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 .


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.


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.


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