Re: [ATM] Loveday alternative to Cassegrain

["vladimir sacek" <vlad@copper.net>]

Richard F.L.R. Snashall

> The umbral shadow of the secondary must cover the hole or light would go
> directly from the middle'ish part of the primary into the hole and hit
> the sensor.

Yes, but not only the umbral shadow for the axial cone,
light of off-axis cones also needs to be kept away. This means that the 
opening at the secondary has to be somewhat smaller than the umbral shadow. 
Its max opening (diameter) is given by Dc^2-2Theta*f1(1-k)
with D being the aperture diameter, "c" the relative obstruction, Theta the 
field angle in radians, f1 the primary f.l. and "k" the min secondary size 
in units of aperture.

For the design posted, with ~40% c.obstruction and field angle of ~0.4 
degrees, the max opening diameter at the secondary is about 52mm - more than 
needed.

Vlad 


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