Does vignetting of the 75% ray in fact mean that all the
light gathered by large  primaries with typical Obsession-style (close to
the secondary) focuser placement is not reaching the eyepiece? 

Confused, as always,
Jay

PS As to the Crayford tube protrusion into the light path, I asked a
similar question a few months ago about filter sliders causing diffraction.
One ATM member replied with a link to a photo of the diffraction effects of
a such a filter slider. I did not build the slider. It depends if you want
to build a diffraction limited scope or not. The diffraction effects of the
Crayford may be minimal but I hate diffraction, even the spikes from
spiders (once you've observed virtually spikeless stars as a result of a
wire spider, you'll never go back to vanes).  






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