by Dave Martin, skyhawk1@pcisys.net
Have you ever wanted to try taking astrophotos but have not been sure where to start? What type of film should you use? What type of camera is needed? How long of an exposure should you use? If you answered yes to any of these questions, then this article is for you. I’ll discuss what to look for in a camera, what films work well and some of the equipment and techniques used in basic astrophotography.
Camera
To take photographs, you need a camera. Of the many types available, 35mm Single Lens Reflex cameras are best suited for beginning astrophotography. They are readily available, have all the desirable features and 35mm film is offered in a very wide range of types. Modern cameras are marvels of mechanical, optical and electronic ingenuity with automatic exposure control, automatic focusing and many other features. However, astrophotography has special demands that most modern cameras cannot cope with. This is one place where automatic features of a camera are not desirable and the older fully manual cameras are better suited.
Features you do not want in a camera are automatic focus and automatic exposure control. Both of these are designed for typical snapshot photography and are not needed or useful for astrophotography. The majority of new cameras being sold today are the fully automatic types that are completely dependent on battery power. Fortunately, as photographers upgrade to a newer camera, they often sell or trade in their older, manual cameras. These used manual cameras may be found at camera repair shops, flea markets and pawn shops. Camera bodies with broken light meters or inaccurate shutter speeds can often be found quite cheaply and may be perfect for astrophotography.
The first feature a camera must have is a mechanical bulb or time setting to allow it to take long duration exposures. Many newer cameras use a solenoid to operate the shutter and will not work if the batteries are removed or dead. Even if the camera has a time setting, the long exposure times will quickly drain the batteries. Mechanical shutters do not rely on batteries and can take the long exposures necessary.
The other essential feature is interchangeable lenses. These allow you to change the magnification of the photo from a low power wide angle shot to a high power close up of the object being photographed. With appropriate adapters, the camera body may be attached directly to a telescope so that the telescope becomes a super telephoto lens.
Two highly desirable features in a camera are an interchangeable focusing screen and mirror lock up. In the 35mm SLR camera, the light enters the lens and is reflected upwards to form an image onto a small glass or plastic focusing screen. This image is then reflected by a prism so it can be seen in the camera viewfinder. Standard focusing screens are designed for typical daylight use with relatively short focal length lenses. At night or when using very long focal length lenses, the images formed on standard focusing screens may be dim and hard to see. Some cameras allow the focusing screen to be replaced with special focusing screens specifically for astrophotography. The images formed by these specialty screens are much brighter and easier to see and focus with. Replacement focusing screens can be expensive but after using one, you won’t want to use a standard focusing screen again. Aside from their cost, the disadvantage of these focusing screens is that they can cause the light meter to read incorrectly. The screen must be replaced with a regular screen when using the camera for regular photography.
At the high magnifications often used in astrophotography, small vibrations can be magnified to giant blurs on the film. Mirror lock up is a useful feature to reduce some of these vibrations. In a SLR camera, a small mirror hangs down in the light path to divert the incoming light to the focusing screen and on to the viewfinder. When the shutter is tripped, the mirror has to move out of the light path to allow the light to strike the film. When the mirror moves, it hits limit stops which may create vibrations that become visible blurs in the photograph. Some cameras have a lever or button to move the mirror before the shutter is tripped. This greatly reduces at least one source of vibrations that can ruin your photograph. Canon, Nikon, Pentax, Olympus and others have manufactured cameras with ideal features for astrophotography. Some of the more popular models are the Canon F-1, Nikon F, Nikon F2, Pentax LX and the Olympus OM-1, OM-2, OM-3 and OM-4 models. The Olympus OM-1 has long been popular for astrophotography as it combines all of the features listed above at a reasonable cost.
