[Dennis G]

I've not read of this before and would like some one to describe it in full. eg the required magnification x? etc.
When replacing the primary mirror I've always been careful never to tighten the clamping screws too tight and although I made a laser collimator I gave it away for a 200mm length of 31.75 mm (1 1/4 inch) aluminium tube with cross hairs at one end and 2 mm hole in the closed cap. Accurately made in a lathe. Using this I assumed it had to be accurate. BUT IS IT?