As with picking a camera, picking the best telescope for your needs will depend on a number of different factors:
Refractor and guidescope on an equatorial mounting.
Experience Level - Are you a beginner or a seasoned expert?
Budget - how much do you have to spend?
Area of Interest - Is there a particular area that you want to specialize in? Do you want to do high-resolution work, or wide-field astrophotography?
Planets - Sun, Moon, Mars, Jupiter, Saturn etc. You need a high-quality scope with a relatively long focal length if you want to do high-resolution photography of the planets.
Deep-Sky - Star Clusters, nebulae, Galaxies. You need a fast, short focal length scope for wide-field work such are large nebulae. You need a lot of aperture and focal length if you want to shoot small planetary nebulae and galaxies.
Observe or Photograph - Do you want to specialize in astrophotography and observe occasionally? Or do you want to use the scope mostly for observing, and just take a couple of snapshots once in a while?
Portability - Whether you intend to observe from your driveway, drive an hour to a reasonably dark sky site, or get on a plane and fly to a world-class dark-sky site, the size and weight of your equipment will become important. Obviously, you don't want to buy a very large and heavy telescope, such as a 16 inch Schmidt-Cassegrain telescope if you are going observing by yourself and it takes two people to lift it. Likewise, you don't want to purchase an 8 inch f/10 refractor if you have to transport it in a compact car. No matter what kind of equipment you decide on, remember that you are going to have to lug it from inside to outside every time you use it, unless you install it in a permanent observatory.
A small scope that gets used often is a better investment than a big scope that stays in the closet.
Almost any telescope can take pictures. How good they turn out will depend on the quality of the equipment (and especially the mount), your own personal standards, your level of expertise, and the amount of work you put into learning and perfecting the craft of astrophotography.
Telescope Attributes - Telescopes are designed to gather light and bring it to focus so that the image can be examined in detail with an eyepiece, or recorded on film or with a digital camera. Telescopes reveal fainter objects than can be seen with the eye because they gather more photons than the eye can gather. Smaller details can be seen because they also magnify objects.
The nomenclature used to describe telescopes and camera lenses can sometimes be confusing. Telescopes are usually talked about in terms of aperture, while camera lenses are usually talked about in terms of focal length. Most people will say they have an 8 inch telescope (meaning aperture), but they will also say them have a 300 millimeter camera lens (meaning 300mm of focal length). No wonder it's confusing! But we can easily sort this out.
Telescopes and camera lenses have three main numerical attributes that we are concerned with in describing them:
Aperture - The aperture is the size of opening in the telescope through which the lens or mirror gathers light. It is the most important attribute of a telescope because light gathering is what telescopes are all about. In astrophotography, the larger the aperture, the more photons can be collected. Aperture, however, is not the only criteria for judging a telescope. Optical quality is just as important. You can have a gigantic aperture and if the optical quality of the telescope is not good, the light won't be very well focused, and the images produced won't be very good. Aperture is the main determinant in how faint of a star you can see with a telescope.
The down side to aperture is that as the size of the aperture goes up, so does the cost and complexity of making the optical system, as well as the weight and size. Bigger apertures also usually mean more focal length, and this makes mounting them, carrying them around and using them more difficult, especially for astrophotography./
Aperture is measured in inches or millimeters (mm). There are 25.4 mm in an inch, so a 4-inch aperture telescope has an aperture of 101.6 mm.
Focal Length - The focal length of a telescope is the distance from the objective lens or mirror at which the light comes to focus. The longer the focal length, the larger the image is that forms at the focal plane, and the higher the magnification of the telescope.
Increased magnification with longer focal lengths is a good thing for small objects like planets and double stars, but undesirable things also get magnified, like poor atmospheric seeing, and imperfections in the telescopes drive and wobble in the mounting.
Focal length is also measured in inches or millimeters. Camera lenses usually give the focal length in millimeters. A simple lens with a focal length of 300 mm will form the image 300 mm behind the lens. Some telescopes have a secondary mirror that bends the light path, sometimes even folding it back on itself, making the physical length of the instrument much shorter than the focal length would imply.
