Re: ATM I need the Dakin Diagonal Offset Calculation Correction

[Aart Olsen <aart@uiuc.edu>]

I like to use a CAD program to measure the diagonal offset (CAD programs
are great to help design telescopes anyway -- they show mechanical
clearances, help to avoid drilling holes in the wrong place, etc).  First
decide what size focal plane you want.  Draw lines from the mirror to the
prime (unfolded) focal plane located out in front of the telescope.  Also
draw lines from the desired secondary plane location, at the focuser, to
the virtual mirror imaged by the secondary mirror; this virtual mirror will
be located outside of the telescope directly opposite the focuser.  Where
these rays cross you'll see a quadrilateral box, and the diagonal of this
box is exactly where the secondary mirror should be.  You can directly see
and measure the offsets from the optical and focuser axes, and see what
diagonal size (major axis) is needed to illuminate the focal plane you
drew.  Note that you can also use this if you want to put the focuser at
other than a 90 degree angle or do something else odd where the formulas
don't work so well. 

Aart Olsen           mailto:aart@uiuc.edu           217-244-4688
Library Systems Office      Univ of Illinois at Urbana-Champaign