Lenses
To go along with the camera body, you should have a couple of lenses to use with it. Camera lenses have two important specifications. First is the focal length in millimeters, such as 50mm or 135mm. The focal length is what determines the image size or magnification of a lens. Unlike in telescopes where you use shorter focal length eyepieces to increase magnification, in cameras the longer the focal length, the higher the magnification. The longer the focal length of a lens, the larger the image on the film will be.
For example, if you take a photo of the moon or Jupiter with a 35mm camera, the image size in millimeters on the negative would be:
| Lens Focal Length
|
Moon Image Size
|
Jupiter Image Size
|
Lens Focal Length
|
Moon Image Size
|
Jupiter Image Size
|
| 28 | .25 | — | 1,000 | 9.1 | .20 |
| 50 | .45 | — | 1,250 | 11.0 | .25 |
| 100 | .91 | — | 1,500 | 14.0 | .30 |
| 200 | 1.80 | — | 2,000 | 18.0 | .40 |
| 300 | 2.70 | — | 2,500 | 23.0 | .50 |
| 400 | 3.60 | — | 3,000 | 27.0 | .60 |
| 500 | 4.50 | — | 4,000 | 36.0 | .80 |
| 600 | 5.40 | — | 5,000 | 45.0 | 1.00 |
| 700 | 6.40 | — | 6,000 | 55.0 | 1.20 |
| 800 | 7.30 | — | 8,000 | 73.0 | 1.60 |
| 900 | 8.20 | — | 10,000 | 91.0 | 2.00 |
| (focal lengths in millimeters, image size in millimeters) | |||||
A frame on a 35mm negative measures 24 x 36 millimeters. It takes a lens with a focal length of 2500 millimeters for an image of the moon fill the short dimensions of a frame. When photographing Jupiter, it takes a focal length of 10,000 millimeters just to get an image 2 millimeters across on the frame. As the focal length of the lens increases, the light captured by the lens is focused over a larger area of the film. This requires that the exposure time increase when the focal length increases.
The other vital measurement of a lens is its focal ratio or f-ratio. The focal ratio is determined by dividing the focal length of a lens by the diameter of the objective element of the lens. A lens with a large diameter will capture more light during an exposure than smaller diameter lens with the same focal length at the same exposure time. A lens with a low f-ratio will require a shorter exposure time and is said to be a fast lens. A lens with a higher f-ratio requires a longer exposure time to capture the same amount of light. For this reason it is called a slow lens.
Use fixed focal length lenses with low focal ratios when possible. Fixed lenses normally offer a higher resolution in the image at lower f-ratios than zoom lenses at the same focal length. The stated f-ratio of zoom lens is measured at it shortest focal length. As the lens is zoomed out to its longer focal lengths, the f-ratio increases as well. Short focal lengths will require shorter exposure times which in turn lead to fewer tracking errors. Low power lenses such as a 28mm or a 50mm lens are useful for taking wide angle photographs of star fields. A 135mm lens in a nice medium power lens and 200mm or 300mm lenses are very useful when photographing nebulae. Lenses for astrophotography should have focal ratios of 4.5 or less. Longer focal ratios require excessive exposure times and should be avoided.
Tele-converters are available that fit between the camera body and the lens. Much like a Barlow lens for a telescope, these devices act to double or triple the focal length of any lens they are used with. While tele-converters can be used for astrophotography, remember that they double or triple the f-ratio along with the focal length of the lens.
Film
The 35mm format offers a very wide range of film types to choose from. Black and white film is an excellent choice for taking high resolution photographs of the moon or when an appropriate filter is used, photographs of the sun. Color film works well to capture the colors in deep space objects that the eye cannot see. Slide films have a simpler developing process and tend to represent actual colors better when used for astrophotography. The developing and printing process for print films introduces more possibilities for the color balance to be distorted. Prints are easier to handle and enlargements are easier to make from negatives than from slides. Which type to use is largely personal preference. Film is rated by its sensitivity to light expressed as its speed or ASA number. An ASA200 film is twice as sensitive to light as an ASA 100 film and requires half the exposure time to capture an image. Likewise, an ASA400 film is twice as sensitive as an ASA200 film. The grain of a film refers to the active particles on the film that capture the image. As film speed increases, the grain of the film also increases leading to lower resolution on the photo. If enlargements are made of a photo taken with a fast, large grain film, the grain may show in the enlargement.
