Diagonal collimation?

[Franklin]

I recently bought a cheapy Chinese 5"achromat off the used market as a lockdown project. It is unbranded but I believe it to be a Phenix Optical scope from my web search. Anyway, the objective is surprisingly quite good but the stock focuser was unsurprisingly quite bad. I replaced the stock focuser with a leftover R&P from a Vixen 4" scope which is  a vast improvement. Using a laser collimator I have managed to get the new R&P spot on, which now holds the laser dot perfectly central on the objective when the drawtube is racked in and out. That is all good but when tried with the diagonal in the laser point is about 8mm off centre. Obviously the mirror in the diagonal is not quite set right at 45 degrees as it should be. This is a 2" diagonal and has no collimation screws, just a back plate and four fastening screws. The only way I can think of adjusting the mirror is to install four small pads of sticky backed foam (0.5mm thick) for the mirror to press against when the backplate is replaced. Adjustment of the tension in the four screws should then alter the angle of the mirror and hopefully allow me to collimate the diagonal with the scope. Has anyone tried using this method of improving the collimation of their diagonal?

Edited August 28, 2020 by Franklin

[John]

I think that way - shims behind the mirror is the best way to do it. There will be some trial and error in it so maybe not sticky shims until you know how much tilt is needed ?. Probably not a lot.

 

[Franklin]

Thanks John. Yeah the mirror sits into the diagonal base and is protected by four small pads of felt but they are so thin I can't see much movement in them. The backplate has a rectangular pad which applies pressure when the screws are tightened. So there's nothing for it to push against, it would just sit flat against the housing which is not collimated. I think I will experiment with paper shims as you suggest to gauge what is needed. 

[Franklin]

2 hours ago, John said:

I think that way - shims behind the mirror is the best way to do it. There will be some trial and error in it so maybe not sticky shims until you know how much tilt is needed ?. Probably not a lot.

 

You were right about the foam pads John, much too thick. Ended up using a micro (0.2mm) Teflon strip along the bottom corners of the mirror. Seems to have done the trick. 

[John]

Excellent - good work