If your running single wavelengths through each telescope your looking for "Aspheric" or " Best Form" lenses to go long distances. Otherwise normal spherical is fine. If you can find an old, old, Melles G catalog the pages on Collimator /Telescope design are worth reading. As achromats are well matched in order to be cemented together, they generally have a better optical form then much cheaper lenses. Beware of putting a laser focal point in the center of the achromat's cement.
If due to budget reasons, only one lens is "Best Form, or Aspheric" make it the one closest to the cavity.
Hope all is well, give a ring if you have further questions.
Steve
Last edited by mixedgas; 07-07-2020 at 19:15.
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