[erick (Eric)]

I have an 8" GSO OTA with a 10:1 GSO focusser.

I want to replace it with a second hand JMI focusser I have bought.

A quick look suggests that the mounting holes are interchangeable between the GSO and the JMI - beaudy!

The JMI is on a flat base, but I also bought a spacer that fits the JMI onto an 8" tube.

Now the JMI is sophisticated - well for me anyway - it has four small grub screws that can be driven into the spacer (loosen mounting bolts first) to tilt the focusser.

So, how do it get it properly squared?

Here is my thinking. The current OTA seems to be fairly well collimated so perhaps the GSO focusser is fairly well squared at the moment. So, remove the secondary, fit my collimation laser to the GSO focusser and see where the spot hits the opposite inside surface of the OTA. Crank the focusser in and out and see if it moves. Mark the relevant positions. Remove the focusser. Accurately measure the location of the centre of the focusser hole (from the mounting holes in case the large hole is off). Now locate that point on the opposite inside surface of the OTA. Does it coincide with where the laser struck? If yes, all is good. If not, then?

Fit JMI and insert laser. Adjust grub screws to have laser strike - where?

Or ignore all this entirely - swap over the focussers then worry about the location of the focusser relative to the optical axis by other means?

Advice welcome - thanks Eric