[ATM] 20" f/4 - f/23 spherical Mangin Cassegrain

[Jean Fejes <JsFejes@Swva.net>]

  Here is a design for a 20 inch all spherical Cassegrain which uses two 
BK7 lenses as a Mangin type secondary lens / mirror.  It has all 
spherical surfaces and a small obstruction, and a short tube.  The back 
focus is long enough so the image could go through a perforated primary 
mirror, or a diagonal mirror could be used with a Nasmyth design.  This 
should be a good telescope for the planetary observer, and I like the 
ability to avoid aspherical surfaces.

  This 20 inch primary mirror is f/4, the resulting telescope is f/23. 
The on axis images are good, and so is the color correction.  The back 
surface of the second lens is aluminized (radius -801.94mm). The design 
below lists the two lenses a second time as the light passes through 
them again before coming to a focus.  The lenses could be ground against 
each other as the steep curves match, and the other two surfaces are 
close to being equal. It is possible to make surface 3 and surface 5 
with a radius of 791.58mm to test the convex surface against the 
concave, then polish the concave to 801.94mm and test directly.

  The aperture shown is half the diameter of the lens / mirror.

              radius   thickness  aperture   type

SRF 1    -4000     -1650      250       reflect
SRF 2       424         -10        45        BK7
SRF 3       791.58      -1        45        air
SRF 4      -424        -12         45        BK7
SRF 5      -801.94      0         45        reflect
SRF 6      -801.94    12         45        BK7
SRF 7      -424           1         45        air
SRF 8       791.58    10          45        BK7
SRF 9       424      1900.115  45        air

Steve Fejes
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