Differences in size and resolution of details can be seen in these shots of Jupiter taken with a Stellarvue SV70ED refractor (left) and a Celestron C11 Edge SCT (right). The SV70ED has just 70 mm of aperture and 1,750 mm of focal length when working at f/25 with eyepiece projection. The C11 has 279 mm of aperture and 8,100 mm of focal length when working at f/29 with a Barlow lens. The larger scope produces a much larger image with much smaller details resolved on the planet.

There are three main attributes that describe telescopes: aperture, focal length, and focal ratio.

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  Aperture - The aperture is the size of the lens or mirror that gathers light in a telescope. The main light gathering device in a telescope is called the objective. Aperture is measured in inches or millimeters. Larger apertures gather more light and give a brighter image. Larger apertures also will resolve finer details on the Sun, Moon and planets, if the seeing allows.

• Focal Length - The focal length of a telescope is the distance from the objective lens or mirror to the focal plane where the light comes to focus. The longer the focal length, the larger the image is that forms at the focal plane. The longer the focal length, the higher the magnification of the telescope. Focal length is also measured in inches or millimeters.

  A long focal length and more magnification are good for small objects like the planets, but we can usually increase the native focal length of our telescopes to whatever we need by using a Barlow or eyepiece projection.

• Focal Ratio - Focal ratio is defined as the telescope's focal length divided by its aperture.

  For example, a telescope that has a focal length of 2,000 mm and an aperture of 250 mm has a focal ratio of 2000/250 = 8. We also refer to the focal ratio as the "f-number". This telescope would have a focal ratio, or f-number, of f/8.

Scopes for Planetary Photography

You can use almost any telescope for planetary photography. You would be amazed at what you can record with just a very modest instrument. On the other hand, if the seeing conditions allow, you will record more detail with a large telescope.

Most observing locations don't have world-class seeing. Most usually have seeing that allows the resolution of detail down to a couple of arcseconds on a good night. You don't need a gigantic telescope to record this kind of detail.

If you are fortunate to live somewhere like Barbados, or the Florida Keys, where you can get exceptionally good seeing, then you are going to need a lot of aperture in a scope with outstanding optics to take advantage of that kind of seeing. This kind of optical quality is rarer than you might think.

As scopes get bigger, they are harder to make with excellent optics. Even if you get a scope with great optics, their actual performance doesn't follow a straight line in accordance to what theory would predict for what they can resolve. This is because larger scopes are more influenced by seeing, and it is harder to keep them collimated and cooled to ambient temperatures because of the large mass of their primary mirrors.

Generally, for most observing locations, 8 to 12 inches of aperture is the most you will need to do good high-resolution planetary work. This doesn't mean you can't do any planetary photography with smaller 4 to 6 inch telescopes. You most certainly can! To take full advantage of locations with outstanding seeing, you need 14 inches of aperture or more.

Scope Considerations

• Optical Quality - For the highest-resolution planetary work, optical quality is very important.

  Dobbins, Capen and Parker, in their book Observing and Photographing the Solar System, note that the Rayleigh specification that was historically adopted for a "diffraction limited" mirror was that it have a figure accurate to 1/4 of a wavelength of light. However, they also note that, at 1/4 wave, 68 percent of the energy goes into the Airy disk and 32 percent into the rings. This, unfortunately, causes a 50 percent loss in contrast. They say that a mirror that is accurate to 1/20th of a wave has a loss of contrast of only 6 percent.

  Most mass-produced telescopes sold these days are of decent, but not exceptional, optical quality. As with most things, you get what you pay for.

• Field of View - Field of view is dependent on focal length and physical size of the sensor. Except for the Moon, having a small field of view is not critical because the planets are small.

• Image size - Image size is dependent on focal length. For the planets, we usually have to magnify the image with a Barlow or eyepiece projection to get a larger image size so we can record small details.

• Effects of a Central Obstruction - Central obstructions cause diffraction. This causes a loss of contrast in an image which will hurt in the recording of small low-contrast details on the planets. The larger the central obstruction, the more diffraction and loss of contrast.

  Rutten and VanVenrooij, in their book Telescope Optics - Evaluation and Design, say a 25 percent obstruction causes a 15 percent loss of contrast, and that a 50 percent obstruction causes a 50 percent loss of contrast.

