Introduction to Astrophotography: Telescopes
Astronomy and astrophotography really boil down to two things: The object you’re trying to photograph and the tools you use to see it. The tools include your telescope, tripod and mount, camera and the software you use to capture the image and process it. Your technique and experience will make a difference in how well your final photos turn out. While you might get lucky with quick choices, success often is determined by thoughtful choices and diligence in their use.
If you’re interested in taking shots of huge swaths of the night sky a DSLR (digital single lens reflex) camera with a wide-angle lens will work just fine. The Milky Way is a perfect example of this type of shot. My friend Adrian Bradley took the photo below of the Milky Way from Michigan’s Lake Hudson State Park with his DSLR on a tracking mount:
If you want to take pictures of smaller chunks of the sky, you’ll want to “zoom in” to make these objects fill your field of view (FoV) as best you can. Telescopes take the place of the camera lens in this situation. With that said, all types of telescopes will do this basic zooming, but there are pros and cons with each type. Picking the right tool for the job will get you the results you’re after.
There are a number of terms and concepts that help to describe the capabilities and limitations of each design, so I’ll be introducing those along the way in the description of each type of telescope.
Refractors
A refractor telescope is the one that everyone thinks of when they think of a telescope. Galileo invented the refracting telescope in 1609 and spent a lifetime discovering objects in the heavens. With his refractor, he researched the Moon, discovered Jupiter and its moons, and discovered the rotation in the Sun. A refracting telescope is the simplest design, and I’ll use it as a nice example to explain some of the characteristics common to all types of telescopes.
All telescopes have an eyepiece at one end. In refractors, the big end of the optical tube has one or more lenses that are referred to as the objective lens. When there is more than one lens, they are usually fused together for better optical qualities. The size, referred to as the aperture, and the shape or curvature of this lens will determine the view you get. You can think of a telescope as a light bucket, where the bigger the aperture is, the more light you’ll catch in the bucket.
The curvature of the objective lens will determine how wide the swath of sky is that the telescope can collect. A wide-angle lens might capture 65 degrees or more of the sky, while a more focused lens might only deliver a few degrees, which is a very tiny patch of the sky. Most telescopes have relatively narrow views of less than 3 degrees.
Light comes in through the objective lens and is focused near the eyepiece, which then refocuses and sizes the image so that it projects onto your eye.
Refractors come in 2 styles, long and short, which is referred to as the focal length of the telescope. In all telescopes, the shorter the tube, the less distance light has to travel, and therefore, the quicker the system delivers light to your eye or camera sensor. This tube length, when divided by the aperture size, will give you the focal ratio of the telescope. For example, if a telescope that is 2000mm long has a 200mm objective lens, it has a focal ratio of f/10. The smaller the focal ratio, the quicker the telescope delivers the image to your eye or camera sensor. This applies to ALL types of telescopes. This speed is a big factor in how long you have to expose your camera sensor to get a usable image, or if your eye will be able to see it at all!
Short-tube refractors are considered fast with focal ratios less than f/6, and have wider views of the sky than other types, due to the curvature of the objective lens needed to focus its light in that short distance. Short-tube refractors are usually used for imaging larger structures in the sky (wider field of view) or very dim objects (fast focal ratios). Conversely, longer-tube refractors with slower focal ratios (f/8 and above) will excel at getting detailed images of planets, the moon and the brighter, smaller deep-space objects (DSOs).
Refractors tend to be relatively heavy for their size, and have a practical size limit of around 6" in aperture. The cost of the lens increases dramatically as the size increases. In spite of these deficiencies, refractors are virtually maintenance-free and very durable.
As light is being bent as it passes through the glass lens in a refractor, blue hued colors do not match up with other colors and produce a side-effect called chromatic aberration. This is a fancy way of saying you get blue or purple halos around bright objects. To correct this problem, designers use a third lens in the objective (known as a triplet) and make them out of more exotic low-dispersion glasses. This eliminates the problem, but as you might suspect, it adds significantly to their cost.
Whew! Still with me? It gets easier from here on out!
Reflectors
A reflector telescope uses mirrors to focus and bend the incoming light to your eyepiece or camera sensor. It was first invented by John Gregory in 1663 independent of the more widely attributed invention of Issac Newton in 1668. Modern reflecting telescopes are known as Newtonian. The aperture in a reflector refers to the size of the main convex mirror that directs incoming light to a flat secondary mirror which directs the light to your eyepiece.
