[gregbradley]

Achromats are no good for imaging and you will get blue halos around stars.

I am not 100% sure refocusing on each filter will stop the blue halos. I think not because you will probably get bloated stars in the blue subexposures but not 100% certain of this point.

Even so who wants to have to refocus for blue. A lot of extra work without automation software which will do it but it now requires an autofocuser software program and just another thing to bug up at 2am.

My scopes have the same focus for each filter. I checked.

Triplets outperform doublets generally. But the ED80 with the FPL53 (not FPL51) is the one that is used a lot and gives great results.

I read a thread about carbon fibre for refractors and Roland Christen seemed to be against it. Tube currents. Also aluminium tubes shrink with temperature but tends to match the shift in the lens with falling temps to some degree. Not conclusive. I wonder if those who are using a carbon fibre tubed APO can comment about tube currents and focus shift with temp changes?

ED80 is also super light which means it is easier for the mount to get better tracking results.

Greg.