In many ways the Moon is the easiest celestial object to photograph, in others it is one of the most difficult. Since it is relatively large and bright it does not require special films or extended guiding techniques. It does, however, contain a wealth of detail that can be frustratingly difficult to record.
Although the Moon can appear quite large to the unaided eye, especially when near the horizon, the eye is more easily deceived than the camera. When photographed with an ordinary camera lens of 50mm focal length the Moon's image on the negative will only be a half a millimeter in diameter. Even if we were to enlarge the image ten times, the Moon's diameter on the resulting print would still be less than a quarter of an inch across. The question is "How do we get a larger image?"
Image size is related to the focal length of the lens being employed. A handy rule-of-thumb is that the Moon's diameter on the negative will be approximately equal to the focal length of the lens (in millimeters) divided by 110. Obviously we need to use a lens of considerable focal length to achieve a lunar image of any appreciable size.
A 500mm telephoto lens will produce an image on the negative that is 5mm across. Enlarged only five times, the final image will be a very respectable one inch in diameter. Obviously a one inch lunar disk will not show a great deal of detail, but not all lunar photos require infinite detail. The photography of lunar appulses, the close approach of the Moon to other celestial objects, does not demand a high degree of resolution but can be a very enjoyable and rewarding pursuit. And, of course, a lunar eclipse can be recorded very nicely with a series of one inch or two inch images.
LONGEST PRACTICAL EXPOSURE
Simply attaching a telephoto lens to your camera, pointing it at the Moon and snapping away, is not going to get you very satisfactory results. Long lenses are impossible to hold steady and must be tripod mounted for acceptable lunar photographs. The Earth's rotation can also be a problem since it appears to carry the Moon across the sky at the rate of one lunar diameter every two minutes. In order to offset the apparent motion of the Moon a suitably short exposure must be used. The longest practical exposure time for a fixed camera (one that is not electrically driven to match the apparent motion of the Moon) is equal to 250 divided by the focal length of the lens being used. For example: a 500mm lens requires an exposure time of no longer than 1/2 second (250/500) to record an acceptably sharp image of the Moon.
Establishing an upper limit on exposure times is only one step in determining the correct exposure for a lunar photograph. Although the focal length of the objective determines the size of the image, it is the focal ratio which determines the intensity of light that is delivered to the film. A 2.5 inch diameter lens with a focal length of 20 inches (f/8) will deliver four times as much light to the film as a 1.25 inch diameter lens with the same focal length (f/16) even though they will both produce an image of the same size. Exposure time, therefore, is dependent on the focal ratio of the lens.
The following are exposure guides for two of the more popular speed films. It should be understood that these are only guidelines and not hard-and-fast rules. Atmospheric conditions and the height of the Moon above the horizon are only two more variables that can directly affect the final image. It is best, therefore, to bracket your exposures by making at least one exposure above and one exposure below those shown. Maintaining a complete record of each exposure will prove to be an invaluable aid in refining your photographic technique.
EXPOSURE GUIDE - ISO 400 FILM
Full Moon Gibbous Quarter Wide Crescent Thin Crescent
at f/5.6: 1/2000 1/1000 1/500 1/250 1/125
at f/8: 1/1000 1/500 1/250 1/125 1/60
at f/11: 1/500 1/250 1/125 1/60 1/30
at f/16: 1/250 1/125 1/60 1/30 1/15
EXPOSURE GUIDE - ISO 100 FILM
Full Moon Gibbous Quarter Wide Crescent Thin Crescent
F/5.6: 1/500 1/250 1/125 1/60 1/30
f/8: 1/250 1/125 1/60 1/30 1/15
f/11 1/125 1/60 1/30 1/15 1/8
f/16 1/60 1/30 1/15 1/8 1/4
Up to this point, lunar photography has been discussed only in terms of telephoto lenses, but the same principles apply to photography through a telescope. In fact, a telescope is nothing more than a large telephoto lens. Or, if you prefer, a telephoto lens is nothing more than a small telescope.
All that needs to be done to photograph through a telescope is to remove the eyepiece and attach the camera (without the camera lens) to the scope with one of the many adapters on the market today. This method of photography, without any intervening lenses between the objective and the film, is photography at the prime focus.
Since telescopes will usually have longer focal lengths than telephoto lenses, larger images can be obtained. Longer focal lengths, however, enlarge more than just the image. Motion is also magnified so steadier mounts and shorter exposures are needed to keep sharpness within acceptable limits. Focusing also becomes more critical and extra care must be used to ensure a sharp image.
The distance scale of a telephoto lens can simply be set to infinity and, unless something is seriously wrong with the lens, the Moon will be in focus. Telescopes, however, have no such setting and the point of exact focus is a matter of care and judgment. The focusing screen of your camera can also make a significant difference in the sharpness of the images you obtain. The finer the 'grain' of the ground glass screen, the easier it is to see fine detail and the more precise your focus will be. Focusing magnifiers that attach to the viewfinders of some cameras can be a great help in achieving a well-focused image.
Little equipment is required beyond the telescope (or lens), a camera (35mm SLR) and a suitable tripod. In fact, the only other necessary accessory is a cable release. One thing sure to ruin an otherwise good photograph is vibration. A cable release, especially an air release, will prevent vibrations from being transmitted to the camera as the shutter is tripped.
