Choosing a camera for film astrophotography.
THE 35-MILLIMETER SINGLE-LENS reflex, or SLR, is undoubtedly the camera of choice for the vast majority of astrophotographers. Virtually unsurpassed in price, versatility, convenience of use, and variety of films and accessories available, the 35-mm SLR camera uses a mirror that allows you to view, focus, and compose your subject through the same lens that takes the picture. This means you can see what you're getting when you use it to take photographs through telescopes, spotting scopes, microscopes, and other optical instruments.
The diagram on the facing page shows how an SLR camera works. The film lies flat at the back of the camera body, just behind the shutter. In front of the shutter is a first-surface mirror that directs the image-forming light to a focusing screen, which you examine through the eyepiece via a pentaprism. When you take a picture, the mirror flips up and out of the way, the shutter opens, and the film is exposed.
Twenty years ago the SLR was popular for taking family snapshots. Today, compact "point-and-shoot" 35-mm and Advanced Photo System (APS) cameras (both unsuitable for astrophotography) are much more predominant, and the amateur who wants to take up serious astrophotography often has to go shopping for an SLR in mail-order catalogs or camera stores that carry used or professional equipment. In this article I'll tell you what to look for.
Today's most advanced SLR models are not necessarily the best for sky shooting: as cameras become more automated for regular picture taking, they become less versatile for photographing the night sky. Automatic cameras, that is, those with electronic auto-focus, auto-light-metering, and auto-exposure-compensation features, can do a lot of things, but astrophotography isn't one of them.
What you need is a camera with full manual control--that is, you are able to control the focus and exposure yourself. At the very least, you need to be able to hold the shutter open (on the "B" setting) for time exposures, from a few minutes to more than an hour. You also need to be able to attach a cable release, which usually screws into the shutter button. The cable not only allows you to trip open the shutter while minimizing vibrations that blur the image, but it also locks the shutter open throughout the exposure.
Newer SLRs use battery power to keep the shutter open; older cameras do not. This is important because during a long exposure, many late-model cameras consume power the whole time the shutter is open, which can quickly drain the batteries, especially in cold weather. You're better off with an older camera whose shutter works without batteries. To check, try using the camera without batteries installed. But if your shutter requires battery power, don't despair; it may be cheaper to stock up on heavy-duty batteries and replace them with every roll of film than to buy another camera!
It's also important for a camera to have interchangeable lenses so you can take the lens off and replace it with a telephoto lens or a telescope for deep-sky work. A few SLRs, such as the Olympus IS-1, won't let you take the lens off. Others, such as some early Minolta Maxxums, won't work without the electronic components in the lens itself. Still others, such as old Exaktas and Mirandas, have lens mounts that don't take modern lenses. If possible, choose a camera for which a wide range of good-quality lenses is still available.
Beyond the Bare Necessities
A removable viewfinder (the "hump" atop the camera that you look into) is desirable. With interchangeable viewfinders, the standard pentaprism can be replaced with a waist-level or magnifying finder that lets you look directly down at the focusing screen. Such finders make focusing easier by offering a brighter, enlarged image. They also often provide comfortable viewing when the camera is attached to a telescope and aimed high in the sky (see page 122). For cameras with fixed viewfinders, you can often buy a right-angle viewer that attaches to the pentaprism eyepiece.
It's also handy if your SLR lets you change its focusing screen--the surface between the reflex mirror and pentaprism where the image is viewed and focused (see the photograph on page 123). The standard screen usually consists of a rectangular ground glass with a microprism or split-image focusing aid in the center. Such screens are good for general terrestrial photography, but they don't work well with dim astronomical subjects and high focal ratios, such as when the camera is attached to a telescope. A plain, very fine matte screen, such as the Beattie Intenscreen, can make focusing much easier. (Beattie, www.intenscreen.com, is an independent manufacturer of special screens for many brands of SLRs.) A non-interchangeable screen, however, is again no cause for despair; many newer cameras already have fine matte screens rather than microprisms, and even the older microprism screens usually have a fine matte ring around the central focusing area.
About Mirror Lockup
Many SLRs have either a mirror-lockup or a mirror-prefire mechanism to keep the slapping motion of the mirror from shaking the camera during an exposure. Mirror lockup lets you manually raise the mirror prior to exposure, using a special lever or button; mirror prefire means that when you use the camera's self-timer, the mirror goes up several seconds before the shutter opens.
For a long time I've considered mirror lockup or prefire to be essential, but now I'm beginning to reconsider. My experiments have shown that in modern cameras, the mirror isn't a particularly big source of unwanted vibration. Even with the mirror locked up, there is still vibration from the shutter curtain itself, which can soften the image and mimic a case of bad focusing.
