RE: ATM Eight inch F8 Design

["Anderson, Jay" <andeja@mail.va.gov>]

So... you have measured your eye's pupil and it is larger than 7mm?
You want all the available light to completely enter the eye that is for
sure.  Most people shoot for a 5mm exit pupil I think.  If you design
around your own eyes then you should allow for 1mm of slack at the exit
pupil to compensate for movement of your head (unless you plan to put
your head in a vice or some contraption to hold it steady.)  IMHO, if
you have a 7mm eye pupil then you should design for a 6mm exit pupil.

Jay D. Anderson, KF4LLZ
 ----------
From: John D. Gwinner
To: atm@shore.net
Subject: ATM Eight inch F8 Design
Date: Tuesday, June 17, 1997 6:44PM


Folks:

  Well, I thought I had everything figured out; I was about to order the
diagonal today (I got a used mirror cell that'll be here on Thursday, so
then I go shopping for tubes).  The tube will apparently be fairly small
in
comparison to the mirror size; it takes 9" tubes.  I'll verify this when
the cell gets here.

  I had been planning on a 1.52" diagonal, or maybe a 1.83, but it seems
like if I go with the longer focus, a smaller diagonal would be better
for
planetary use (I later plan on going with a 12" thin mirror after I
finish
the 8", so I'm planning on 'starting' with a 'planetary scope' so I
won't
feel like I've wasted the time doing the 8").

  However, I was running the design of my 8"F8 designed for high
contrast
planetary images through NEWT and DIAGONAL and got the following results
from Diagonal that seemed a bit scary: see the results below.

  To me, it's the 22% and the 1.33 inch radius that seems a bit bizarre.
Does this mean with the 68 degree eyepiece, and 7mm exit pupil, the
image
will look very dark at the edges?  I don't see a place in DIAGONAL.EXE
to
put the focuser size, but I am planning on a 2" focuser.  With a 1.25"
focuser, would the illuminated area required be smaller?  In this case,
maybe I'd be better to forget the glass grenades if I want a true
'planetary scope'.

  If this whole setup is a problem, then I think I'll just grind a bit
more
and go with the 8" F6, 'super portable' scope.  Just wanted some
comments
before I buy a diagonal that's too small.

BTW, the blank for this scope is a piece of Zerodur that I'm getting
surplus.  This will also be the 'too cold to lug out the big scope'
telescope.  It gets COLD up here in Upstate NY.

Comments?

                == John ==

Results:

                       NEWTONIAN DIAGONAL ANALYSIS         by Mel
Bartels
       This program calculates off-axis illumination and diagonal shift.

    Primary mirror diameter (inches) = [8      ]
    Primary mirror focal length (inches) = [64     ]
    Diagonal size - m.a. (inches) = [1.52   ]
    Diagonal to focal plane distance (inches) = [6      ]

Off-axis rad.   illum.   light loss    Off-axis rad.   illum.   light
loss
===================================
===================================
0.00 inches    100.0 %    0.00 mag.    0.70 inches     65.9 %    0.45
mag.
0.10 inches    100.0 %    0.00 mag.    0.80 inches     50.5 %    0.74
mag.
0.20 inches    100.0 %    0.00 mag.    0.90 inches     35.8 %    1.12
mag.
0.30 inches    100.0 %    0.00 mag.    1.00 inches     22.2 %    1.63
mag..
0.40 inches    100.0 %    0.00 mag.
0.50 inches     94.0 %    0.07 mag.
+------------------------+
0.60 inches     80.9 %    0.23 mag.             | Please select:
|
                                                |-----------------------
-|
Illumination may drop to 70 % for visual use.   | a. another analysis
|
Max field for visual use with 68 deg eyepiece   | b. change diag. size
|
  and 7mm exit pupil = 1.33 inch radius.        | c. change diag. to
|
                                                |       focus distance
|
Diagonal offset = -0.041 inches || to focal     | d. quit
|
  plane, or -0.058 inches on diagonal surface.
+------------------------+