Focal Ratio - The focal ratio is the relationship between the aperture and focal length. This is true for telescopes and camera lenses. The focal ratio is defined as the focal length divided by the aperture. For example, a refractor with a focal length of 800mm and an aperture of 100mm has a focal ratio of 800/100 = 8 or f/8.
The focal ratio gives the relative "speed" of the optical system. This is important for recording extended objects such as nebulae and galaxies. A faster focal ratio will record an image faster (with a shorter exposure).
Focal ratio is also known as the f/ratio, and is described by the f/number.
For example, a 4 inch refractor has an aperture of about 100 millimeters. If the focal length of this scope is 500 millimeters, then we can determine the f/number by dividing the focal length by the aperture, which in this case is 500 / 100 = 5. So we say this scope has an f/ratio, or focal ratio, or f/number of f/5.
F/5 is a mid-range f/number. Mid-range f/ratios are usually about f/5 to f/8. "Fast" f/ratios are usually considered about f/4 or lower, such as f/2.8 or f/2. You won't usually find f/ratios this fast in a telescope, but you definitely will in camera lenses. Slow f/ratios are anything bigger than f/9 or so.
F/ratios are also known as f/stops in photography. Each f/stop is equal to a doubling or halving of the amount of light. For example, an f/ratio of f/4 lets in twice the amount of light as an f/ratio of f/5.6 and requires half the exposure.
The full f/stop series, in one stop increments is:
These numbers continue on each end of the scale, but these are the practical working range of f/stops.
Each of these f/stops is equal to a one-stop difference in light getting through. So every time you change the f/stop by one full increment, you also have to change the shutter speed, or exposure time, by doubling or halving the exposure to compensate.
For example, at the same ISO (Film speed or digital camera sensitivity), a 1 second exposure at f/5.6 would equal a 2 second exposure at f/8, or a 1/2 second exposure at f/4. All would be equivalent.
Here is a list of equivalent exposures, all allowing the same amount of light to reach the sensor:
For simplicity in the short exposures, the higher shutter speeds are rounded off, such as 1/32nd sec is rounded to 1/30th sec, 1/64th to 1/60th, 1/128th to 1/125, 1/256th to 1/250th, 1/512th to 1/500th and 1/1024th to 1/1000th. The differences are so small as to be inconsequential.
If you take a camera lens with a fixed focal length, and stop down the lens, and look at the lens from the front, into the camera, you will see the size of the hole made by the diaphragm blades gets smaller as the f/number gets bigger. f/32 is a very small hole compared to f/2.8. f/32 is a "slow" aperture because the small hole does not let a lot of light get in over the same time exposure as a larger hole. It's "slow" because it requires a longer exposure.
Long focal length instruments with slow focal ratios will work well for bright objects like the Sun, Moon and planets. You can get by with scopes with high f/numbers because the exposures will still be reasonably short. Long focal length instruments also have small fields of view.
Short focal length instruments have wider fields of view and usually have faster focal ratios and can record faint extended objects faster.
Optical System Types - Refractive Optics
The "objective" lens is the main light gathering component in a refractor. The lens is located on the sky end of the telescope tube and light is focused out of the back end of the bottom of the tube.
Because the lens is permanently aligned and mounted in a cell , there is little maintenance required, and the sealed optical tube reduces problems with local seeing effects such as tube currents that can plague other types of scopes such as Newtonians.
Refractors were the first kind of telescope to be invented in the early 1600s. Galileo made the instrument famous when he began observing the sky with it in 1609, and published Sidereus Nuncius in 1610. Galileo discovered the satellites of Jupiter and was the first to observe craters on the Moon, spots on the Sun, and the phases of Venus.
The first telescope consisted simply of a plano-convex lens at the sky end, and a plano-concave lens at the eye end.
Light is refracted, or bent by a curved lens. The amount of bending depends on the wavelength of the light and the refractive index of the glass. Different wavelengths bend by different amounts, so it is impossible to bring all wavelengths of light to the same focus with a single lens. When this happens, light can be focused for a single wavelength, but all of the other wavelengths are out of focus, degrading the image and causing color halos.
