Maksutov Newtonian is a rare and unique design in the telescope world. It is not so well known, but it is starting to get the attention that it deserves.
Maksutov Newtonian is a combination of the Newtonian reflector and Maksutov correcting lens. Standard Newtonians suffer from coma around the edge of the field of view. The Maksutov corrector plate is correcting this coma and producing coma-free stars across the field. It also corrects the spherical aberration of the primary mirror.
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Maksutov Newtonian vs Maksutov Cassegrain
Don’t get mistaken. The Maksutov Newtonian is not Maksutov Cassegrain. What they share is only the Correcting lens and mirrors, but the design is completely different.
Maksutov Cassegrain is a catadioptric design with a spherical primary mirror with a hole in the middle and convex secondary mirror. Something like the reflector but the light path is different. The light travels through the optical tube then it is reflected by the primary mirror to the secondary mirror that is flat to the primary mirror. The secondary mirror is reflecting the light into the hole of the primary mirror where the eyepiece is installed. The light travels twice as much as in reflector, and the convex curvature of the secondary mirror is magnifying the focal length further. It results in very long focal length but also a very short optical tube. You can read more about the catadioptric design in my article where I’m comparing the Schmidt Cassegrain and the Maksutov Cassegrain telescope.
Maksutov Newtonian has a standard reflector design with a primary mirror, and 45 degrees tilted secondary mirror at the front of the optical tube. But because it is using a spherical primary mirror, it suffers from spherical aberration. That’s why it is equipped with the Maksutov correcting lens. Standard reflectors use a more expensive parabolic mirror that is free of spherical aberration, but they also have the issue with coma.
What Is Coma
An imperfect mirror shape causes coma in the reflector, and it means that you see deformed stars around the edge of the field of view. The stars are elongated. They look like small comets- dots with a tail. Coma is present in all Newtonian reflectors and can be fixed with coma corrector screwed onto the eyepiece or a camera. So even if you have a more expensive parabolic mirror in your telescope, you still need a coma corrector. The coma is not really an issue for visual astronomy, but for astrophotography, it is a problem, and you need to correct it. With the cheaper spherical mirror in the Maksutov Newtonian, you don’t have spherical aberration, and it is coma free because all this is already corrected with a single Maksutov correcting lens.
Maksutov Correcting Lens
Maksutov Correcting plate is a multicoated spherical lens with an equal but opposite spherical aberration to the spherical primary mirror. This lens is correcting the spherical aberration and the coma. When light travels through the lens, it is refracted, so it hits the primary mirror at the right angles to produce a crisp image. That’s why you don’t need any other correcting equipment with the Maksutov Newtonian telescope.
But to manufacture the correcting lens is not easy for bigger apertures and as a result, the Maksutov Newtonian is more expensive. You may ask why not use a standard reflector with a parabolic mirror and use the coma corrector instead. Well, a high-quality coma corrector can cost hundreds of dollars. The Maksutov correcting lens is made exclusively for the given telescope, so the image is much better in the Maksutov Newtonian than in the standard reflector with a coma corrector.
The Advantage Of The Maksutov Newtonian
There are few advantages of the Maksutov Newtonian over the standard reflector. As I mentioned above, the lens is built to perfection for a given telescope, and the big difference is that the secondary mirror is not spider mounted at the front of the optical tube. On the Maksutov Newtonian telescope, the secondary mirror is mounted on the correcting lens itself. Hence no diffraction spikes. However, I must say that some astronomers like the diffraction spikes on the stars, and some astrophotographers using refractors are adding diffraction spikes to their photos in post-processing of the image manually.
Next advantage is the balance of the OTA. The correcting lens is adding additional weight in the front of the tube, and it creates a good balance. The standard reflector is heavy at the rear with the primary mirror only, and you have to push it a little bit forward when balancing the OTA on the mount. Maksutov Newtonian is easier to balance on the mount because the weight is distributed between the primary mirror at the back and the correcting plate at the front.
Another advantage which is also a disadvantage is that it is a closed system. It’s great that you don’t have to worry about the dust in the optical tube. On the other hand, it will take much longer to thermally equalize for the outside temperatures. And it will take increasingly longer with the bigger apertures. So yes, it is a little drawback. Make sure that you take your telescope out long enough before the observing or shooting session.
The Maksutov Newtonian is primarily made for astrophotography. It’s the field where it excels. Although you can also use it for visual astronomy, you just need to use an extender with the eyepiece to reach the focus. The extender usually comes with the telescope, so you don’t have to buy it separately.
Majority of telescopes used for astrophotography are refractors for many reasons. But the reflector astrophotography community is also strong with me included. And then, there is something in the middle, and it is Maksutov Newtonian astrograph.
There are many reasons why the refractor group is big, but the Maksutov Newtonian is the most refractor-like of non-refractor telescopes if you know what I mean. These scopes were born to do astrophotography with pinpoint stars across the field and coma-free sharp image. They also have a small focal ratio, so they are pretty fast scopes gathering light faster than a slow refractor. Because of this, you are ok with shorter exposures, and you will collect the same data as a slow refractor with longer exposure time.
And there is also the price. Yes, they are more expensive than a typical Newtonian reflector, but if you want a top quality ED Apochromatic refractor for astrophotography, the Maksutov Newtonian is much cheaper for a given aperture.
Best Maksutov Newtonian Telescopes
There are few manufacturers on the market making excellent Maksutov Newtonians like SkyWatcher, Orion or Explorer Scientific. My favorite is as always the SkyWatcher.
They make a great 190mm Maksutov Newtonian telescope. It has a very small central obstruction, and it’s equipped with 5 internal tube baffles that are providing a high contrast image by preventing stray light from reaching the primary mirror. It has a wide field of view because of the small focal ratio of F/5.3 with a focal length of 1000mm. Check the images made with this telescope on Astrobin.
The Explorer Scientific is also making an excellent scope called Comet Hunter with the carbon fiber tube which makes it much lighter, but lacking the internal baffles and other features compared to SkyWatcher.
If you are only starting with the astrophotography and you are deciding between buying the refractor or reflector, the Maksutov Newtonian is a perfect option in between. It has the advantages of both refractor and the reflector. For me, this type of design is the ultimate astrophotography telescope.