## FR equivalents for apo, semi-apo, achromatic?Posted By Vladimir Sacek |

The simplest way is to go with polychromatic Strehl. If we assume that an apo should have it 0.95 or better in the visual spectrum (400nm-700nm), an achromat pretending to this low chromatic error should be at this level.

For the standard Fraunhofer, polychromatic Strehl is closely approximated by ~1.3(F/D)^0.25 for (F/D)<0.25, and by (F/D)^0.07 for 0.25<(F/D)<1, where F is the focal ratio number, and D the aperture diameter in mm.

For 80mm f/15 that gives polychromatic Strehl of 0.86. Considering that most standard achromats - including 4" f/12 - are bellow 0.8 PS, it could be regarded as a semi-apo level.

For 0.95 PS, the 80mm would have to be f/40. But it would be real close at ~f/30 already.

Vlad

For the standard Fraunhofer, polychromatic Strehl is closely approximated by ~1.3(F/D)^0.25 for (F/D)<0.25, and by (F/D)^0.07 for 0.25<(F/D)<1, where F is the focal ratio number, and D the aperture diameter in mm.

For 80mm f/15 that gives polychromatic Strehl of 0.86. Considering that most standard achromats - including 4" f/12 - are bellow 0.8 PS, it could be regarded as a semi-apo level.

For 0.95 PS, the 80mm would have to be f/40. But it would be real close at ~f/30 already.

Vlad