Astro-Physics 130mm aperture
Doublet Apochromatic Refractor
In 1733 Chester Moor Hall discovered how to bring more wavelengths to a common focus by using two lenses, one a concave flint glass and one a convex crown glass, in a doublet achromat. This type of refractor produced much sharper images because most of the visual wavelengths were brought to a relatively common focus.
In 1892 H. Dennis Taylor, an optical designer for T. Cooke & Sons, designed the first triplet apochromat for the Cooke Photo Visual telescope objective. Today the best high-end refractors are triplet apochromats made of special ED (Extra Low Dispersion) or Fluorite glasses.
Refractors provide high contrast views and are excellent for lunar and planetary work, and all types of deep-sky astrophotography.
High-quality refractors are more expensive per inch of aperture than Newtonians or catadioptric telescopes. Their maximum size is limited because of price considerations as well as difficulty in design and obtaining the exotic glass necessary for optimum color correction. Because of this limitation on aperture size in amateur refractors, they can't see or record objects as faint as can be detected with a telescope with a larger objective. For astrophotography however a refractor in the 4 to 6 inch size can record remarkably faint objects. Also, the stellar resolution of a good refractor in this size range, around one arc second, matches up very well with the resolution allowed by atmospheric seeing, especially on long-exposure deep-sky images.
Inexpensive doublet achromatic refractors suffer from secondary color. This can be partially corrected for astrophotography with the use of a minus-violet filter. Doublet fluorite and ED glass refractors have better color correction but are more expensive. Triplet fluorite and ED glass refractors are color free and classified as apochromats, but are very expensive. High-end apochromats are excellent for high-resolution visual observing and deep-sky astrophotography, although for the really faint stuff there is no substitute for aperture.
Most refractors, except those of the Petzval design, have a curved field and require a field flattener for critical astrophotographic work.
Optical System Types - Reflective Optics
The "primary" or main light gathering component of the telescope is a concave parabolic mirror. A secondary flat mirror may also be used to change or divert the light path so that the image can be viewed at a more convenient location.
Most reflectors are an open tube design with the primary mirror at the bottom end of the tube. Light comes in the open top end of the tube, hits the primary and is reflected to a secondary flat mirror at the top of the tube suspended by a thin vane "spider", and is directed out of the side of the tube for viewing or photography.
The reflecting telescope was produced by Sir Isaac Newton in 1668. It solved the problems of color correction in refractors because a mirror focuses all wavelengths at the same point. The problem with early reflectors was that they are made of metal instead of glass, and their reflectivity was poor. Today reflectors are made of glass polished to a parabolic curve, the accuracy of which determines the optical quality of the mirror.
12 inch f/4 Newtonian Reflector
Newtonian - Newtonians provide great value for the price and the lowest cost per inch of aperture. If properly constructed, they can provide excellent images, but require work to maintain them. They must be collimated every observing session and because the mirrors are exposed to the atmosphere, the coatings eventually deteriorate and must be re-coated periodically. They can provide very good visual images of both deep-sky objects and planetary views.
For astrophotography, they must be off-axis guided. Depending on the focal ratio, they may require coma correctors for photography. The biggest problem with attaching a camera to a Newtonian telescope that is made for visual use, is that they usually don't have enough "back focus". This means that you cannot rack the focuser in far enough to get the camera to come to focus (the focus doesn't go back far enough to reach the camera's focal plane and sensor). The typical solution to this problem is to move the mirror up in the tube.
Ritchey-Chretien - This is a telescope that has two mirrors, but the primary is a hyperboloid, and the secondary is also a hyperboloid. They are corrected for coma and spherical aberration and were developed to have a well-corrected field over a relatively wide area for astrophotography from f/6 to f/9. Ritchey-Chretiens are the telescope of choice for professional observatories. The Hubble Space Telescope is a Ritchey-Chretien. Light is usually reflected from the primary to the secondary and then back out of the bottom of the tube through a hole in the primary.
