Astrophotography Equipment: Telescopes

This is part of a larger article series on the basics of astrophotography and its subsection on equipment types. In it, we will review the main types of telescopes that a beginner would usually consider, and comment on their suitability for various forms of astrophotography.

Terminology

First, let’s review some of the terms used frequently when discussing telescopes for astrophotography.

Aperture

A telescope’s aperture is the diameter of the main lens or mirror. (This will be slightly less than the outer diameter of the main tube.) Amateur telescopes have apertures from 60 mm to 400 mm or more. Very large apertures are very expensive, and common amateur telescopes will be in the 100 mm to 250 mm range. Also, very large aperture telescopes (e.g. 600 mm Dobsonians) are really intended for visual use, not photography. For practical purposes, let’s assume the limit for photography is the limit that can be carried by a traditional equatorial or fork mount.

Aperture is important for two reasons.

  1. Light Gathering: The larger the aperture, the more light is gathered and concentrated, and the dimmer the target that can be observed. The light gathering power is proportional to the area of the lens or mirror, which depends on the square of the radius, so it increases rapidly with increased aperture. For example, a 200 mm telescope does not gather twice as much light as a 100 mm telescope – it gathers about four times as much.

    Aperture is very important for visual observation (i.e. using your eye and an eyepiece) because it determines what you can see. For astrophotography it is slightly less important, because you can use longer exposures to gather more light — something your eye cannot do. That is why you can take images with a small aperture telescope that show far more detail that you can observe visually with even a considerably larger scope.

  2. Resolution: The diameter of the main lens also determines the resolving power of a telescope – the smallest or closest objects that can be distinguished from one another. For beginner-level astrophotography this is not a critical factor but, as you become more advanced, you may want to image very small objects, or very fine detail on large targets, where the greater resolving power of a larger aperture is important.

As a beginner to astrophotography, you can get good results with any aperture from about 80 mm and up. So you should select an aperture that meets your visual observation needs, and not worry too much about it for astrophotography purposes. Remember that large aperture reduces the portability of your gear, and may also create capacity and balance problems for your mount that make it less stable.

Focal Length

A telescope’s focal length is the distance the incoming light travels from the first lens or mirror to the detector (your eye at an eyepiece, or the film or electronics in a camera). For refractors and reflectors, this is approximately the length of the main tube. For compound telescopes (e.g. Schmidt-Cassegrains) the light travels back and forth inside the tube more than once, so the focal length will be considerably longer than the main tube.

Refractors usually have focal lengths in the 400mm to 1000mm range, Newtonian reflectors in the 1000mm to 1500mm range, and Schmidt-Cassegrains in the 1200mm to 4000mm range.

Focal length determines the magnification of your telescope-camera system. (Note: serious astrophotographers hate the word “magnification” and never use it. We’ll discuss the correct terminology later, but for now this will give you the idea.) Longer focal lengths will give you larger images of given objects, or enable you to image smaller objects. Shorter focal lengths will allow you to image very large objects that may not fit in the field with a longer focal length.

Longer focal lengths are, generally, harder for beginners to use for astrophotography since the long focal length also magnifies the imperfections in your system, especially instability and tracking errors in your mount.

Focal Ratio

Focal ratio is a calculated value: the ratio of focal length to aperture. It is written in the form “f/6” and typical focal ratios are f/5 to f/10. Small refractors tend to be around f/6, while Schmidt-Cassegrain Telescopes tend to be f/10.

Smaller focal ratios (e.g. f/6) are called fast, while larger focal ratios (e.g. f/10) are called slow. These words refer to the photographic exposure times required. All else being the same, a faster focal ratio requires shorter exposure times than a slower focal ratio.

For beginners, then, a faster focal ratio makes astrophotography easier since shorter exposure times are needed, and this reduces problems such as mount instability that are associated with long exposures. f/6 refractors are very popular among astrophotographers.

Field Curvature

Many optical designs suffer from a problem called field curvature, which is illustrated in this diagram. After passing through a lens, or reflecting off a mirror, incoming light is spread out to cover the detector (the camera film or CCD). The detector is placed at an appropriate distance from the lens, to bring the image into focus.

However, the edges of the detector are farther from the lens than the centre of the detector, and the light has to travel farther to get to the edge than it has to travel to reach the centre of the detector. This means that, with such optical designs, you can’t have both the centre and the edges of your image in perfect focus at the same time.

There are three possible solutions to this problem:

  1. Use a curved detector. While some professional observatories do this, it is not practical for the amateur.
  2. Use a different optical system that produces a “flat field” by sending light that needs a little farther to focus at the edges than in the middle. Some telescopes, such as the Televue NP series (e.g. NP-101) do this automatically, but they are very expensive. Or, additional optical adaptors called field flatteners can be inserted in the light path of any telescope.
  3. Ignore it. Make sure your target is in the centre of your image, and ignore or crop away the slightly out-of-focus edges.

#3 is the best option for the beginner: don’t worry about it. You can always add a field-flattener later.

Telescope Optical Designs

Now, let’s review the most popular telescope designs that would be used by a beginner. If you haven’t bought a telescope yet, and are interested in astrophotography, this may help you make your decision. If you have already purchased a telescope, this may help you decide the types of astrophotography for which it is well-suited.

