astro_nutt
11-05-2006, 03:04 PM
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.:D
(I burred one of the originals when I tried to loosen them off for the first time).:eyepop:
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.:thumbsup: The result gives greater fine-tune collimation of the secondary mirror.
(I burred one of the originals when I tried to loosen them off for the first time).:eyepop:
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.:thumbsup: The result gives greater fine-tune collimation of the secondary mirror.