For best results, pictures taken through the telescope should be at an exposure either fast enough to "freeze" vibration (1/250 second or shorter) or slow enough for you to do the "hat trick." The latter simply means you hold an opaque card (or your hat!) in front of the telescope. You open the shutter and lock it with a cable release and, after vibrations settle down, you quickly uncover the scope for the exposure without touching it. Then you put the card back and close the shutter. The shortest hat-trick exposure that I can manage is about 1/5 second.
Features You Don't Need
For astrophotography, a camera doesn't need automatic exposure or even a light meter. If present, auto exposure is sometimes helpful with eclipses of the Moon (see page 122) or close-ups of lunar landscapes, but it shouldn't be relied on.
Since auto focus is a function of a camera's lens, it doesn't work with telescopes, and auto-focus cameras often lack full manual control, so approach them with caution. High-end auto-focus cameras such as the Nikon F5 and Canon EOS-1 are as full featured as their manual predecessors, but they are quite expensive.
Other nonessential features include a motor drive for advancing/rewinding the film automatically, as well as a data back for imprinting the date or time on each frame.
Some Classic SLRs
"So which one should I buy?" you may ask. Dozens of models of SLRs are good for astrophotography; you don't have to get a particular one. Nor do you have to have all the features that I mentioned as desirable. Still, a few models are particular favorites of experienced astrophotographers. Here are some notes I've accumulated over the years.
The Olympus OM-1 is one of the best astrophotography cameras on the market --the secondhand market, that is. Made from 1973 to 1987, this small, lightweight camera revolutionized SLR design; earlier SLRs were much heavier. The Olympus viewfinder is large, bright, and reasonably comfortable for eyeglass wearers. The camera has a mirror lock, and its shutter produces remarkably little vibration. There's a broad range of good lenses available for wide-field imaging. The OM-1 MD and OM-1N are later versions, equivalent to the OM-1, as far as I'm concerned. The newer OM-3(T), still in production, is much more expensive and omits the self-timer and mirror lock; I don't recommend it.
The OM-2S (alias OM-2SP) and the OM-4 (later OM-4T or OM-4Ti) resemble the OM-1 but with the auto-exposure feature added. I have used these cameras to take auto-exposed shots of lunar eclipses and even Comet Hale-Bopp. They have mirror prefire, but it requires lots of battery power. The shutter doesn't require battery power on "B" or on the 1/60-second setting.
The earlier Olympus OM-2 lacked the mirror lockup or prefire. The mass-market OMs (OM-10, -20, -30, -F, -G, and -PC) lack interchangeable focusing screens and require battery power throughout a time exposure, but they're often very cheap and are handy as backup camera bodies. They're also distinctly less reliable, however, so check used models carefully for any sticking shutters.
The Nikon FM was introduced in 1977 to compete with the OM-1. It's a compact, lightweight camera, but the focusing screen is not changeable. The FM accepts all Nikon lenses, from the oldest (pre-AI) to the present. The later FM2 and FM2N and the current FM3A have interchangeable screens, but they don't take pre-AI lenses.
The heavy but versatile Nikon F3 is now abundant on the secondhand market as professional photographers switch to auto-focus cameras. It was recently discontinued after 20 years of production, making it one of the longest-lived SLRs ever made, and spare parts will be available, secondhand if not new, for a very long time. It takes all Nikon lenses.
The F3HP is an F3 with a high-eye-point (long-eye-relief) viewfinder. The whole viewfinder prism comes off, and astrophotographers often replace it with the DW-4 "stovepipe," a 6x magnifying finder (see the picture below). This is equivalent to the 1.2x setting of the Olympus Varimagni Finder accessory coupled to the 5x magnification of the Olympus OM-1 eyepiece. The difference is that the DW-4 gives a much brighter view, and I enjoy using it. For really high magnification, you can aim an 8x monocular into the DW-4's eyepiece; then you have 48x.
Nikon makes a bewildering variety of focusing screens. After trying several others, I settled on the Nikon F4 Type B screen for general use in my F3. It's an all-matte screen, similar to a Beattie Intenscreen but much less expensive, with a fine-matte spot in the center. For astrophotography, the Nikon Type D screen is better because it's fine matte all over; you don't have to place your target in the central spot. However, it doesn't work well with ordinary lenses for general photography.
The earlier Nikon F2 may actually be preferable for astrophotography because it takes all Nikon lenses and its "B" setting consumes no battery power. Like the F3, the F2 has interchangeable screens and viewfinders. So does the original Nikon F, although on that camera, use of mirror lockup requires sacrificing one frame of film. Moving in the other direction, the F4 and F5 are like the F3 with auto focus and motor drive added. They are very expensive and too heavy to attach to most portable telescopes.
