Astrophotography: Fundamental Questions

This is part of a larger article series on the basics of astrophotography.

Introduction

There many ways to approach astrophotography, many types of equipment, and many techniques. Which should a beginner use? The answer (which may not seem helpful at first) is, “it depends”.

Depends on what?

Many things: what kind of targets you would like to photograph, what kind of results you want, how much you are willing to spend, how much automation you need vs. how much manual work you are prepared to do, and many other factors.

On this page, we’ll have a quick look at some various options for capturing images, then focus on one way to divide the world of options in half, simplifying the decisions you are facing.

With or Without Telescope?

You can do astrophotography with normal camera equipment and without a telescope, and can produce some very attractive images. Many books recommend you start with one of these options because they reduce the complexity and lower the learning curve. We’ll quickly review a couple of these options here, but won’t dwell on them since this series is primarily about astrophotography through a telescope.

Without a Telescope

Star Trails

If you have a tripod and any camera capable of taking long manually-set exposures (several minutes or more) you can produce a unique kind of image called star trails. Most modern DSLRs are suitable for this. Such an image shows a broad field of sky that includes the North (or South) Celestial Pole. For people in the Northern hemisphere, this means the image includes the sky around Polaris, the North Star. The images will show many semicircular arcs centred on the celestial pole, giving the impression of a large number of concentric circles in the sky.

The technique is simple. Mount a camera on a sturdy tripod, and take a long exposure with the camera pointing toward the celestial pole (near Polaris). The longer the exposure the better but an exposure of at least several minutes is needed. Use your widest-angle lens, and set the focus to “infinity”.

The rotation of the earth makes the sky appear to rotate around the North Celestial Pole, but your camera is stationary, so the result is that all of the stars appear to trace circular paths in the image. The long exposure will also make nearby objects visible, even in the dark of night, so the most attractive star trail images have something interesting in the foreground – typically a tree line, hills, or some other reference. You will need to experiment with the aperture (f-stop) setting of your camera and the sensitivity (ASA or ISO) setting to find values where a long exposure does not over-expose the sky to the point where the stars are washed out.

Constellations and Milky Way

With the same equipment used for star trails, you can also take moderately-long exposure photos of the sky, capturing constellations or the milky way. With a wide-angle lens, you should be able to take 30- to 60-second exposures without getting any noticeable streaking of stars, and this is enough to capture the brighter constellations, or the milky way if your skies are very dark. Much longer exposures will begin to show star trails and would require mounting your camera on a motorized mount, just like a telescope.

Telephoto Lens

You may also consider attaching a normal telephoto lens to your camera. This will work well and is, in fact, optically the same as attaching the camera to a telescope of the same focal length. For example, many imagers use a short-focal length telescope with a focal length of 480mm. Attaching a traditional 400mm or 500mm telephoto lens to your camera is, in theory, equivalent. However, there are some additional considerations:

  • The telescope may be “faster” than an equivalent telephoto lens. e.g. the popular 480mm telescopes are f/6, while a long telephoto lens, unless it is huge, may be f/8. The slower lens will require longer exposures.
  • Without the attached finder accessory of a telescope, you will find it very difficult to find dim targets with a camera and telephoto lens. The viewfinder is generally too dim to see faint objects. On the other hand, it will work quite well for the moon.
  • Focusing a camera and telephoto lens on a star field is difficult. Your camera’s auto-focus feature will not work on dim objects like stars – you will have to focus manually. (Of course you also have to focus a telescope manually, but if you thought an “auto-focus” telephoto lens would eliminate this step, it won’t.)
  • Telescopes have fewer moving parts than camera lenses, and will stay focused where you set them. The focus or zoom settings on a camera lens may move, causing your photo to blur.
  • Just like a telescope, long exposures through a telephoto lens will require a motorized, stable, tracking mount.

So, although a telephoto lens on a DSLR camera can be made to work quite well, it is not a short-cut that avoids the complexities of astrophotography. You still need to solve the same problems of mount stability, tracking, finding targets, and focusing.

With a Telescope

There are many good articles and books on the subject of doing astrophotography without a telescope – star trails etc. However, I assume you are reading this series of articles because you are interested in taking images through a telescope, so the rest of this series will focus exclusively on that.

Film or Digital?

For many years amateur astrophotography meant connecting 35mm cameras to telescopes, and using high-speed film. There was (and still is) a whole sub-hobby of special ways to pre-treat film to raise its sensitivity, and special development techniques to bring out the captured details.

There are still people using film and getting spectacular results but I know nothing about that science and won’t be mentioning it further here. Most amateur and all professional astrophotography is now done with electronic imaging, replacing hyper-sensitized photographic film with highly sensitive electronic sensors, usually CCD chips. The rest of this article series deals exclusively with a variety of electronic ways to capture images.

By the way, purists use the word “imaging” for capturing astronomical images with electronics, and reserve the word “photography” for film. I use both interchangeably. Sorry if that offends anyone.

Target Size and Brightness

It turns out that one of the easiest ways to simplify the complex field of astrophotography is to consider the type of target you will be imaging. Especially, whether the target is relatively bright or relatively dim, and its size.

This simple distinction allows us to divide the field of astrophotography into a subset that requires relatively simple equipment and technique, and a subset that requires more complexity.

Bright Targets

Bright targets are those you can easily see with your naked eye: the Moon, and the brightest planets (Venus, Jupiter, and Saturn). Mars and Mercury are also bright, but are more challenging because they are small and, in the case of Mercury, harder to find.