Other characteristics of film are reciprocity failure and color balance. Most films are designed for exposure times one second or less. When exposed for longer than one second the film does not respond to light in a linear manner. An exposure of five minutes may not capture five times as much information or detail on film as a one minute exposure. To capture five times the information, an exposure of ten, twenty or possibly thirty minutes may be necessary. This nonlinear response is called reciprocity failure. Fortunately, with the excellent films available, the beginning astrophotographer will not need to worry much about it.
Different films do not respond the same way to colors. Poor color balance can cause your astrophotos to have dark green or dark brown skies instead of black or very dark blue skies. When used for astrophotography, some films, notably Kodak, tend to be slightly more sensitive to red shades than they are to blue or green. Fuji films on the other hand tend to be more sensitive to the blue and green shades and somewhat less sensitive to red. An easy way to remember this is to look at the boxes. Kodak film comes in bright red and yellow boxes while Fuji films come in blue and green boxes.
Advanced astrophotographers may treat their film with a high pressure hydrogen and nitrogen gas at high temperature to increase the sensitivity of their film in a process called hypering. Another common method to increase sensitivity is a cold camera which uses dry ice in contact with the film. Fortunately, the currently available off the shelf consumer films from the local drug store are better than the best professional astrophotography films of just a few years ago and these advanced techniques are not necessary for the beginning astrophotographer.
Several commonly available films with acceptable speed, grain, color balance and reciprocity failure make them ideal choices for astrophotography. For slides good choices are Kodak Ektachrome Elite II 100 and Fujichrome Provia 400. For prints, the Kodak ASA400 and ASA1000 Royal Gold are good choices. A favorite print film of many amateur astrophotographers is Fuji Super G+. These films are available in ASA400 and ASA 800 and have very good reciprocity failure, color balance and exceptionally fine grain for their speed. Regardless which you choose, try to stick with the same type and speed for a few rolls as you gain some experience. This makes it easier to compare results from one session to the next.
Camera and Tripod
The simplest way to begin taking astrophotos requires nothing more than a suitable camera and a sturdy tripod. With short focal length lenses you can take exposures of up to around thirty seconds before star trails begin to show in your photograph. Many striking astrophotos have been taken by using long exposures to create deliberate star trails. This technique is especially effective when a stationary object is in the foreground of the photo. If the image is deliberately defocused, the star trails can create vivid color streaks in the photo.
A long exposure with the camera aimed at Polaris can create circular star trails on the photo. When doing stationary astrophotography, the formula to determine exposure times before star trailing blurs the image is to divide 250 by the focal length of the lens being used. If some blurring in the photo is acceptable, then multiply the value by four. Using this formula yields these suggested exposure times:
| Focal Length
|
No Blur
|
Some Blurring
|
Focal Length
|
No Blur
|
Some Blurring
|
| 28 | 9 | 35 | 200 | 1 | 5 |
| 50 | 5 | 20 | 300 | .5 | 3 |
| 135 | 2 | 7 | 400 | .5 | 2 |
| (Focal length in millimeters, exposure times in seconds) | |||||
Barn Door Mounts
OK, you want to take longer exposures without star trails and without spending a lot of money to do it? The answer is a Barn Door mount, also known as a Scotch or Haig mount. This is little more than two boards and a hinge. The bottom board is held stationary while a camera is attached to the moveable top board. The axis of the hinge is aligned with the celestial pole and a threaded rod is rotated to force the top board to move. This movement counteracts the rotation of the Earth and allows the camera to remain fixed relative to the stars.