Refractors

To discuss refractors for astrophotography, we need a little more terminology.

Colour Correction

Refractors use lenses to focus light and lenses made from ordinary glass bend light of different colours different amounts. Bright objects (such as the moon, planets, or bright stars) viewed through certain fast refractors will show false colour, usually a violet fringe around the bright object.

Achromatic refractors, or achromats have this problem. (The name, which means “no colour”, is ancient and refers to the improvement this design had over the original simple glass lenses, which were really bad.)

Lenses using multiple layers of different glass alloys can reproduce colours accurately. These are called Apochromatic telescopes, or apos for short.

(There are specific technical definitions of achromatic and apochromatic which have to do with how many different wavelengths of light in the visual spectrum are brought to correct, aligned focus. 2 for achromats and 3 for apochromats. This level of detail is not important for the beginner.)

Apos are significantly more expensive than achromats – hundreds to thousands of dollars more.

Unfortunately the refractor field has become cluttered with marketing terminology and misleading claims. Telescope quality is a function of more than just the kind of glass used – the care taken in lens grinding, mounting, alignment, and other considerations are all a factor. Some companies are managing to take ED (enhanced dispersion) glass which, theoretically, is apochromatic, and produce low-quality telescopes with poor image quality. Others are producing telescopes that are, theoretically, achromats, but whose image quality rivals some apos. It’s not magic – price really is a good predictor of performance, and reading reviews of a scope you are considering is a good practice.

Slow (large focal ratio) refractors suffer much less from false colour, and are another approach to colour correction.

Advantages of Refractors for Astrophotography

Apochromatic or good quality achromatic refractors, with small focal ratios, are favoured by astrophotographers for wide fields and large targets such as large galaxies and nebulae, for several reasons:

  • They have fairly short focal lengths, thus giving low magnification and allowing wide areas of sky to be imaged.
  • The low magnification means that imperfections in mount stability and tracking are less significant.
  • They tend to be fast – f/6 or f/7 – which allows shorter exposures to be used.
  • Small refractors are compact and are easy to mount solidly.
  • Because the lenses on refractors are unobstructed, they provide clear views and good contrast. Because there is no 3- or 4- armed vane holding a secondary mirror, refractors do not produce “diffraction spikes” (see “Reflectors”, below, for a discussion of diffraction spikes). Stars are clear pinpoints in the field.

Disadvantages of Refractors for Astrophotography

Refractors also have some disadvantages:

  • As already mentioned, apochromatic refractors are expensive, and achromatic refractors, while cheaper, can show false colour on bright objects.
  • The low magnification of small refractors means they are not suitable for small targets such as planets without using additional optics to boost the magnification.
  • Large refractors can be very long. E.g. a popular 150 mm-aperture refractor is about 1.5 metres long. This can be a strain on the mount, since the long refractor exerts significant leverage, and is more susceptible to wind.

Reflectors

Newtonian reflectors are also very common beginner scopes, either mounted on traditional mounts such as the one shown here, or mounted in Dobsonian mounts for ease of use and pointing. We’ll talk about using a Dobsonian mount for astrophotography in another section. For now, let’s consider just the optics.

Advantages of Reflectors for Astrophotography

Good-quality reflectors are good astrophotography scopes for a number of reasons.

  • They are cheaper to build than refractors so, for a given price, you get more aperture. As mentioned above, aperture in astrophotography yields higher resolution.
  • Mirrors reflect all colours of light in the same manner, so reflectors do not suffer from the “false colour” problem of some refractors.
  • Reflectors come in a variety of focal ratios, including fast f/5 and f/6 units that yield short exposures for imaging.

Disadvantages of Reflectors for Astrophotography

Beginner-level reflectors also have some disadvantages for astrophotography:

  • The most common problem with reflectors not specifically designed for astrophotography is that there may not be enough travel in the focuser. Once a camera is mounted in place of an eyepiece, it may not be possible to move the focuser far enough in or out (“in” being a bigger problem) to bring the image into focus. This is especially common with large DSLR cameras, since the focal plane is quite a bit farther back than an eyepiece would have been. You will definitely want to check with an experienced user or dealer whether the camera you have in mind can focus with the reflector you are considering.
  • Very low-priced reflectors may not have correctly shaped mirrors. A correctly shaped mirror will be a parabola but these are expensive to produce. Inexpensive reflectors may have mirrors that are spherical instead. These are much easier to produce but don’t reflect light to an accurate focus. Some spherical reflectors add a “corrector lens” to compensate for this, essentially becoming “compound telescopes”, and losing some of the advantages of reflectors.
  • Reflectors need a secondary mirror suspended in the tube near the top. The four thin support arms that hold the mirror cause bright stars in the image to have “diffraction spikes” – 4 thin arms of light that protrude from the stars. You may not find these objectionable – they are quite pretty, and astrophotographs with diffraction spikes are so common that many people think stars are supposed to look that way, and you even see them drawn that way in some art or sketches. Common or not, pretty or not, these spikes are considered an optical aberration, and are not supposed to be there.
  • Field Curvature (see above) and other forms of optical distortion are common in inexpensive reflectors, and can reduce the useful size of the field.
  • The long bodies of reflectors, with most of the weight – the mirror – concentrated at one end, can make mounting them securely, and balancing the mount, more challenging, and the moderately long focal length means that imperfections in the mount will be more noticeable.

