Collimating a refractor??

[Lee]

I'm after a bit of advice...... since changing the focuser of my ED80, I have noticed what looks like coma at the edge of the field - comet shaped stars.... I'm pretty sure it wasn't like that before. The focuser looks straight/square.... I have no idea how I would go about adjusting anything anyway!

[Roger Davis]

You will find that the objective hasn't moved, but the focuser isn't square to the optical and mechanical centres of the telescope.
What you need is a Cheshire eyepiece (without crosshairs), your front objective dust cap on, and a well lit surface.
Put the Cheshire into the focuser with the port to the 45° pointing at the ceiling or some other well lit surface. Looking into the eyepiece you should see four reflections from the surfaces of the front objective.
Loosen the three focuser retaining screws a tad (1/16 of a turn) and gently move the focuser to bring all the circles co-incident. You are now collimated.
Further reading: The Adjustment & Testing of Telescope Objectives by H. Dennis Taylor.

[Lee]

Cool - I thought that might the problem..... will give that a try, thanks.....