View Single Post
Old 18-05-2018, 04:39 PM
Billyboy78 (Bill)
Registered User

Billyboy78 is offline
Join Date: May 2018
Location: CRESTMEAD Brisbane
Posts: 103
Making a 20" Gregorian.

Hi, new guy to the forum. Why would you want to make a 20" Greg you ask?? Over the last 50yrs I've made an 8", 10", 16", and a 6" Berry refractor, started a 12" classic cassegrain nasmyth plus a purchased 8"Celestron XLT. The 16" was near the size that I wanted but at my age climbing a ladder with a heavy camera was impossible, besides it was too flimsy, all the time adjusting the counterweights and I hated the reverse image in that if the image drifts to the right commonsense tells you to move left, WRONG!, then you've lost it and you start again.

All this started me thinking a Gregorian with Nasmyth/ Coude' focus would be the go, squirting the image down the dec shaft and then the polar shaft of a GEM to a comfy chair where I can hang all sorts of equipment on the eyepiece and not upset everything. At this point in time to test the concept I've constructed a 18 point mirror cell, the 6 sided cage of 25mm square tubing to hold the tertiary diagonal in front of the primary, the dec shaft made of 3" steam pipe inside of 3" roller bearings in pillow blocks. The intersection of the dec/polar shafts will house the 2nd diagonal. The polar shaft is similar to the dec shaft with provision for the slow motion drive and goto encoders. When I described this idea on the CN forum I was politely told to make a CAD drawing and take a course in optical engineering.
They did however raise the question of the secondary and at the time I wasn't 100% sure what an ellipse was, the idea of 2 focal points made my head spin. So, now I've ordered an 8" blank to test whether I can grind it to the desired shape. To this end I made a turntable driven by 2 drill presses, an accessory belt and pulleys off my old Jeep, plus a KE tester. Did I mention I'm on a tight budget?
Reply With Quote