ATM atm: Barlows again

[Klaus Peter Schroder <schroder@ast.cam.ac.uk>]

Hi Doug,

the magnification x of a Barlow (or any other negative lens)
goes as

  x = (d/|f-Barlow|) - 1

where d is the distance of the eyepiece from the barlow. (The latter is
assumed to be in the right place, i.e. in which the focus of the combined
optics -Telescope-lens/mirror plus barlow- is achieved at the field stop
of the eyepiece.)

Klaus-Peter