Choice of telescope optical system

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Today, engineers and opticians have created such a huge number of optical schemes, types of design and installation of a telescope that an astronomer who has to choose his first instrument for observing the sky is completely at a loss from such an assortment. A person who thoroughly approaches the choice of a telescope needs to understand the essence of the operation of a particular optical scheme, take into account many points related to where and how observations will take place and what operating conditions the optics will be subject to. Only by dealing with this kind of things, you can make a truly conscious and correct choice of a telescope.


Thanks to the optics available for mass production and the low price, classic design achromat refracting telescopes have become very popular with both beginner astronomy enthusiasts and experienced observers.

The lens of a refractor, as a rule, consists of two lenses, the so-called doublet – one element is made of optical crown glass, and the other is made of flint. The use of glasses of different brands and, as a result, having different dispersion, can significantly reduce the effect of chromatic aberration.

In a refractor, the tube is closed, which prevents the appearance of convective air flows along the tube walls, which can noticeably spoil the image. It is less demanding on adjustments and careful transportation than its mirror counterparts, which are described below.

Refracting telescopes are usually made long-focus, with a relative aperture of the order of 1/10-1/12. This makes it possible to reduce the influence of many aberrations and achieve a sharper and more contrasting image at high magnification observations. These telescopes are great for observing the moon, planets, and binary stars.

But recently, shorter-focus modifications of refractors with a relative aperture of 1/5-1/6 have also appeared. Although boosted refractors are more prone to chromatic aberration, at the same time they are much more compact and are great for wide-angle observations of the amazing scattering of stars in the Milky Way, diffuse nebulae and star clusters.

ED refractors

The device of these telescopes in many ways resembles the device of ordinary achromats, but their lenses use a special low-dispersion ED-glass ( extra – low dispersion ) instead of the classic crown and flint. The use of such glass guarantees a high-quality and better image corrected from aberrations. Due to the enhanced mechanics of such refractors, they are often very popular with fans of astrophotography. At reasonable prices, the ED refractor will provide excellent observational results, but it will always be unpretentious in alignment and easy to maintain.

Apochromat refractors

Apochromat telescopes are something of a hi-end in the world of amateur astronomy. Apochromat lenses in most cases contain elements of even more expensive and perfect low-dispersion fluorite glass. The frame incorporates complex systems using more than three lenses, which provide the best aberration correction and accurate convergence of beams, both in wavelength to correct chromatic aberration, and in lens zones to correct spherical aberration, coma and astigmatism.

Such optically serious instruments, of course, are equipped with equally excellent mechanics. These are high-precision lens frames with the ability to accurately install and align lenses, and light-alloy metal or carbon fiber pipes made with precision precision,  focusers with a load capacity for installing photographic equipment, and powerful finders.

Although these telescopes have the highest cost per millimeter of aperture of any system, apochromats are preferred by many observers seeking image perfectionism or superior results in astrophotography. 

Newtonian reflectors

Newton’s telescope is the most popular instrument among experienced astronomy enthusiasts. He deserved such popularity with an amazing combination of price / opportunity. It is in Newtons that the aperture unit has the lowest cost, and as we already know, the aperture is the main factor determining the power and capabilities of the telescope.

In Newtonian telescopes, the lens is a high-precision concave spherical or parabolic mirror, which is located at the lower end of the tube. The light collected by the primary mirror is reflected onto a flat secondary mirror mounted at the front end of the tube at an angle of 45°, which in turn reflects the cone of light towards the tube wall into the eyepiece assembly. Newtons, as a rule, have aperture ratios from 1/8-1/10 to the most forced modifications from 1/4-1/5.

It turns out that for the manufacture of a Newtonian telescope, it is necessary to accurately manufacture only two optical surfaces – the surface of the primary mirror and the secondary, instead of the four surfaces of a conventional achromat refractor. But at the same time, the reflecting telescope is completely devoid of chromatic aberration, the main problem of refractors, and at moderate apertures they provide excellent image quality.

The image of a reflector telescope will be only slightly less contrast than the image of a refractor with a similar aperture due to the presence of central shielding in the first one and higher light scattering on the reflecting surfaces of the mirrors. But this slight difference can be fully compensated by a 25-30% larger aperture, of which Newton really has a lot to offer!

Cassegrain system and its modifications

At the end of the 17th century, the French sculptor and artist Guillaume Cassegrain suggested modifying the previously proposed Gregory two-mirror system. In the Gregory system, a concave secondary mirror was installed behind the focus of the main mirror, which reflects the beam of light back into the central hole of the main mirror, behind which the eyepiece assembly is located. Cassegrain also proposed to install a secondary mirror in front of the focus of the main one and give it a convex shape. Thanks to this, the system turned out to be more compact. Today, the Cassegrain system in its classical sense is practically not produced by world manufacturers, because the manufacture of its optics is associated with many difficulties. But in practice, except for a slightly more compact size, it does not give a significant advantage over the much simpler Newton.

But now there are many modifications of this system. The original Cassegrain contains a primary concave parabolic mirror and a secondary convex hyperbolic one. At the beginning of the 20th century, George Ritchie produced a modified and faster Cassegrain system with a hyperbolic primary mirror, proposed earlier by the optician Chrétien. The scheme was called Ritchie-Chrétien. This system provides a sufficiently high luminosity and a high-quality photographic field, thanks to which it has gained immense popularity among professional astronomers. This is the first mirror aplant – a telescope free from coma and astigmatism. Telescopes from hundreds of observatories around the world, including the Space Telescope. Hubble are made according to the Ritchey-Chrétien system.

Lately, amateur astronomers have not lagged behind professionals. Many well-known brands specializing in the production of serious astronomical equipment offer amateur Ritchey-Chrétien telescopes from 200-300mm to half a meter or even a meter in diameter. Of course, a serious telescope must be permanently installed in a place with a good astroclimate, and models of such a complex system are also quite expensive. But for many amateurs, this is the only way to get truly outstanding results in artistic or pseudo-scientific astrophotography.

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