In
most reflecting or catadioptric telescope designs, a
secondary mirror
is
required in the optical
path to send the light from the main mirror
to the
correct position for
viewing. This mirror and its holder obstructs
some of
the light entering the
telescope, and for larger secondaries, this
has the
potential to cause some
problems.
A
central obstruction affects the image in two ways:
1. Causes a light loss due to the blocking of light entering the telescope.
2. Introduces
diffraction effects which can cause a slight loss of
both light
and contrast for high
power images if the secondary is too large.
As far as light loss is
concerned, the best human eye
can just detect a
difference of 0.1
magnitudes, so with obstructions less than 30 percent
of
the aperture diameter
(9 percent of the area), the actual light loss
caused
by just the blockage is
essentually undetectable visually. Indeed,
many
people have trouble
seeing a magnitude difference of 0.2 magnitudes,
so for
obstructions of less
than 41 percent of the aperture (16.8 percent
of the
area), the light loss
due to the obstruction is not all that noticable.
Of these two effects, the second (diffraction)
is more significant. The
obstruction from the
secondary and its cell slightly alters the disk
and ring
diffraction pattern of
stars, taking a little light out of the central
Airy
disk and putting it
into the rings (mostly the first ring out from
the Airy
disk for common-sized
obstructions). If the secondary is large
enough, this
energy redistribution
can result in a slight reduction in the contrast
of
fine detail for high
power images of the moon and planets. How
much a
problem this is for the
observer depends on how big the obstruction
caused by
the secondary
is. In practice, if the secondary obstruction is
less than 20
percent of the main
mirror's diameter (1/5th of the mirror is obstructed),
the effect on the image
is negligable. For example, a six inch
with a 1 inch
secondary mirror (16.7%
obstruction) would perform about as well as
if the
obstruction wasn't
there at all. Indeed, the secondary's obstruction
can be
somewhat larger without
hurting high power images all that much.
As you use larger and larger secondary sizes,
eventually, high power
images will tend to
gradually aquire a slight "softness" to them, which
may
make fine low-contrast
detail a bit harder to see. A certain
limit comes
when the secondary
obstruction does become somewhat more noticable,
and that
point is when the
obstruction reaches about 25 percent of the main
mirror's
diameter.
This amounts to a six percent light loss, and becomes
significant,
especially for daytime
use, when the shadow of the secondary mirror
may
become visible in the
eyepiece at low power. Only Rich-field
instruments or
those requiring large
fully illuminated fields should have secondaries
which
obstruct more than 25
percent of the aperture. One frequent claim
by some
authors is that a
larger secondary can help increase the apparent resolving
power of a
telescope. This is somewhat of an exageration.
While the
diffraction caused by
the secondary obstruction does cause a reduction
in the
diameter of the Airy
disk, the actual amount of reduction for common
central
obstruction sizes is
slight, and would not significantly improve the
ability
of the telescope to
resolve close double stars. Indeed, the diffraction
disk
of a telescope with a
20 percent central obstruction is only about
four
percent smaller than
that of an unobstructed instrument. Even
a 33 percent
central obstruction
would only yield a 10 percent reduction in the
Airy disk
size, so for common
central obstruction sizes, the "improvement" in
effective
resolution is
negligable. The amount of energy put into the first
ring by
the obstruction would
negate any alleged resolution increase on extended
objects, so it is still
best to keep the secondary obstruction at 25
percent
or less if
possible.
As long as you keep the secondary's minor
axis size below 1/4 of your
primary mirror
diameter, the telescope should yield good images.
However,
even at a 25 percent
level, the image degredation is far from fatal,
and the
telescope will still
function. As for resorting to ultra-small
secondaries
to improve the
telescope's high-power performance, this can backfire.
You
don't really get
significant image quality improvement by using a secondary
size much under 20
percent of the main mirror's diameter, and you may
actually lose light
with a secondary mirror which is too small to catch
all
the light from the
primary mirror. In addition, low profile focusers
used
with such small
secondaries may allow some external scattered light
to get to
the eyepiece without
being blocked by the diagonal. Small secondaries
of
high optical quality
can also be hard to find or to make. You
need to
balance the desire for
high power contrast with the need for proper
overall
design, and for this,
the 20% to 25% obstruction guideline is often
a good
rule to
follow.
Overall optical quality is more important in the
long run than is how
small a secondary
mirror your telescope uses. A quarter wave
of spherical
aberration has about as
much effect on the overall energy in the Airy
disk of
an imaged star as a
nearly 33 percent central obstruction does.
The effects
of a quarter-wave
(peak-to-valley wavefront error) mirror, while just
meeting
the Rayleigh Limit,
would basically overshadow the benefits gained
by the
use of a small
secondary mirror. Resorting to a tiny secondary
mirror while
tolerating a
lower-quality mirror makes little sense to those who are
trying
to get the best
planetary images from their scopes. Indeed, many
of the
often talked about
performance differences between various telescope
designs
are due more to lack of
mirror quality than to the problems caused
by the
presence of a secondary
mirror.
The "modified" Schmidt-Cassegrain telescope needs
a much larger secondary,
often obstructing 33 to
35 percent of the primary mirror's diameter.
This
does cause a visible
loss in contrast for high power images and a slight
reduction in limiting
magnitudes for stars, but overall, the telescope
still
performs
adequately. Indeed, many planetary observers do successfully
use
SCTs. The
tradeoff is in contrast verses telescope compactness.
The SCT
does offer a very
convenient package for people who want portability
or ease
of use for photography,
so the secondary obstruction isn't the only
factor to
consider.
Once again, optical quality is the most important thing
to have
when it comes to an
astronomical telescope.