
210 9. Auxiliary Optics for Telescopes
The coma due to the field flattener lens can be largely removed by reducing the
corrector-mirror separation by about 2%, but this displacement does not, of
course, affect the spherical aberration. For this example it turns out that higher-
order aberrations are not negligible. Fifth-order spherical aberration compensates
in part for third order and the result from ray tracing is an on-axis blur diameter of
about 1.4 arc-sec. This blur can be reduced to a negligible value by adding an
aspheric term of higher order to the corrector, as noted in Section 7.2.
Comparing the effects of
the
flattener lens in thes ...