Classical Cassegrain - A classical Cassegrain uses a parabolic primary mirror, just like a Newtonian, but the secondary is a convex hyperbolic mirror. Light is reflected from the primary to the secondary and then back out of the bottom of the tube through a hole in the primary. Astigmatism and coma are problems in a classical Cassegrain, although astigmatism is generally small and coma can be corrected with an additional optical lens element corrector near the focal plane. Cassegrains are usually relatively slow optical systems around f/12 although they can be designed to be as fast as f/8. They also suffer from curvature of field, like many other scope designs.
Optical System Types - Catadioptrics
A catadioptric system uses both refractive and reflective optical elements.
These compound scopes use a mirror as the primary light gathering component and a secondary mirror to magnify the light and send it back out of the bottom of the tube assembly. They also have a lens element at the front end of the tube to correct aberrations in the optical system.
Schmidt Cassegrain - In a Schmidt Cassegrain Telescope (SCT), light first passes through a thin aspheric corrector lens at the top of the telescope tube and then is collected by a spherical primary mirror and reflected to a secondary mirror which then reflects the light back through a hole in the primary and out the back end of the telescope tube to the focal plane. The aspheric corrector plate corrects for the aberrations in the spherical primary mirror. This folded-light-path design results in a very compact scope compared to the aperture, and makes for a very portable telescope.
Meade LX200 12 Inch SCT
For astronomical use, SCTs can provide good images, but suffer from some drawbacks.
Most SCTs are a compromise design that allows large changes in the location of the focal plane so many different photographic and optical configurations can be used such as cameras, eyepiece projection, off-axis guiders, and flip mirror devices. These designs also allow for large movements of the primary mirror so that close focus can be achieved for use such as in bird watching.
Focus is obtained in most SCTs by moving the primary mirror. However, one of the most critical optical considerations in an SCT in terms of optical performance is the spacing between the primary mirror and the secondary mirror, and focusing changes this distance. Therefore, an SCT is rarely used at the correct primary - secondary spacing, resulting in less than optimum performance.
For top-notch astronomical use, the optimum spacing between primary and secondary should be determined, and then the primary mirror locked at this position. A focuser should then be placed at the back of the tube for critical focusing. Locking the mirror down also helps with the problem of mirror "flop" or movement during an exposure, but an off-axis guider should be used to eliminate this problem.
SCTs, like many other telescope designs, do not have a flat focal plane. This is not a major drawback for visual use, but can be a problem for astrophotography. If a field flattener is not used, then the image should be focused on a star about 1/3 of the way from the center of the field to the edge.
It is also difficult to use a telecompressor with an SCT to make the optical system faster and provide a wider field of view without vignetting.
Schmidt-Cassegrain telescopes are usually around f/10 which is relatively slow for deep-sky astrophotography. They are compact and portable for their aperture size and are reasonably priced because they are popular and mass-produced.
Good pictures can be taken with SCTs, but they require a lot of work and patience. The vast majority of SCTs sold these days are sold with computerized Go To altazimuth mounts that limit exposures to about 30 seconds before field rotation becomes a problem. To use an altazimuth mounted Go To SCT for long-exposure astrophotography, you will need a wedge.
Maksutovs - The Maksutov optical system design is similar to a Schmidt Cassegrain in that it uses primary and secondary mirrors and a corrector plate, but the corrector lens is a spherical meniscus which is deeply curved and much thicker and heavier than the corrector in an SCT. A small spherical secondary mirror or a silvered spot on the back of the corrector is used to fold the light path back through a hole in the primary mirror, just as in an SCT.
A Maksutov can provide excellent optical performance, but the f/ratios are usually too slow for astrophotography of anything except the Sun, Moon, planets and bright double stars, and can suffer from the thermal problems because it takes a long time for the mass of the corrector to reach thermal equilibrium with the ambient atmospheric temperature.
Dall-Kirkham - This is a simplified Cassegrain design the uses a ellipsoidal primary mirror and a spherical secondary mirror. The classic Dall-Kirkham design is not well corrected for off-axis coma, and the overall speed of the optical system is fairly slow at f/15 or slower. Newer modifications to the design offer faster systems that are corrected for coma.
