Re: ATM Aberrations

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Post Mortem (I think)

Thanks to all who responded.  From the responses, this is
what happened, I think.

While spherical aberration is controlled at the original f#,
decreasing the the f# apparently increased the relative
thickness of the lenses deterioting spherical aberration
correction, so that the original glass selection was no
longer adequate.

Selecting another glass type corrected the spherical aberration,
but introduced off-axis aberrations (or they were already
present in the larger angular field).  In addition,
the glass was not quite as good a color match, introducing
longitudinal error in the IR.  Thus, although the situation
was better than with the original glass selection, it was only
slightly better.
 
Some of this was confirmed by allowing some figuring to one
surface; while the original selection was better when figured,
the amount of figuring was larger than with the alternative
glass selection, but also improved performance more.

In this geometry (an ED sandwich and two flint cookies), it
was thought that, by balancing the reselection of both
glasses around the ED, another match could be found. Another
such pair was not found.

                        Rick S.