Telescope design

A telescope is nothing more than a device that gathers photons (light) in sufficiently large quantities to create a useful image of a distance, and often faint, object in the night sky. The bigger the aperture, the more light the telescope gathers. In the last 400 years a lot of varying designs have been created, all of which are useful in one way or another. None of them are perfect in every way. This page lists the most common designs used in the world of amateur astronomy together with their common usage. If you are interested how a telescope creates an image and how this image can be adjusted, you can go to the Telescope Optics page.

Achromatic Refractor
This is probably the most known and iconic design out there, a single lens made of glass in a tube. Unless it's from a department store the lens is made out of achromatic glass, meaning glass that doesn't disperse colors as much as regular glass (See Optical Aberrations for more information). On the market there are a variety of different sized refractors ranging from small ones with an aperture or opening of 60mm (2.4 inches) to as much as 300mm (12 inches) or more. To limit the amount of color fringing the focal ratio is usually quite high, around f/8 to f/10 in the smaller range and up to f/15 in the upper range. This can make them quite long and difficult to transport. Achromats with smaller focal ratio's such as f/5 do exist however these cannot create sharp images due to the color fringing. Telescopes of this design owned by amateur astronomers usually have an aperture somewhere around 100mm (4 inches) to 150mm (6 inches) range.

Achromatic Doublet
Instead of using a single lens, a doublet uses two lenses that counteract the color fringing of each other. This allows the telescope to focus two colors at the same time.



Apochromatic Refractor
On the outside this design looks identical to an achromatic telescope however the lens at the front is made up of three separate pieces of glass acting as a single lens. This makes it possible to bring three different colors into focus at the same time rather than two with an achromatic lens. The main disadvantage is costs, as you're effectively buying three times the amount of polished glass. Because this design doesn't disperse colors as much as an achromatic refractor, the focal ratio can be lower thus creating a shorter and lighter telescope. Commercial designs usually have apertures ranging from 60mm to 150mm. Apochromatic telescopes larger than 150mm are quite rare. Telescopes of this design owned by amateur astronomers usually have an aperture somewhere around 80mm to 120mm. The focal ratio's are usually around f/6 to f/8, depending on the size.

It is worth noting that some manufacturers market their doublets as "ED-APO" or apochromatic, this is misleading as they're made up of 2 elements rather than 3. This makes it physically impossible for the telescope to focus 3 colors at the same time.

Newtonian Reflector
As the name suggests, this design reflects rather than refracts light by using a mirror instead of glass. Unlike refractive telescopes a reflective telescope doesn't disperse colors at all however there are other optical aberrations. The most prevalent of these is coma, which creates v-shaped stars pointing towards the center. This means that only the center of the image is sharp. The amount of coma depends on the focal ratio, however there are correctors out there that remove coma however these can be as expensive as the telescope itself. A reflective telescope is often cheaper than a refractive telescope with the same aperture or opening. However this design requires collimation, a process where the primary and secondary mirror are aligned to each other. If this isn't done every once in a while the quality of the image produced can degrade rapidly. The secondary mirror, along with casting a shadow on the primary mirror, creates diffraction effects which decreases the amount of detail the telescope can resolve. The aperture of commercial designs vary wildly, from 150mm to 500mm and even bigger. Telescopes of this design owned by amateur astronomers usually have an aperture somewhere around 200mm to 400mm. Focal ratio's vary ranging from f/3 to f/10.



Dobsonian telescope

A 'dob' as it's commonly called is a newtonian telescope attached to a dobsonian mount. This is an interpretation of the standard altitude-azimuth mount. The dobsonian mount is a compact design with a rotating base (azimuth) and a pivot point somewhere on the telescope tube (altitude). Due to the simple and inexpensive but highly effective and stable design of the dobsonian mount, a greater portion of the complete telescope's value can be devoted to the reflective optics. Thus, dobsonian telescopes offer the greatest value in mid- to large-apterture telescopes available today.