With the correct dimensions of the boards and the rod, a barn door drive is suitable for exposures of up to ten minutes. Not bad for a few dollars worth of materials! More elaborate versions of barn door mounts have been made with motor drives, multiple hinges and guide scopes. Some of these are capable of accurately tracking the stars for up to thirty minutes. Plans for making barn door mounts are available in many astronomy books and on the world wide web.
Afocal Projection
The next level up is afocal projection. The camera is fitted with a normal lens and attached to a tripod. The camera lens is then placed as close as possible to the eyepiece of a telescope and then focused on the image formed by the eyepiece. This yields much greater magnification than using a camera lens alone. This increased magnification significantly limits the exposure time before blurring begins however. Surprisingly good results may be obtained if care is taken to focus well and if stray light is prevented from entering the camera lens.
For longer afocal exposures, the camera can be mounted to a motorized telescope. To calculate the overall focal length of an afocal projection setup, multiply the magnification of the telescope and eyepiece combination by the focal length of the camera lens. For example; using a telescope with a focal length of 2000mm and a 32mm eyepiece produces a magnification of 2000 / 32 = 63x. Using a 50mm lens on a camera produces an effective focal length of 63 / 50 = 3125 mm. The f-ratio of this setup is the effective focal length divided by the telescope lens or mirror diameter. By using the example above, an 8 inch or 200mm scope is 3125 / 200 = 15.6 f-ratio.
Piggyback Mounts
When a camera and a normal lens is attached to a telescope or other moveable platform, the technique is called piggyback mounting. In this case, the camera goes along for the ride while the telescope tracks the stars. With a motorized telescope, very long exposures may be taken using a piggyback mount. While piggyback mounting is normally done with a motorized telescope, beautiful astrophotos have been taken by piggyback mounting a camera to a Dobsonian telescope and then manually guiding the telescope for up to thirty minutes. This method requires large amounts of patience. A camera and tripod can be set on a motorized equatorial platform for exposures of several minutes.
Prime Focus
Moving up in difficulty is prime focus. This is where the camera is attached directly to the telescope so that the image formed by the telescope optics falls directly on the film. The basic hardware to attach the camera is called a tele-extender which is nothing more than a fancy name for a hollow tube. The tele-extender attaches to the telescope, either by fitting into the focuser or by threading onto the back of a Schmidt-Cassegrain telescope.
The other end of the tele-extender is threaded to attach a T-Ring adapter. T-Rings are available for most cameras and are necessary to attach the camera to the tele-extender. By changing the T-ring, many different cameras can be used with the tele-extender or other accessories. Obtaining successful prime focus astrophotos depends a great deal on the accuracy of the polar alignment of the telescope, the accuracy of the drive motors and the duration of the exposure. For prime focus photography, the focal length and f-ratio of the telescope are used to calculate exposure times.
Guiding
A variant of prime focus is guided exposures. If the telescope drive has a controller to adjust tracking, then long duration photos may be taken by correcting tracking errors during the exposure. Guiding can be done with a separate telescope mounted to the main telescope or device called a radial guider may be used. This device attaches to the telescope and has threads for a T-Ring to attach a camera, same as the tele-extender. In the radial guider a small prism projects slightly into the light path. The prism diverts a small amount of light to an eyepiece at a right angle to the light path.
After aligning and focusing the telescope and camera on the object to be photographed, the guide scope or prism is adjusted so that a bright guide star is visible in the guiding eyepiece. The eyepiece used normally has an illuminated reticule or crosshairs. For the duration of the exposure, the tracking of the telescope is adjusted with the controller to keep the guide star centered on the cross hairs of the guiding eyepiece.
For long exposures, this can be an extremely tedious process. Many astrophotographers now use a specialized CCD camera called an autoguider to take their photographs. The CCD tracks the guide star and adjusts the telescope tracking automatically. After the first few minutes of manually guiding a long exposure, a CCD autoguider may begin to seem like a necessity rather than a luxury.
