Can an apo triplet be made from an achro doublet?

I have been wondering if a corrector lens could be ground and added to a doublet thus making an apo triplet? If so, does anybody do this kind of one off work?

Hi Bill,

I assume you are already aware of the Chromacor, but perhaps you are looking for a more permanent and specialized solution than that.

Take care,

Lee

Since a triplet requires careful selection of three glasses that, when combined, give very little secondary color, and have the proper radii to reduce other aberrations to unobtrusive levels, I suspect adding a third element to an achromat is pretty much out of the question.

You can add a corrector well removed from the main objective. The Chromacor has been mentioned, and is probably the best and easiest choice if you want to improve an existing achromat. Roland Christen detailed a sub-aperture "Tri-spaced" corrector many years ago in Sky & Telescope.

Clear skies, Alan