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Re[2]: [ATM] convex secondary
Hi Jonathan,
Tuesday, March 1, 2005, 4:15:28 PM, you wrote:
JL> Why wouldn't the secondary be 4", I tought the only difference between
JL> Cassgrain and RC was the figure of both primary and secondary mirrors?
If you are still using your tertiary diagonal mirror and can get
away with the -2" back focal length, then to cover field of your
camera sensor the secondary needs to be 4.12" diameter. A normal
RC, with a Cassegrain type focus, has a much longer back focal
distance and this puts the size of the secondary up for a give F
ratio. This isn't necessarily an optimum configuration, but you
are just diffraction limited up to about +/-0.215 degrees field
which is all you need. There are other configs that would
shorten the overall length by 6" or so at the expensive of
needing a bigger secondary. Have you tried ray tracing this in
MODAS or similar?. The figures I'm quoting above are based on :
Surf Type Radius Thickness Glass Diameter Conic
OBJ STANDARD Infinity Infinity 0 0
1 STANDARD Infinity 940 110 0
STO STANDARD -2438.4 -927.1 MIRROR 406.4 -1.07002
3 STANDARD -876.3 876.3 MIRROR 104.6809 -4.9863
IMA STANDARD -299.3343 27.45498 0
This is my metric translation of your imperial figures, so
watch out :)
JL> I know anything about interference testing, does it need special equipment?
JL> How accurate is it?
In this case just a monochromatic light source. A cheap laser
pointer with its lens removed would probably do. You grind,
polish and figure a concave version of your desired secondary
then you figure the convex secondary by comparing the two under
monochromatic light and looking at the interference fringes.
--
Best regards,
Richard mailto:cnc@cncservo.co.uk
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