Optical System Types - Hybrid Designs
Schmidt-Newtonians basically combine a Newtonian optical configuration with a Schmidt corrector plate at the front of the tube. Instead of a spheroidal secondary, a small flat secondary mirror is used to send the focal plane out the side of the tube, as in a typical Newtonian. The small secondary leads to less light and contrast loss, and performance is nearly as good as with a refractor, but it is difficult to fully illuminate a wide field with this design, and it has little back focus.
Maksutov - Newtonians have a thick meniscus (concavo-convex) lens as a corrector at the front end of the optical tube, a spherical primary mirror and a small flat secondary mirror which directs the light out of the side of the tube like a Newtonian. This design offers optically excellent images in a relatively fast optical configuration, but because of the small secondary, which reduces contrast loss, it is difficult to fully illuminate a large image circle at the focal plane. The design also offers very little back focus.
Maksutov - Cassegrains combine a thick meniscus (concavo-convex) lens as a corrector at the front end of a Cassegrain optical configuration. The focal plane comes out of the back of the optical tube through a hole in the primary mirror, just like in an SCT. Maksutov-Cassegrains can provide excellent images, but the f/ratio is usually too slow to use for deep-sky astrophotography.
Hyperbolic Astrographs offer extremely fast optical systems as fast as f/2.8 by combining a corrector lens group made of ED extra-low dispersion glass and a hyperbolic primary mirror. The scope is in a Newtonian configuration so a secondary mirror is used to send the focal plane out of the side of the tube near the top.
Cassegrain - Newtonians have interchangeable secondary mirrors that allow switching between a Newtonian configuration where the image comes out of the side of the tube or a Cassegrain configuration where the image comes out of the back of the tube through a hole in the primary mirror. The Cassegrain setup gives a much longer focal length in a slower optical system.
Tips on Buying a Telescope
Stay away from any telescope that brags about it's power or magnification on the box.
Stay away from any telescope that you can buy in a department store.
Remember that you get what you pay for.
The old cliché in astronomy is that there is no substitute for aperture, but this is not necessarily true for astrophotography. With good optical quality you can do a lot in astrophotography with a modest aperture.
There is no substitute for optical quality. The optical quality of a telescope is one of the most important attributes of a telescope. The biggest and fastest telescope won't work very well if its optical quality is not very good.
The mount is as important as the scope.
Astrophotography can get to be as expensive a hobby as you want to make it. Some people spend, literally, hundreds of thousands of dollars on prime real estate in Arizona and New Mexico and build completely automated remote observatories and then stay in the comfort of their homes and download images over the internet. For those with the resources to do it, the sky is the limit on how much you can spend.
You don't have to spend a fortune to have a million dollars worth of fun in the hobby of astrophotography. Modestly-priced equipment can take images that will give you immense satisfaction and provide years of fulfillment and enjoyment.
Telescopes for Astrophotography - 60mm to 85mm Aperture
Telescopes for Astrophotography - 90mm to 110mm Aperture
Telescopes for Astrophotography - 125mm to 140mm Aperture
As you can see, I admit to being prejudiced in favor of refractors. They are excellent for astrophotography. The lens is permanently mounted and usually does not require collimation. Refractors cool down quickly compared to Newtonians and other type designs. The tube is closed so that thermal currents are never a problem and dust is kept to a minimum in the optics and in the camera.
Most experienced astrophotographers find that a good 4 or 5 inch apochromatic refractor is an excellent choice for deep-sky astrophotography because of its versatility. Such a scope can keep you busy for a long time and last a lifetime.
For beginners, a small 66mm, 70mm or 80mm refractor is an excellent choice. A small telescope can more easily be mounted on a less-expensive equatorial mount, and because it has less magnification, problems are not magnified as much either.
Astro-Physics makes some of the best refractor telescopes on the planet, but they are difficult to obtain, with several years required on a waiting list to obtain one new from the manufacturer. Unlike most other scopes, they almost always appreciate in monetary value over time. They can sometimes be found used on Astro-Mart.
Other excellent manufacturers of refractors for astrophotography are Thomas Back, Takahashi, Telescope Engineering Company and Tele Vue.
