Folded refractor: Optical collimation.

*

It is some time since I last tried to collimate the iStar 7" [180mm] f/12 R35 refractor. Which might explain my difficulties in trying to align the optics. [collimate] Struggling to reach the objective's collimation [push-pull] screws with the short dewshield in place was becoming a bore. Nothing I tried with the screws seemed to change anything in the torch reflection from the objective that I was seeing through the empty focuser.

So I removed the dewshield and immediately discovered my problem. My fear of the objective falling off the tube adapter had driven me to fit Nyloc nuts to the cell's tension screws. These nuts were jammed between the two bayonet plates. The objective had to come off the bayonet plates. Which meant removing the plates from the OTA.

Guess who forgot the balance of the OTA without the objective in place? As soon as I removed the heavy lens the OTA did a somersault. It nearly hit the objective out of my hands but somehow I clung on.  

I drilled the marks where the nuts had been rotating against the rear bayonet plate. Now the nuts were free to move inwards and outwards. Re-assembly, without the dewshield, resulted in my being able to collimate with clear changes after each cell screw adjustment.

Remembering which screw did what was quite another matter after a tour around the mounting between each trial adjustment. Eventually all the reflections overlapped as I moved my eye around the small LED torch I was holding in the focuser.  A subsequent check with the Cheshire in the focuser confirmed that all the reflections were hidden under the cross-hairs.

I'm now looking at ways to attach the dewshield without trapping it behind the objective cell. There should be enough meat in the rear rim to allow me to trim a larger hole. While still allowing me to make screw holes to fix it independently to the backplate.

The sharp eyed will notice the damp and sooty mold on the inside of the plywood dome from condensation. I have researched the issue and will try a bleach solution. Forum members were polarized for and against painting the dome inside.

Click on any image for an enlargement.

*