Nikon's rival among professional photographers is Canon. Present-day Canon SLRs all have auto focus, and most models are not ideal for astrophotography, but manual-focus Canons, such as the F-1, F-1N, A-1, and AE-1, are still abundant secondhand, and some of the lenses Canon made for them are superb.
Another plentiful secondhand SLR is the Minolta SRT-101. If nothing else, Minolta deserves a stability award for introducing its bayonet lens mount four decades ago and keeping it in production, completely unchanged, to the present day. Manual-focus Minoltas are still being built and still have the same Minolta MD lens mount (which is not the same as Minolta uses on its Maxxum auto-focus cameras). The flagship of the fleet is the rugged SRT-101, which was sold for more than 15 years.
The earliest SRT-101 cameras had mirror lockup, but most do not. All have a fixed focusing screen with a fine-matte ring around the central microprism or split-image area. Later manual-focus Minoltas have similar features; all of them have a reputation for reliability.
Finally, there's the Pentax K-1000, the Volkswagen Beetle of photography, used by a generation of photography students, and now one of the easiest SLRs to find secondhand. It's as small and light as an OM-1 but somewhat simpler mechanically and quite rugged. There is no mirror lockup or prefire, but it is sometimes possible to pretrigger the mirror by pressing the shutter-release button halfway down. The focusing screen is not interchangeable. The K-1000 takes Pentax K-mount lenses, which are abundant but no longer in production.
New Bargain-Price SLRs
In the late 1990s several camera makers introduced surprisingly low-priced, all-manual SLRs. Among these are the Olympus OM-2000, the Nikon FM10, and the Yashica FX-3. Some Ricoh, Vivitar, and Phoenix cameras also belong to this family; all of them are actually made by Cosina.
They are close relatives, but not identical sextuplets. Each takes the lenses of its name brand (Olympus, Nikon, and so forth). The Yashica FX-3 takes the Zeiss lenses made for Contax; if you come upon one of these excellent lenses, this camera body is a cheap way to put it to use.
These cameras' prices are right. I bought my OM-2000 from a New York discounter for $160 new (body only). That's a tenth of the price of a new Nikon F3HP--yet the OM-2000 is solidly built and works reliably. The other cameras in the family are equally affordable.
The features are almost ideal. None of the cameras have interchangeable focusing screens, but all, as far as I know, have mirror prefire and can take long exposures without consuming battery power. Unfortunately, when doing this you have to remove the battery; otherwise an LED (light-emitting diode) in the viewfinder remains lit throughout the exposure, which consumes battery power and can leak red light onto the film. The OM2000 takes the Olympus Varimagni Finder; the Yashica apparently does also; the Nikon does not. The OM-2000 is the only Olympus OM-series camera that permits deliberate double exposures, important for some astrophotography projects.
I found that the OM-2000 has a good bit more shutter vibration than the OM1; but then, so do almost all cameras. If you can live with its limitations, an OM2000 or FM10 is a very inexpensive way to get a brand-new SLR. Allowing for inflation, these are the least expensive good-quality SLRs that have ever been marketed.
Where to Go Shopping
Before you go shopping for an SLR, check carefully whether you already have one in your closet or attic. I'm not kidding! Many of us have family members or relatives who bought a decent SLR in the 1970s and stopped using it as soon as "point-and-shoot" cameras became available. Those cameras are just waiting to be used, and most of them are very good for astrophotography.
Next, check the used-camera section at your local camera store. Some photographers are selling off their old SLRs, and that means bargains for us. A good online source of camera bargains, as well as comparison prices, is KEH Camera Brokers (www.keh.com). For a real bargain, look for a camera whose light meter doesn't work or uses mercury batteries that are no longer available. That's no problem for astrophotography, and substitute batteries do exist if you're willing to do some digging around. Another good source of cameras is swap meets at star parties and other gatherings. (I once bought a whole box of Pentax lenses at a ham radio convention.) You can also try your neighborhood garage or church sales.
What if you have an older camera but no instruction book? Sometimes the camera manufacturer will supply instructions on request (Olympus always does). Alternatively, you can buy third-party instruction manuals from Silver Pixel Press (Magic Lantern Guides) and other publishers. Inquire at any large bookstore or camera store.
Happy bargain hunting!
MICHAEL COVINGTON can be reached at email@example.com. He is the author of Astrophotography for the Amateur (1999). His two latest works, How to Use a Computerized Telescope and Celestial Objects for Modern Telescopes, will be available from Cambridge University Press this summer. You can visit his Web site at www.covingtoninnovations.com.
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|Author:||Covington, Michael A.|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 2002|
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