|
From the graphic illustrations above it is readily
apparent how advantageous the use of a field flattener can be
for the wider field. Planetary work, if centered on the optical
axis (using optics that are accurately collimated), does not
benefit from such a flattener. But if wide field imaging with
1.5"+ CCD chips is the goal, then a field flattener can
greatly benefit the optical system. |
|
|
The RMS spot size at the edge of a 2.0" diameter
field is 26.9 microns without a flattener and only 6.2 microns
with the flattener used. This is greater than a four fold reduction
in RMS spot size. Moreover the first chart, RMS spot size, shows
that only at the very edge of the field does the RMS spot size
cross above the airy disk size. A telescope with such performance
would always be seeing limited, unless adaptive optics were employed. |
|
|
Defocus of the uncorrected
system allows for a maximum reduction in spot size to about half
of the 26.9 micron inherent RMS spot size, at the cost of increasing
the on-axis spot size past the airy diameter. Use of defocus
on a field flattened system only degrades the optical performance. |
|
|
A field flattener offers a much greater reduction
of the RMS spot size over the entire field, while keeping the
on-axis RMS spot sizes as small as they would be if focused for
on-axis (without a field flattener). A field flattener benefits
the optical system far greater than using defocus on an uncorrected
system. Although the latter is an excellent technique to maximize
performance of an uncorrected system. |
|
|
|
|
|
There are numerous mentions on the Web regarding Ritchey-Chretien
telescopes having a flat field. This is simply not the case for
every Ritchey-Chretien. In other words, the term Ritchey-Chretien
does not inherently imply "flat field." A Ritchey-Chretien
Cassegrain can be designed with its secondary and primary
mirrors having the same radius of curvature, thus yielding a
true flat field, but this is NOT how most commercially produced
telescopes are designed and made. To do so increases the central
obstruction (of the secondary mirror itself, not the baffles)
typically 50% or higher. The SLOAN
Digital Sky Survey is one Ritchey-Chretien (2.5m) that uses
such an optical design. The Henrietta
Swope Telescope is another example (1.0m). |
|
|
Ironically it is much more feasible to make a Ritchey-Chretien
set that does NOT have a primary and secondary of the same radius
of curvature. There are numerous reasons for this. One is that
both systems benefit greatly from the use of correctors, for
moderate to wider field use. The corrector for a flat field Ritchey
is much more complex and therefore more expensive than a simple
field flattener that is required on a more conventional (curved
focal plane) Ritchey-Chretien systems. |
|
|
|
A conventional Ritchey-Chretien can have a secondary
that is 37.4% central obstruction for a f8 system. The flat field
Ritchey-Chretien (utilizing mirrors of the same radius of curvature)
needs a 46.3% central obstruction (secondary mirror) plus the
secondary itself is extremely hard to fabricate, which again
drives costs for such a system skyward... The zero Petzval f8
Ritchey-Chretien (utilizing mirrors of the same radius of curvature)
is also about 13" longer than the conventional Ritchey-Chretien
that has an inherent curved focal plane. |
|
|
|
A 20" f8 zero Petzval Ritchey-Chretien has a
15.8 micron RMS spot size at the same 2.0" diameter field
edge. The conventional (curved focal plane) Ritchey-Chretien
has a RMS spot size of 26.9 micron at the same 2.0" diameter
field edge. So there is definitely some gain in spot size quality
with the zero Petzval choice but it still doesn't compare to
6.2 micron RMS spot sizes of the field flattened conventional
Ritchey-Chretien. The latter, even with the cost of an expensive
field flattener (say $2000-$4000), would still cost far less
than the zero Petzval optical set. The zero Petzval Ritchey-Chretien
can benefit from a corrector but that corrector is more complex
and therefore more expensive than the field flattener required
for the conventional Ritchey-Chretien. So the zero Petzval optical
set with a dedicated corrector becomes an extremely expensive
endeavor. Plus central obstruction (of the secondary itself prior
to baffling) is usually ~50%. A conventional f8 Ritchey-Chretien
typically has a central obstruction (glass only) of ~38%. |