Bright targets have a number of advantages for the beginning astrophotographer:

  • Since they are bright, short exposures can be used when imaging them.
  • Since short exposures can be used, a motorized mount with accurate tracking is not necessary (although motorized tracking still helps a lot).
  • Simpler cameras can be used. Your household camera can be made to work for the moon, and an inexpensive video camera (like a $30 webcam) can be used for imaging the bright planets.

These targets still pose some challenges:

  • Although these targets are bright, all but the moon (i.e. all the bright planets) are also small. If you just hook a simple camera to your telescope you will be surprised, and may be disappointed, by how small the planets are. You will have to use one of several techniques to increase the magnification for imaging planets.
  • Although simpler cameras can be used for these targets, anything except the most basic moon images will still require a computer, software, and some processing skills.

I omitted the Sun from the list. It is, of course, a bright target, and it can be imaged with equipment similar to what you would use for the Moon, with the addition of highly specialized filtering to reduce the brightness to a safe level. Because of the danger, however, solar astrophotography is not a good starting point, and I will not be mentioning it further. In particular, the small, dark, eyepiece solar filters that come with some inexpensive “department store telescopes” are not safe for any purpose. If you have one, you should throw it away immediately. The only safe way to view the sun is with a full-aperture filter that goes on the objective end (the “front” end) of the telescope, so the intensity of sunlight is reduced before it enters the telescope.

Dim Targets

Dim targets are everything else: galaxies, nebulae, and star clusters. Dim, deep-space objects attract amateurs because they are icons of astronomy, and most of the stunning astrophotographs you have seen in magazines are of this nature.

Unfortunately, they are more challenging for a number of reasons:

  • Dim targets require long exposures (minutes or hours). Long exposures produce several challenges:
    • Because the rotation of the Earth causes the sky to appear to move, long exposures require a motorized mount that can very smoothly track the moving targets. Even a motorized mount that seems to track smoothly when used for visual observation may have minor imperfections in its accuracy, and these will be amplified by long exposures and become apparent in the resulting images.
    • Because objects in the sky appear to move in circular paths, the mount must not only track, but must mimic the circular motion. The popular fork mounts can track objects but do not mimic the circular motion so, without adding additional technology, are not suitable for long exposure astrophotography.
    • Electronic camera chips accumulate various kinds of electronic noise during long exposures so additional electronics, or additional process steps, are necessary to remove the noise.
  • Like bright objects, dim objects also come in a variety of sizes.
    • Many interesting dim targets are quite large, so low magnification is sufficient and small, wide-field telescopes, or even camera lenses, are sufficient.
    • Many more objects, including the majority of galaxies, appear small as well as dim, so long-focal length telescopes are needed to image them. The high magnification of long-focal length telescopes amplifies the effect of equipment imperfections, requiring even more attention to tracking and stability.

Size and Brightness vs. Difficulty

Target brightness divides astrophotography into two categories. Bright targets can be imaged with simpler equipment, and are a good entry point. Dim targets require more complex equipment and more highly developed skills. Likewise, large targets (the moon and large galaxies or large nebulae) are generally easier to image than small targets of the same class.

This suggests good order in which to approach astrophotography: Because of the simpler mounts and simpler cameras, bright objects are generally a better target for beginners to start with, building up to the challenging dim objects as experience develops.

The most difficult aspect of astrophotography is imaging small dim objects using long exposures through a long focal-length telescope. If you attempt such targets too early you will likely be frustrated by the numerous difficulties encountered.

Visual Astronomy vs. Astrophotography

Most beginners will start with Visual Astronomy: observing astronomical objects “live” with an eyepiece. Beginner-level equipment can also generally be used for simple astrophotography with pleasing results.

Eventually, however, you may be forced to make a decision about whether your priority is visual observation or astrophotography.

  • Equipment well-suited for visual use, and well-suited for beginners, can be used for astrophotography, but will not be optimal and may have limitations.
  • Equipment best-suited for astrophotography may not be well-suited for beginners and may be quite limited in what it can see when used for visual observation.

Some examples of this trade-off:

  • The classic telescope recommendation for a beginner to Astronomy is a 150mm to 200mm (6-8″) Dobsonian, probably the ideal beginner scope for visual use. For astrophotography, this telescope can be used for moon snapshots and, with some practice, for video-based imaging of bright planets. However, it is not suited for long-exposure astrophotography of dim objects. Even the tracking systems available for larger Dobsonians are designed to provide tracking as a convenience for long visual observation; they are not stable enough for long-exposure astrophotography.
  • Computerized Fork-Mounted Catadioptrics are another very popular beginner scope, are motor driven to track objects, and usually incorporate “go-to” features to find targets. But the geometry of movement of these mounts means they are still not suitable for long-exposure astrophotography. They can be made suitable with the addition of accessories (equatorial wedges or field de-rotators) but this reduces the beginner advantages of intuitive set up and pointing.
  • The classic astrophotography mount is the equatorial mount. But for beginners, these mounts are harder to transport, harder to set up, and less intuitive for finding objects.
    • Probably the worst option for astrophotography is the lightweight, cheap equatorial mount. These have all the disadvantages of equatorial mounts for beginner visual use, and still aren’t useful for astrophotography since they do not track accurately. They look like “serious, scientific, astrophotography mounts”, and are often sold as such, without actually being so.

Conclusion

Read the rest of this series before buying your first scope if you have astrophotography in mind – you may want to adjust your choice of equipment type.

Ideally, pick a great beginner’s scope and either delay astrophotography or set your goals on a type of astrophotography well-suited to the chosen equipment. Then plan to upgrade your equipment if and when your interests in imaging deepen, keeping your beginner scope for convenient visual use.

If you are reading this series in order, the next article is an overview of the wide array of equipment types for astrophotography.

 

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