Compound / Catadioptric

Catadioptric or Compound telescopes (collectively called CATs) use a combination of lenses and mirrors. One result is that they are much more compact for a given focal length. The common “stubby-looking” telescopes that are so popular as beginner scopes are compounds – usually Schmidt-Cassegrain Telescopes (SCTs) or Maksutov-Cassegrain Telescopes (MCTs or Maks).

Advantages of Catadioptrics for Astrophotography

For astrophotography purposes, Catadioptric telescopes have some advantages:

  • 200 mm SCTs are probably the most popular beginner telescope on the market today. These mean more people getting into Astronomy and attempting astrophotography, and it means there are a large number of accessories suitable for scopes of this class.
  • Mid-sized CATs are compact, which can make them easier to balance on a mount.
  • Since there are no support vanes holding the secondary mirror (it is attached to the back of the front lens, either mechanically or with glue), images taken with CATs have no diffraction spikes.
  • CATs have very long focal lengths for their size. The high magnification that results makes them suitable for planetary photography and for other small targets.

Disadvantages of Catadioptrics for Astrophotography

And, of course, CATs also have disadvantages:

  • While compact, CATs are heavy and, to keep the price of all-inclusive sets down, some CATs are sold with mounts that are barely large enough to hold them steady for visual use, and are not large enough to hold them steady for imaging. The SCT shown above, for example, was sold with a mount (not shown) that was inadequate for long-exposure astrophotography, and the mount had to be upgraded to the heavier one shown as a first step on the road to imaging.
  • The high magnification resulting from the long focal length is often too high – many famous targets, such as the M31 Andromeda Galaxy, are too large to fit in the field of common CATs. (Accessories are available to reduce the focal length.)
  • Most CATs are slow – with a focal ratio of f/10 being very common. The slow focal ratio means longer exposure times are needed.
  • The long exposure time combined with the high magnification means that long-exposure imaging through a CAT is very challenging. Every minor alignment, tracking, and stability imperfection in the mount shows up in the image. Experienced astrophotographers produce stunning images with long-focal length CATs, but it is hard.
  • Most CATs are focused by a knob that moves the primary mirror. For astrophotography this is problematic for several reasons:
    1. It is hard to fine-focus in this way. The focus knob can usually turn through a dozen revolutions or more, but the area of focus of interest for astrophotography may be only 1/8 of a turn. Combined with backlash in the focus mechanism, very fine focusing can be quite difficult.
    2. Since the mirror must be moveable, it can move at unwanted times. This is especially common if, during a long exposure, the scope crosses the meridian – i.e. it goes from leaning toward the East to leaning toward the West. Mirror motion at this time is called “mirror flop” and can ruin an image. (Some larger CATs have a control to lock the mirror in place once focused.)
    3. Moving the mirror also moves the image. The amount of motion is not noticeable when using the telescope visually, but in astrophotography, changing the focus may move the target entirely out of the camera’s field of view.

    Adding a separate focuser can resolve problems #1 and #3. Problem #2 is generally dealt with by paying careful attention to where the balance point of the telescope is and ensuring that no shift happens during an exposure.

Astrographs

You may see the term Astrograph in advertisements for telescopes. This isn’t a specific type of telescope. An Astrograph is any telescope that has been optimized for astrophotography. Features would typically include:

  • Optical design that produces a large field (suitable for large CCD chips) that is free of common aberrations such as coma and field curvature.
  • Wide field of view.
  • Fast focal ratio.
  • Specifically designed to hold cameras – may not support visual observing with an eyepiece at all.
  • Compact, well-balanced design for ease of stable mounting.
  • Fine focus capability.
  • Designed to prevent motion of optical components (e.g. mirror flop).
  • Electronic assistance for things such a focusing and temperature control.
  • Standard connection points for easily attaching guide scopes and other accessories.

Astrographs are special-purpose instruments, not really intended for beginners or for visual use, and I won’t be discussing them further in his article.

Summary

As you can see, there are a large number of factors to consider in selecting a telescope for astrophotography, and there is no clear winner. Any scope can be made to work, for any kind of astrophotography; but some combinations are much less problematic than others.

Click the table to enlarge it to readable size.

For a beginner who has not yet picked a telescope, or who has not yet picked a camera, we can recommend an ideal scope for different types of imaging targets. This may help you decide where to start. When more than one scope is listed, my primary recommendation is first, with the second being a good alternative.

Target Recommended scope Reason
Moon Refractor (apo or good achro) or Reflector Good low and medium power
Bright Planets Catadioptric or Reflector Good high power
Outer Planets Catadioptric High Power & light gathering
Large DSOs (M31, large nebulae) Refractor Low power, ease of use
Small DSOs (M57, small galaxies) Reflector or Catadioptric Power & light gathering

The next part of this overview of equipment types deals with cameras for astrophotography.

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