Testing an Achromat lens

Hi Folks,

I did a quick search of the old Astromart threads and could not seem to find a description of how to bench test an achromat lens.

I have access to optical flats, Ronchi screen, filters, etc. I kind of got the idea a lense could be tested like a mirror if a flat was placed behind it?

Maybe there is an ATM thread on this also ?

Thanks !

If you have a very good parabolic mirror with a diameter somewhat twice the diameter of the achromat you wish to test you can make a beam of "parallel" light rather easily by putting a bright point light source at the focal point. The returning beam can be used to test your lens with a knife edge, ronchi grating etc. Just put your filters behind the knife edge. I have been using this method for many years and it works great on small lenses too, which don't easily lend themselves to autocollimation techniques.
Duane Jaecks