Grub screws for secondary mirrors.

[astro_nutt]

Having just changed the 3 secondary mirror grub screws on my (chinese made) 10 inch dob, I found Coventry Fasteners, (www.coventryfasteners.com.au), most helpful.
(I burred one of the originals when I tried to loosen them off for the first time).
The screw size is 4mm x 20mm reccessed hex head, (80 cents each) and is made of a tougher grade steel which is less likely to burr. The bottom of the screw has been finished in a conical shape and requires to be ground flat. I did this by threading the screw into a 1 cm length of 6mm plastic spline, (the same type used to secure flywire), secure it handtight into a drill chuck then gind down the screw against a wet sharpening stone using the slowest speed. Move the drill in a circular fashion accross the stone, then clean throughly with an old toothbrush soaked in metho, then apply a little WD-40 and wipe clean. Exchanging one screw at a time will save time collimating. The result gives greater fine-tune collimation of the secondary mirror.

[spacezebra (Petra)]

Hi there

I did the same - excellent same company here in Albury!!

Cheers Petra

Originally Posted by astro_nutt

Having just changed the 3 secondary mirror grub screws on my (chinese made) 10 inch dob, I found Coventry Fasteners, (www.coventryfasteners.com.au), most helpful.
(I burred one of the originals when I tried to loosen them off for the first time).
The screw size is 4mm x 20mm reccessed hex head, (80 cents each) and is made of a tougher grade steel which is less likely to burr. The bottom of the screw has been finished in a conical shape and requires to be ground flat. I did this by threading the screw into a 1 cm length of 6mm plastic spline, (the same type used to secure flywire), secure it handtight into a drill chuck then gind down the screw against a wet sharpening stone using the slowest speed. Move the drill in a circular fashion accross the stone, then clean throughly with an old toothbrush soaked in metho, then apply a little WD-40 and wipe clean. Exchanging one screw at a time will save time collimating. The result gives greater fine-tune collimation of the secondary mirror.

[Roger Davis]

NO NOT FLAT!
Think of the surface area and the torque on that surface area against the flat back of your secondary cell. The bolt will attempt to drill into the back and it will burr the surface against which it bears! The best shape is a spherical one as the torque is applied tangentially and there is no lateral movement of the bearing surface. That's why all the lock knobs on all of the scopes I have made over the years were spherical and why all those commercially made ones I centre popped and epoxied a ball bearing to!

[astro_nutt]

Thanks Roger..I'll re-shape the bottoms of the screws to spherical!

[spacezebra (Petra)]

Hi there

I had the same warning - my technical advisor kindly did the job for me so I would not dig into the secondary.

Cheers Petra