Schmidt-Newtonian
A variant of the newtonian design where the primary mirror is spherical rather than a parabolic. These are easier to manufacture than parabolic mirrors. Instead of coma they suffer from spherical aberration and rely on an asymetrical glass element at the front of the telescope to correct this. This glass element is known as the Schmidt corrector plate. This design is somewhat rare and the aperture of commercially available schmidt-newtonians almost never exceeds 250mm.



Maksutov-Newtonian
Almost identical to the Schmidt-Newtonian except it uses a meniscus corrector plate in place of the schmidt corrector plate. Meniscus lenses are easier to manufacture but are much thicker and therefore much heavier and slower to cool to ambient temperature. However it does have one benefit over that of the schmidt corrector plate in that it corrects for field curvature as well.



Cassegrain
A cassegrain is similar to a newtonian telescope however the secondary mirror bounces light back trough a hole in the primary mirror rather than out towards the side of the telescope. This design is almost never used by amateur astronomers however a few variants are. Read more about those in the next sections.

Schmidt-Cassegrain
Just like the Schmidt-Newton this is a cassegrain with a schmidt corrector plate at the front of the telescope. This is a very compact design making them popular among amateur astronomers. With this design the primary mirror isn't fixed as the focusing mechanism involves moving the primary mirror as a whole. This can make the primary mirror tilt slightly as the telescope moves. For visual use this is never a problem however for astrophotographers this can ruin long exposures. Like newtonians these require collimation but only the secondary mirror needs to be adjusted. Commercial designs have apertures ranging from 200mm to 350mm with focal ratio's ranging from f/10 to f/11.

Maksutov-Cassegrain
Almost identical to the Schmidt-Cassegrain except it uses a meniscus corrector plate in place of the schmidt corrector plate. These are easier to manufacter but are much thicker and therefore much heavier. The focal ratio's are usually around f/10 to f/13.

Ritchey–Chrétien
A design loved by astrophotographers. It uses complex and expensive hyperbolic primary and secondary mirrors. Only recently these have become available at price tags that compete with schmidt-cassegrains.

Fastar/HyperStar
A reflecting telescope without a secondary mirror. Instead, a set of lenses creates a flat and corrected image circle where the secondary would normally reside. This takes prime focus literally, as it's the focus point of the primary mirror. This has a number of advantages, most notably the focal ratio is increased from from f/10 or higher to somewhere around f/2. The downside is that it can't be used for visual observation as your own head would block the majority of the incoming light, the same goes for any camera housing that's bigger than the size of the original secondary obstruction.

Currently the only manufacturer to offers this design of telescope is Starizona. They sell a conversion kit for popular SCT's. For more information, please go to their product page here: http://starizona.com/acb/hyperstar/index.aspx

Astrograph
Astrographs are telescopes that have been optimized for astrophotography. Optimizations often include cooling fans, built-in corrective optics and fast focal ratio optical trains, but astrographs are generally modeled after the designs listed above.

On the cs.astronomy.com forums, user "chipdatajeffB" has this to say:


 * "With astrographs, a lot of the difference depends on specifically which astrograph it is and its basic design type. But, generally, the terms means an astronomical "camera" ... that is, a telescope that's designed to attach to a camera. While just about any telescope can function as a camera lens, most telescopes are designed for visual observing. As such, their field of view need not be as "perfect" as that of a telescope meant to be used for photography.


 * Taking a Schmidt-Cassegrain as a typical example, it bears noting that the field of view often has misshapen stars around its periphery. Stars in the center of the field of view will be sharp points, and quite round (as they should be), but near the outer edges of the field of view they may look a bit like teardrops and be slightly larger and/or blurry. This is an inherent part of the Schmidt-Cassegrain design.
 * Other reflectors (such as "fast" Newtonians) may suffer similar aberrations. And refractors can suffer from chromatic aberration.


 * These aberrations can be tolerated more easily by visual observers, but they will ruin a photograph.


 * To get around the problems, astrographic refractors are generally apochromatic designs and/or employ what are called "field flatteners", and reflectors will have built-in "field correctors" ... and the degree to which these correct the aberrations generally depends on the specific scope's design and the price.


 * Astrographs generally also employ focusers which will handle more weight (since cameras are generally heavier than eyepieces). They also generally are sold as optical tubes only, without mounts."