Other Telescope Designs for Astrophotography
Of course, it is possible to take astrophotos with other types of telescope designs. A Newtonian with a coma corrector can be an excellent low-cost entry into the hobby. Astro-Tech's 8 inch f/4 imaging Newtonian, or Orion's 8 inch f/4 Newtonian are inexpensive scopes for imaging if you already have a good mount that can handle their 800mm of focal length.
The Boren-Simon PowerNewt Astrographs are very fast Newtonians with a 4-element optical corrector incorporated into the focuser that provide a coma-free flat field at f/2.8.
High-end designs, such as Ritchey-Chretiens, are designed especially for astrophotography. Previously, these types of scopes were very expensive. However Astro-Tech and Orion have both brought in Chinese-made Ritchey-Chretiens that are reasonably priced. Orion sells a 8-inch f/8 RC. Astro-Tech offers a 8-inch f/8 RC, and a 10-inch f/9 RC. Ritchey-Chretiens, while having a well-corrected field for coma and astigmatism, do not have a flat field. You will usually have to use a field-flattener for optimum performance in the corners of the field.
Celestron's Edge series and Meade's ACF (Advanced Coma Free) series are new variations of the traditional Schmidt Cassegrain designs that are more corrected for a flat field for astrophotography. Celestron uses a Aplanatic Schmidt design to achieve a flat, coma-free field. Meade uses a hyperbolic secondary mirror with a corrector-lens-and-spherical-primary-mirror in a combination that performs as one hyperbolic primary.
The problem with Schmidt-Cassegrains and Ritchey-Chretiens is that they are very slow photographically and have long focal lengths and require long exposures, which, in turn, require better mounts. SCTs and RCs are not really recommended for beginners in astrophotography unless you already own one. The long focal length in these scopes will magnify every problem that you will have in terms of mount tracking accuracy and stability.
Another consideration for a beginner in astrophotography is that with short focal length instruments of about 500mm and less with a focal ratio of f/6 or faster can be used unguided initially. Most decent mounts will allow exposures of several minutes with a small refractor, so multiple exposures can be stacked together to improve the images quality by improving the signal-to-noise ratio. Some exposures may be trailed, but a sufficient number of usable frames can usually be made without guiding, an important consideration for beginners.
Another unique design is the Takahashi Epsilon 180ED Hyperbolic Astrograph which has an very fast system speed of f/2.8 and a focal length of 500mm. But this expensive scope can be difficult to collimate and hold collimation.
Boren-Simon PowerNewt Astrographs are a type of Newtonian that is specialized for astrophotography. They realize a large aperture with a Newtonian optical configuration with a 4-element corrector that provides a very fast f/2.8 focal ratio and a flat coma-free field of view. These astrographs are available in 6, 8, 10 and 12-inch apertures.
Eyepieces for Visual Observing
Finder - A finder is a small telescope or aiming device with a wide field of view that is used to aim the main telescope to help find celestial objects.
Barlow - A Barlow lens is a lens that multiplies the focal length of the optical system, giving more magnification and increasing focal length, but at a slower system speed. For example, a 2x Barlow will make turn a 1,000mm f/8 optical system into a 2,000mm f/16 optical system.
Focal Reducer A focal reducer, or telecompressor, is a lens assembly that shortens the length of an optical system making it faster and giving a wider field of view. For example, a 0.75x telecompressor will turn a 1,000mm f/8 optical system into a 750mm f/6 optical system.
Field Flattener - A field flattener takes a curved focal plane and makes it flat. A field flattener is a very useful accessory for astrophotography because most optical designs do not have a flat field. This includes refractors and Schmidt-Cassegrains.
Coma Corrector - A coma corrector corrects for the optical aberration of coma, which gets worse near the edges of the field, in fast optical systems. It is very useful in fast Newtonian optical systems.
Minus Violet Filter - A yellow filter which filters out violet and blue light in a refractive optical system that is not perfectly corrected for color. It is very useful in achromatic doublets and other non-apochromatic refractors.