As with a refractor, a reflector telescope has a very direct path to the eyepiece, which is mounted on the side of the telescope tube. From the refractor discussion above, you’d be correct in guessing that reflecting telescopes have relatively low focal ratios and are considered fast telescopes. Additionally, no light is lost when it passes through the telescope (since it has no lenses internally) until it gets to the eyepiece. Both of these attributes make them excellent at observing or photographing dim objects in the skies.
One of the main advantages of reflecting telescopes is that mirrors are much cheaper to manufacture than lenses, so it’s possible to purchase very large apertures (up to 24" mirrors!) in a telescope without having to sell your house.
On the downside, reflecting telescopes need to have their mirrors aligned (collimated) frequently, or the image quality suffers. The design is generally considered to be less durable than refractor or Cassegrain types of telescopes.
The thin supports that carry the secondary mirror can produce optical effects in brighter stars that appear to twinkle, which are known as diffraction spikes. Reflectors tend to be very bulky to transport, even though they’re not particularly heavy compared to other designs. Because the mirrors are out in the open, they are subject to dust, dirt, dew and other moisture and require periodic cleaning.
Cassegrains
Cassegrain telescopes are hybrid designs that combine mirrors and lenses. The first notion of these hybrid designs is attributed to Laurent Cassegrain in an excerpt from a letter that was published in 1672. They deliver apertures larger than refractors (commonly up to 12") and use mirrors to fold and focus the light resulting in a very compact design, often not much more than 18" long. Focal lengths aren’t considered fast (typically f/10 to f/16), but reasonable enough that with accessories like a focal reducer, can change an f/10 to an f/6.3, giving you roughly a 35% improvement in speed and field of view.
There are 2 main types of Cassegrains: the Schmidt and the Maksutov (“Mak”):
- Schmidt types are lighter and can be made in larger sizes than the Maksutov, but since they have a secondary mirror, require occasional collimation.
- Maksutov types are often heavier and generally don’t come larger than 5–6 inches in aperture. They do not require collimation, since the objective lens has a fixed secondary mirror on the rear surface of the objective lens, so there is nothing to adjust. Consequently, it is also relatively complex to grind and therefore more expensive. Maks have slower focal ratios and have a relatively small aperture for their focal length.
Cassegrains are popular because they’re a good all-around telescope. As you might expect in a hybrid design, they borrow characteristics of both refractor and reflector telescopes. They don’t excel in any one area, but neither are they poor performers in any area. They tend to have around 1 degree fields of view, so you often need a focal reducer to get wider or faster views. They offer larger apertures without the higher maintenance of a reflector.
Pros and Cons
I’m sure you all were taking notes as you were reading, but if not, here are some handy pro/con lists.
Refractors
- Pro: Rugged, no collimation necessary
- Pro: Fast focal ratios
- Pro: Short-tube varieties offer wide fields of view and even faster focal ratios
- Con: Chromatic aberration (color distortion at the edges)
- Con: Relatively expensive ($/inch-of-aperture) due to size of objective lens
- Con: 5" practical maximum aperture
- Con: Long-tube refractors can be very long, heavy and cumbersome
Reflectors
- Pro: Inexpensive
- Pro: Time-tested design (since Newton)
- Pro: Fast focal ratios
- Pro: Great for DSOs
- Pro: Apertures get really big! (24” or more)
- Con: Bulky to transport
- Con: Diffraction spikes
- Con: Periodic collimation required
- Con: Periodic cleaning of primary and secondary mirrors required
- Con: Less durable than other designs
Cassegrains
- Pro: Inexpensive for under 12” apertures
- Pro: Long/slow focal ratios
- Pro: Great for planetary imaging (Sun, Moon, planets, bright DSOs)
- Pro: DSOs bring extra challenges due to field of view and focal ratio
- Pro: Compact compared to refractors of the same size
- Con: Somewhat heavy and bulky
- Con: Get expensive after 8”
- Con: Maximum practical aperture of 12"
- Con: Schmidt cassegrains need occasional collimation
Conclusion
Whew! I’ll admit that this article went longer than I’d have hoped, but splitting the various types into separate articles seemed like a bad idea since many of the concepts are used for all 3 types.
Hopefully, you gained some insight into the different characteristics and capabilities of each type of telescope, so that when you want to look at Jupiter or the Orion Nebula, you’ll have some idea of which tool to pick out of your toolbox!
The next article will deal with the different types of telescope mounts, which I promise will be shorter!
