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

Back to the 16.25" F/12.5 main page....