Building a 20 in (50 cm) F4.3 newtonian – Part I The Software Tools

Posted by Paulo Cacella on 13th November 2010 in Observatory, Serrurier 20in (50cm) Reflector
I have ordered a 20 inches mirror from Pegasus Optics to build a 20 inches F4.3 newtonian. The instrument will have basically three roles in the observatory : Visual observation of deep sky objects, imaging of faint objects and spectroscopy. The first step is to design the telescope. It is not really possible to design an optimized system for all these functions. We have two possible options : A single compromise design or a changeable secondary to allow these functions to be optimal in each case. I am not sure at this point the real differences I may find, then a field test will be necessary before reaching a final conclusion.

To design the telescope we may calculate everything manually or, in a better way, use a free software to do this. The most simple and useful for the first design is NEWT for Windows.

We have basically two commercially available secondaries for this telescope. One is the 3.5 in and the other is 4 in.

Design 1 – 2 inches focuser and a 3.5 diagonal

The obstruction by area was 3% and by diameter 18%, what keeps Airy disks in a reasonable shape.  With a camera there will be no 100% illumination anywahere and the 75% illumination will reach a diameter of 1.7 inches.

Telescope dimensions would be

When not considering the 2 inches additional space for the camera we have a 0.2 inches 100% illuminated field and a 1.92 inches for 75%. Full data would be

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