Cassegrain
Design for CUAS 16.25" scope
At a meeting on October 14, 2004, the club voted to accept my
recommendations regarding the specifications of the optical
system. The scope will be a classical Cassegrain design, with a
focal ratio of F/12.5. The design parameters are listed
below. The PowerPoint presentation I gave at the club meeting can be accessed here.
Overall
Focal
ratio:
F/12.5
Primary
diameter:
16.25"
Primary focal
length:
61.125"
Focal plane to primary surface:
14.75"
Secondary diameter, 0.6"
FIF: 4.85"
Percent
obstruction:
29.8 %
Secondary
ROC:
50.21"
Secondary
power
(gamma): 3.32
Secondary
conic constant (b2):
-3.46
Secondary
deviation from sphere: -25.2
microinches
Overall Focal
Length:
203.1", 5159 mm
Lowest power (w/55mm
plossl): 83.2 X
This design represents a good tradeoff between low power viewing and
secondary obstruction. At around 30%, this will affect planetary
contrast, but images will still be quite good, since the optics will
smooth and accurate. The difference between 30% and 25%
isn't much.
Overall, the current tube is long enough and in good condition.
It and the for need some paint, though. Also, we will need fans
to cool the primary in order to get the best performance from
it.
Optically speaking, the conic constant indicates that the correction of
the secondary mirror is 3.5 times that of a paraboloid of similar
curvature. With a radius of 50.22", this is F/5. So,
figuring the secondary will require a correction of 3.5 times the
amount of a 5" F/5 paraboloid.
The
final specs for the finished
optics:
Focal
ratio:
F/12.56
p
(sec. mirror to primary
focus): 17.49"
p' (sec. mirror to system
focus): 58.39"
Secondary
diameter, 0.6"
FIF: 4.83"
Percent
obstruction:
29.7 %
Secondary
ROC:
49.92"
Secondary power
(gamma):
3.34
Secondary
conic
constant
(b2): -3.44
Focal
length:
204.1" (5184 mm)
Lowest power (w/55mm
plossl): 83.6 X