  As a very general rule of thumb, to get a ballpark feeling for how much effect this is, subtract the size of the central obstruction from the aperture of the telescope. In other words, a 12-inch telescope with a 2-inch central obstruction will perform approximately like a 10-inch unobstructed telescope.

• Resolving Power - The resolving power of a telescope describes how well small details can be recorded. Larger apertures will resolve smaller details if the scope's optical quality is excellent and if the seeing allows. But the effects of a secondary obstruction also come into play for the resolution of low-contrast planetary detail.

Lets look at the strengths and weaknesses of some of the most popular telescope designs for use in planetary photography.

Optical Designs

Newtonians

A well-made Newtonian with good optics offers the biggest bang for the buck in most types of visual astronomy as well as planetary photography. The price-performance ratio can be outstanding with a Newtonian telescope.

Newtonian telescopes are perfectly apochromatic and do not suffer from any false color.

Some Newtonians however, made primarily for visual observing, will not reach focus with a camera at prime focus because they don't have enough "back focus". Back focus is the distance from the telescope to where the focal plane is. Not enough back focus means you can't rack the focuser into the tube far enough to reach focus. These scopes were designed for eyepieces to come to focus, and most cameras need a couple of inches more to reach focus.

If your camera won't come to focus at prime focus with your Newtonian, you can move the mirror up in the tube a couple of inches. Carefully determine how much extra distance you need before drilling holes in the tube. If you have a truss-tube design, you can also try shortening the poles.

Be aware that there are compromises involved in moving the mirror up so your camera can reach focus.

Moving the mirror up in the tube, or shortening truss-tube poles will cause the eyepieces to focus farther out, so you may need an extension tube for them to reach focus.

Another consideration is that the diagonal mirror may no longer fully intercept the entire light cone from the primary mirror. A larger secondary will solve this problem, but a larger secondary will cause more diffraction. You generally want a smaller secondary for planetary work anyway, so you should be able to use the original secondary. A loss of illumination around the edges of the field is not that critical for planetary work, although you may see it on lunar and solar images if it is severe.

If your camera won't come to focus and you don't want to move the mirror up, you might be able to reach focus with a Barlow lens. This will give you more magnification, which is usually a good thing if you are trying to do high-resolution planetary work.

The secondary mirror in a Newtonian, and the secondary supports, will cause diffraction. The larger the obstruction, the more diffraction, which will reduce contrast in fine planetary detail. Newtonians can be designed with slow focal ratios which allow a small secondary obstruction and large well-corrected field and a longer focal length with more magnification.

Newtonian telescopes require frequent collimation, although this is not difficult to do.

Newtonian telescopes must be acclimated to the ambient environmental temperature or they will not perform at their best. A small fan blowing on the back of the primary mirror usually helps tremendously in this regard.

A Newtonian telescope with good optics and a small secondary mirror can make an outstanding planetary telescope.

Refractors

Inexpensive doublet achromats usually have a lot of false color on bright objects like the Moon and planets. They can be used for planetary photography with a minus violet filter.

Many modern inexpensive, small-aperture, refractors have fairly fast focal ratios. This means they don't have a lot of aperture or focal length. They are great for shooting full disk images of the Sun (properly filtered) and the Moon, but are not going to be able to capture the finest small details on Mars or Jupiter.

Larger high-end triplet apochromatic refractors can have excellent optics. Scopes in the 4 to 6 inch range will do an exceptional job on the Sun especially because daytime seeing rarely allows the high resolution that would require more aperture. While these scopes can be outstanding, they are also exceptionally expensive.

Refractors generally do not require collimation. Their closed tube design helps minimize internal tube currents, and they usually cool down to the environmental ambient temperature fairly quickly.

Schmidt-Cassegrains

Schmidt-Cassegrain Telescopes, also known by the acronym SCTs, are a good compromise design that combines a reasonable amount of aperture in a compact tube.

SCTs usually have decent optics and can take some good high-resolution planetary images because they have a slower focal ratio with a longer focal length. This produces a good image size on smaller objects like the planets.

The secondary mirror in an SCT is usually much larger than in a similarly sized Newtonian, causing more diffraction, which is not optimum for high-resolution planetary work. Some excellent high-resolution planetary images have been taken with SCTs however.