Eyepiece Projection
The most difficult form of astrophotography is called eyepiece projection. The difficulty is due to the very long focal lengths and large f-ratios involved. These demand a high degree of tracking precision and freedom from vibration to capture a sharp image on film.
Eyepiece projection uses the same tele-extender tube used for prime focus astrophotography but now an eyepiece is fitted inside the tube. The camera is then focused on the image produced by the eyepiece. The combination of telescope and eyepiece create a very long effective focal length and magnification.
To calculate the focal length and focal ratio, you must first determine the magnification factor. First, measure the distance from the eyepiece lens to the film plane in the camera. Subtract the focal length of the eyepiece from this distance. Divide this value by the focal length of the eyepiece to yield magnification. By using the example above and an eyepiece to film distance of 110 millimeters results in: (110 – 32) /32 = 2.4x magnification (over prime focus), 2.4 x 2000 (focal length of telescope) = 4875 millimeters focal length, 4875 / 200 (telescope aperture) = 24.3 focal ratio.
Replacing the 32mm eyepiece with an 11mm eyepiece, these numbers shoot up to magnification = 12, focal length = 24,545 mm and f-ratio is 122.7.
Accessories
Most hobbies require a few accessories to get the most out of them and astrophotography is no different. A must have accessory is a locking shutter release cable. This is what keeps the camera shutter open for those long exposures. Get a long cable as they transfer less vibration to the camera than shorter cables. Things to look for in a cable release are flexibility in the cold and ease of use, especially when wearing gloves.
For shorter exposures, especially when doing eyepiece projection, an air bulb shutter release is very nice to have. Using a long vinyl tube with an air bulb and a pneumatic plunger to trip the shutter, they transmit very little vibration to the camera.
A timer to track the exposure times is necessary. In a pinch, exposures can be timed by counting one one thousand, two one thousand, that’s not a very accurate method. A watch can be used but having to check it often is not very convenient. Many electronic countdown timers are available that are almost perfect for timing astrophotos. The desired exposure time is entered into the timer and a start button is pressed. A beep or buzzer indicates the end of the exposure time. Simple, accurate and convenient. Suitable timers are available from Radio Shack and other electronics stores at reasonable cost.
A notebook is a must have item. Faithfully record all details about each exposure, writing down the subject, date, time, lens used, exposure time and any other relevant notes. Reference these notes as a starting point for the next round of exposures. Without reference notes, you are shooting blind each time you go out. More convenient in the field but more work when transcribing notes is a small cassette recorder. Just speak the information into the recorder.
Some cameras have an attachment that fits over the viewfinder to form a right angle viewfinder. These devices make focusing of the camera much easier, especially when photographing objects high overhead. When using a camera in conjunction with a telescope, a simple focusing aid is to cut two holes in cardboard or other suitable material to form an aperture mask. When placed over the objective of the telescope, it creates two images. As the telescope is focused, the images will move. When they overlap, the telescope is perfectly focused.
9
There are many books, magazine articles, and web sites on astrophotography. Sky and Telescope and Astronomy magazines run frequent articles on various aspects of astrophotography ranging from how to choose a camera to reviews of the latest films. The image galleries can provide starting points for exposure times and lenses to use.
Often a local library or used bookstore may have books on astrophotography ranging from simple introductions to advanced texts. An excellent book for astrophotographers of all levels is Michael Covington’s Astrophotography for the Amateur. Much of the information for this article came from this book. All of the topics in this article are covered in much greater detail in the book. It also has examples of the necessary math to calculate focal ratios, exposure times and much more.
Astrophotography–An Introduction by H. J. P. Arnold also covers the topics discussed here in greater detail. A recently published book for beginning astrophotographers is Splendors of the Universe by Terence Dickinson and Jack Newton. While its coverage of technical details is limited, this book is full of beautiful astrophotos, many taken with simple equipment.