Recommended Optical Accessories
Except for the AT65EDQ, which has a flat field by design, you will need a field flattener for your refractor. A focal reducer is also nice to have as it gives you, essentially, two telescopes for the price of just the reducer.
Tube Rings - Some telescopes come with a way to attach them to a telescope mounting, such as a simple 1/4 - 20 socket for a tripod, or a small dovetail bar that mates with a particular mount.
Larger telescopes generally don't come with tube rings, so you'll need to purchase these separately to put the scope on the mount.
Dovetail Plates The tube rings can be mounted onto a dovetail plate for easy removal of the rings and scope from the mount by use of a dovetail saddle.
There are different systems for forming an image at the focal plane of the camera.
In all of these systems, focus can be accomplished with the methods outlined in the section on focusing.
Direct Objective / Prime Focus
Direct Objective System / Prime Focus
In this system, light is focused directly on the sensor from the telescope objective or camera lens. This is usually called "prime focus".
For prime focus photography through a telescope, the camera is connected directly to the telescope, with no lens on the camera, and no eyepiece in the telescope. The telescope takes the place of the camera lens.
In prime focus photography, the focal length of the telescope simply becomes the equivalent focal length of a camera lens. For example, my 130mm aperture refractor is f/8 and has a focal length of 1040mm, so the telescope can be thought of as a 1040mm f/8 camera lens.
Focal Reducer / Telecompressor
A focal reducer / telecompressor makes the focal length of the telescope shorter, and the field of view wider. It also makes the focal ratio faster.
For example, with a 0.8x focal reducer, my SV70ED f/6 refractor, with a focal length of 420mm, becomes f/4.8 with a focal length of 336mm.
In this system a positive lens group is used in between the telescope and the camera. No eyepiece or camera lens is used.
Magnification is determined by the spacing of the focal reducer and the sensor.
An Afocal Setup
This setup consists of a telescope with an eyepiece focused at infinity, combined with a camera with the lens on it also focused on infinity. Afocal setups are usually used to produce more magnification in an optical system. This also makes the equivalent focal ratio slower and the equivalent focal length longer.
The camera is held up to the eyepiece and the photograph is made. Care must be taken to hold the camera so that the sensor plane is perpendicular to the optical axis and it does not move during the exposure. The camera should be at the focal length of the camera lens away from the eyepiece so that the exit pupil of the scope is located at the iris diaphragm of the lens.
For long focal length eyepieces, it may be a problem making sure the eyepiece is focused on infinity because of the eye's accommodation. To solve this problem, a finder scope or binocular can be focused on infinity first, and then held up to the telescopes eyepiece. The telescope's eyepiece is then focused through this auxiliary instrument.
Positive Projection (Eyepiece Projection)
Eyepiece Projection Adapter and Eyepiece
Here an eyepiece is used in the telescope but no lens is used on the camera. The eyepiece projects the image directly onto the sensor. This produces more magnification, a smaller field of view, a longer equivalent focal length, and a slower equivalent focal ratio.
Special adapters are sold for this setup ensuring the sensor is perpendicular to the optical axis. Usually an orthoscopic eyepiece is a good choice for positive projection photography.
In the picture at right, the eyepiece is placed inside of the adapter and secured with the thumbscrew. The 1.25 inch adapter at upper right of the eyepiece-projection adapter goes into the focuser of the telescope. The camera is attached to the T-mount adapter on the other end of the eyepiece-projection adapter.
The amount of magnification is determined by the spacing of the eyepiece to the focal plane of the camera.
Barlow / Teleconverter / Tele-Extender - Negative Projection
Negative Projection with a Barlow
A Barlow, teleconverter, or tele-extender is used to increase the magnification of the telescope. It produces a smaller field of view and slower focal ratio. For example, a 2x Barlow used on a telescope with 100mm of aperture at f/8 with 800mm of focal length will become an f/16 optical system with 1,600mm of focal length.
In this system a negative lens, usually a telescopic Barlow or camera teleconverter, is used in between the telescope and camera. No eyepiece or camera lens is used.
Magnification is again determined by the spacing of the Barlow and the sensor.
*Prices and availability of all items are subject to change without notice by the vendors and manufacturers.