Maksutov-Newtonian

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The Mak-Newt as it sometimes called, is a combination of Newtonian flat secondary and focus position, plus the spherical primary and negative meniscus corrector associated with Maksutov telescopes. As in a normal Newtonian, the focal length of the primary mirror is the focal length of the telescope. The meniscus lens is there solely to correct for lower order spherical aberration created by the spherical primary. Collimation, if required, is via movement of the primary and flat just as in a normal Newtonian. Mak-Newts can be designed to have very small secondary mirrors, so that the image at the centre of the field of view rival that of APO refractors of the same aperture. Unfortunately there is marked vignetting with this design of Mak-Newt, so its application for all aspects of visual astronomy and astrophotography is limited. Having a larger secondary is more beneficial to modern amateur astronomy, as it permits a much greater unvignetted field for deep-sky observing and astrophotography. It also permits a wider range of cameras to reach focus at infinity by positioning the secondary slightly closer to the primary. 

There are advantages and disadvantages to the Mak-Newt compared to the same aperture Newtonian. Compared to a Newtonian of the same aperture the Mak-Newt can be designed to be aplanatic, (free from lower order spherical aberration and off-axis coma) provided the meniscus lens is placed at the correct distance from the primary. This means for astrophotography the Mak-Newt can provide pinpoint star images toward the very edge of the field of view. Newtonians have parabolic primaries, so can be free from spherical aberration but will have off-axis coma, the severity depending on the focal ratio of the primary.

The Mak-Newt is a closed tube telescope so there is less of an issue with moving air currents within the tube. However, the cool-down time for a Mak-Newt may be considerably longer than the same aperture Newtonian. The Mak-Newt has a meniscus which has a different cooling rate to the primary, as they are usually made of different glass types. Differential cooling of major optical components can reduce fine resolution in the image until they reach the same temperature. The Mak-Newt is heavier than a Newtonian of the same aperture and focal ratio, and is usually much more expensive to purchase. However, once the telescope has reached thermal equilibrium, image resolution over the off-axis field for wide-field observing and astrophography is usually superior.

Consider the Mak-Newt as a capable tool for delivering first class photographic images of deep-sky objects, the moon, planets and the Sun, when excellent resolution is required across the entire field. If deep-sky and planetary observing is more important, the Mak-Newt aperture for aperture is first class. However, the price of a 190mm aperture Mak-Newt tube assembly also buys a 250mm or even 300mm Newtonian tube assembly.