A bigger problem with SCTs is that any defects in the quality of the image produced by the primary mirror is amplified by the magnification factor of the secondary mirror. A normal SCT's secondary mirror usually has an amplification factor of around 5x. So to maintain 1/4th of a wave optical quality at the focal plane, the primary must be accurate to 1/20th of a wave. This is difficult to accomplish in commercially-made mass-produces telescopes. Additional degradation in the image may also be introduced by the figure of the secondary mirror and corrector plate.

A huge problem with using SCTs for high-resolution planetary work is that the primary mirror in most stock SCTs is very difficult to cool down to ambient temperature because it is in a closed tube and the mirror has a lot of mass. Cooling down the scope and mirror to ambient temperature is critical for high-resolution planetary imaging.

Because most SCTs focus by moving the primary mirror, the image can shift a lot when focusing. At high magnification, it can shift the image of a planet right out of the field of view. A Crayford focuser can solve this problem along with locking down the mirror.

Focus in an SCT can also change because the tube contracts when the temperature drops. This can actually happen with any telescope, but the effect is magnified by the power of the secondary mirror in an SCT.

SCT corrector plates are prone to dewing in humid observing environments. Since the corrector is at the top of the tube exposed to the sky, it is very easy for it to radiate all of its heat and drop below the dewpoint, and then dew will form on it. A long dewcap is an absolute necessity in humid conditions and not a bad idea anyway to keep stray light out of the scope that could hurt image contrast. On large SCTs, once the entire scope has reached thermal equilibrium, it may also be necessary to use a heating element for the corrector plate, in addition to a dewcap, to prevent dewing.

SCTs must be collimated for critical high-resolution planetary work, usually before each imaging session for the highest quality results. This is something that most SCT owners do not appreciate the importance of.

Other Designs

Other telescope designs, such as Ritchey-Chrétiens, classical Cassegrains, Maksutovs, Maksutov-Newtonians, Schifspeiglers, and such, can, of course, all be used for planetary photography.

Each has its own considerations and peculiarities, but these scopes are not in as wide use as are Newtonians, SCTs and refractors.

Telescopes vs Camera Lenses

Telescope and camera lenses are very similar. They all have apertures, focal lengths and focal ratios.

The nomenclature used, however, can be confusing. Camera lenses are usually referred to by their focal length, usually in millimeters. A 200 mm camera lens has a focal length of 200 mm. Telescopes are usually referred to by their apertures. A 200 mm telescope would have an aperture of 200 mm.

Camera lenses usually also reference the maximum focal ratio. For example, you may have a 200 mm f/2.8 lens. That means the maximum focal ratio is f/2.8.

Aperture is usually never referred to for camera lenses. This is because almost all camera lenses have a diaphragm or iris that can be stopped down, changing the effective aperture of the lens. When a lens is stopped down like this, its focal ratio also changes.

Telescopes are very rarely stopped down. They are usually used wide-open at maximum aperture. The focal length and focal ratio are normally fixed also. Someone may say they have a 6 inch f/8 telescope. It is easy to figure out the focal length, which is just the aperture multiplied by the focal ratio. So a 6 inch f/8 telescope would have a focal length of 48 inches.

While telescopes are not usually stopped down, their focal lengths may be increased by using a Barlow lens, or eyepiece projection to provide more magnification. Keeping the aperture fixed while increasing the focal length will also increase the focal ratio. If we double the focal length of a 6 inch f/8 telescope with 48 inches of focal length with a 2x Barlow to 96 inches of focal length, the focal ratio will increase to f/16 (f/8 * 2 = f/16). Doubling the focal length, and increasing the focal ratio by two stops from f/8 to f/16, requires 4x more exposure.

A good telescope will usually out-perform a good camera lens for astronomical objects. Telescopes are made to be diffraction limited at infinity for a smaller field of view than most camera lenses cover. Camera lenses are compromise designs that have to work at a variety of subject distances with well-corrected large fields.

There really isn't much choice between a telescope and a camera lens for high-resolution planetary photography where we need a lot of aperture and focal length to resolve fine planetary detail. A good telescope will win every time. On the other hand, for wide angle scenics, only a camera lens will work.