Re: [ATM] Schmidt Cassegrain

["vladimir sacek" <v959@msn.com>]

Jean-Guy,
 
Of course, the first choice is the RC. There is also an interesting 
alternative, a three-mirror Laux design described in Wilson's book 
(p239#1). It consists of concave primary, convex secondary 
(in a regular Cassegrain arrangement) and concave tertiary in front 
of primary forming final image behind the secondary. I've put together 
a flat Petzval arrangement around your primary mirror (D=600mm
f/2), for an f/3.3 final system. 

primary: R=-2400mm, 720mm to secondary
secondary: R=-960mm, 600mm to tertiary
tertiary: R=-1600

I checked out the possibility of spherical primary, but seems it 
doesn't work well. Next, I went with aspherized primary
and tertiary to correct for spherical aberration and coma.
Needed conics for it are K1=-0.88, K2=0 and K3=2.13.
It gave decent perfomance with main aberration being 
astigmatism (some residual coma) and blur of ~20 microns
10mm off-axis. Astigmatism results in curved best image
surface, somewhat better than 1000mm.

And with aspherized primary and secondary (needed conics
for canceling spherical and coma are K1=-1.3, K2=-1.52,
K3=0), astigmatism is also near-zero, for nearly flat field 
and ~10 micron blur 10mm off-axis.

For comparison, a RC with identical secondary position in
somewhat slower system (f/4.5) reqiuires K1=-1.263 and
K2=-10.6, has curved best image surface (~600mm), 
and a curved surface blur size 13.5 mm off-axis ~20 microns 
(at 10mm off-axis it falls in between the two above systems in 
regard to blur size).

The second three mirror arrangement looks pretty good. Not quite 
as Paul-Baker, but w/accesible image). It is probable that it can be 
bettered - I've only spent half an hour, or so, figuring these 
two variants out.

Guess not really what you're looking for, but - who knows...

Vlad


i have a 24" f/2 spherical primary, what secondary do i need
to cover 24mm dia. field with about 10 microns star images ?
willing to use a field flattener simple lens "near" focus,
aspherizing the primary an option,
final f/ ratio less than 8 (?)
no Schmidt plate please,

i don't mind 50% obstruction as it is for photographic use only,
cass or gregorian,

thanks,

Jean-Guy
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