I'm no expert, but I've heard that when star testing APO's, that its
important to place a filter (yellow?) in the optical train.

Hank, I think you have it the other way around--- if anything, you want
to test an achromat with a yellow green filter to see its true quality,
an achro is not supposed to have full spectrum correction across the red
and blue to the same degree as an apo, but to more accurately test /any/
refractor, you really should do so in the color for which it was nulled.

Bud