Compound
Telescope Designs, Some Pros and Cons.
Three of the most popular designs
for compound ( having lenses and mirrors) telescopes
are the Schmidt
Cassegrain or SCT, the Maksutov Cassegrain, Rumak
and Gregory versions and the Maksutov Newtonian.
Other compound scopes such as the Schmidt Newtonian
do of course exist, but the general consensus of
opinion is that the first three designs offer the
best images. Mass produced SCT design telescopes,
offer good value on a “dollar per inch of aperture” basis.
However many amateurs who have owned these scopes
have noticed, that under less than ideal seeing conditions
which plague us all, for the majority of our observing
life, the stellar images soon become bloated and
fuzzy as the first diffraction ring is absorbed into
the Airey disk and image contrast is lessened especially
on low spatial frequency targets such as Jupiter
and Mars and many nebula. While many fine photographs
have been taken with the SCT, the visual images compared
to the other designs such as the Mak Cass. and the
Mak Newt. leave something to be desired. This lack
of contrast on detailed images is probably the main
reason while some amateurs still prefer to use refractors
but give up considerable aperture and ability to
see faint objects and at a considerably increased
dollar cost. This leaves us with two main options
to consider, the Mak Newt and the Mak Cass.
The Search for the Perfect Scope.
This has been the Holy Grail for
professionals and amateurs alike for decades and
it must be realized
that no perfect scope exists and that some compromises
have to be made according to one’s needs and
preferences. Here is a brief overview of some of
the pros and cons to be considered when choosing
a Mak Cass or Mak Newt .
MAKSUTOV NEWTONIAN.
Although many observers would say
that the very best images come from apochromatic
refractors, this
may only be true up to 5” or 6” beyond
which size, the sheer complexity and cost of producing
the many accurate surfaces from completely distortion
free glass conflicts with the desire for greater
aperture and a more compact design. Enter the Mak
Newt. This design although developed in the nineteen
forties was popularized in the nineties and has gained
much acceptance since. Similar in appearance to a
Newtonian reflector, the presence of the Mak Newt
corrector plate reduces the troublesome tube currents,
coma and diffraction spikes experienced with a Newtonian
reflector. The size of the secondary is in many cases
smaller. The Mak Newt offers a larger flatter field
of view than a comparable Newtonian or Mak Cass and
the smaller central obstruction yields images every
bit as good as the best apochromatic refractors in
the world at a fraction of the cost and in a more
compact form. The cost of owning an eight inch or
larger apochromatic refractor is beyond the means
of most enthusiasts, as is the cost of the mount
to hold it. Some of the best views that can be obtained
from Earth especially of the Moon and Planets can
be seen through a Mak Newt.
The central obstruction is typically around 20% (on
a diameter comparison) or less and is largely responsible
for the sharpness, contrast, colour and clarity of
the images obtained. The downside is that the shorter
back-focus available on most Mak Newts. means that
additional lenses may have to be used to bring the
focus to a point where bino-viewers can be used.
The shorter F ratios yield magnificent wide field
views and good quality barlows can be used such as
the 4x Powermate for the additional magnification
needed for planetary studies without any loss in
sharpness of the image. A slightly larger mount will
be required to compensate for the longer tube and
should be sturdy enough to overcome wind effects
if imaging is a priority.
MAKSUTOV CASSEGRAIN.
A Mak Cass for many amateurs is as
close as it gets to the “perfect all round
telescope.”
The Mak Cass has been produced in
a variety of configurations, the two most
common being the original Gregory version,
where the secondary mirror is a silvered spot
on the inside of the meniscus. This design also
has
it’s optical limitations and is time
consuming and costly to produce in apertures
over six or seven
inches. The other version is the Rumak Mak
Cass (Rutten Maksutov) where the secondary
mirror is
attached to the meniscus on a separate holder
similar to the SCT design. This allows the
optician a greater
degree of flexibility to produce a larger flatter
field of view with better correction than would
be obtained with the Gregory version. This
scope although
a bit heavier than a similar SCT retains most
of the compactness of the SCT while providing
much superior
and tighter stellar images, improved image
contrast and the ability to allow higher magnifications
for
a given set of seeing conditions. The central
obstruction is often slightly smaller than
an equivalent SCT
but yields stellar images approaching those
of the Mak Newt, but in a more compact form.
The large amount
of back focus available allows easy use with
bino-viewers and reduced need to use extension
tubes when imaging.
Use of a focal plane reducer offers excellent
imaging and visual capabilities in the faster
F6 range. Cool-down
time for the larger scopes can be approximately
30 mins or less when using the built in fans
that are
common with these designs. A slightly smaller
mount
will suffice due to the shorter tube in the
Mak Cass.
If you could only ever own one scope in your life,
it would be hard to argue that a Mak Cass should
not be that choice. If you want the absolute ultimate
in image contrast, then the Mak Newt would have no
rival especially in any medium or larger aperture
size and at an affordable price.
Happy decision making to you all.
Brian Morse
Stellar Optical Corporation.
MAKS R US |