Teleconverters are lenses that can be attached to most 35mm SLR camera bodies to increase the effective focal length of any lens being used. A 2X teleconverter will double the focal length of a 400mm lens and turn it into an 800mm lens, but there is a price to pay. In addition to doubling the effective focal length, the focal ratio is also doubled. A 400mm f/8 lens becomes, therefore, an 800mm f/16 lens and the exposure time must be adjusted accordingly. A quick check of the exposure guide provided above shows that this means a four-fold increase in exposure. In some cases, this can cause you to exceed the longest practical exposure recommended for a fixed (undriven) camera. Another word of caution when using teleconverters: Be sure and check your focus carefully. Teleconverters can sometimes cause a shift in the focus of a lens and the infinity mark on your telephoto lens may no longer be the point of best focus.
WHY PRIME FOCUS?
Some people wrongly believe that only high magnification photographs of the Moon are of any interest or value. Photographs that are taken at prime focus, however, can be quite useful and a lot of fun.
Moon moves through the night sky it often comes with a few degrees of some other
celestial object. These close approaches, or appulses, are best recorded at
prime focus because of the relatively wide field of view provided by this
method. Eclipses, as previously mentioned, are also excellent candidates for
prime focus photography. When recording your observations of the Moon, with
either sketches or close-up photographs, it is useful to record the position of
the terminator with a full disk photo of the Moon. And, for those in search of
lunar transient phenomena, Earthshine is a prime hunting ground and a fine
subject for prime focus photography.
So, give prime focus photography a try. I'm sure that you will find it a rewarding addition to your observing program
Photography of the Moon at prime focus can be a very enjoyable and useful pursuit but it is close-up photography that presents the greater challenge and rewards. The size of the lunar image on film is in direct proportion to the focal length of the optical system used. A typical 8-inch SCT has a focal length of 2,000 mm which will produce a lunar image approximately 18mm in diameter at the prime focus. Enlarged four times (standard for photofinishers), the image is still less than three inches across. A "respectable" close-up of the Moon would need to be three or four times that size, but how does one acquire a telescope with a focal length of 6,000mm or more?
The answer is, of course, that you don't need to buy a telescope with a longer focal length you simply need to extend the focal length of the instrument that you already own. The method of increasing the focal length and, therefore, the image size is the same for photography as it is for visual work. You simply add an eyepiece.
STANDARD EYEPIECE PROJECTION
Eyepiece projection is achieved by coupling the camera body (less lens) to the telescope (with an eyepiece in place) by way of a photo adapter tube. The focal lengths obtained by this method can be calculated in the following manner: With all components in their proper positions (eyepiece, adapter tube, and camera) measure the distance from the eyelens of the eyepiece to the film plane of the camera. Divide this distance (in millimeters) by the focal length (also in millimeters) of the eyepiece you are using. The number obtained is a magnification factor which can be applied to the original focal length of the telescope to give the new focal length. This factor can also be applied to the original focal ratio to obtain the new focal ratio of the system.
For example: If the eyelens to film plane distance is 150mm, and you are using a 25mm eyepiece, the magnification factor is 6. If your telescope is a six inch reflector with a focal length of 1200mm (f/8), your new focal length will be 1200 x 6 = 7200mm and your new focal ratio will be f/8 x 6 = f/48.
The distance from eyelens to film plane will vary with the design of your eyepiece and the type of adapter you purchase. Some adapters, in fact, are of variable length. If you want to calculate the potential capabilities of your telescope, but do not yet own an adapter, use a distance of five to six inches. This is typical of most adapters and will work well enough for rule-of-thumb calculations.
THE AFOCAL METHOD
The second means of eyepiece projection is the afocal method. In this method an eyepiece is used on the telescope and the camera lens is left on the camera. The telescope and camera do not have to be physically linked. In fact, some presentable photographs have been made by using high speed film and simply holding the camera to the eyepiece and tripping the shutter. It is far better, however, to mount the camera on its own tripod to eliminate camera shake and ensure that all optical paths are aligned. Misalignment will result in a lack of uniform sharpness across the photograph.
To use the afocal method you simply focus the telescope as you would for normal viewing and set the camera lens at infinity. Move the camera into position at the eyepiece, keeping the camera lens well centered and close to the eyepiece. Final refinement of the focus can be made with the telescope's focuser, not the camera, but often is not necessary.
The focal lengths obtained by the afocal method are very easy to calculate. Simply multiply the magnifying power of your telescope by the focal length of the camera lens (in millimeters). Your new focal ratio will, of course, be the new focal length divided by the diameter of the objective.
For example: If you are using a 75mm refractor with a focal length of 900mm (f/12), a 25mm eyepiece (36X), and a camera with a 50mm lens your new focal length will be 36 X 50 = 1800mm, and your new focal ratio will be f/24.
A word of caution: Since the focal length of the camera lens has such a profound effect on the focal length of the system, it is tempting to increase magnification by using a telephoto lens instead of a normal 50mm lens. Resist the temptation. It doesn't work very well and here's why. A 50mm lens sees the image formed by the telescope much the same as your eye does. A telephoto lens, however, magnifies the image and any errors in focus that your eye could not detect. The result is usually an unsharp image. It is best, therefore, to use a 50mm lens and record the image as seen by your eye.
FACTORS AFFECTING SHARPNESS
As the focal length of your system increases, exposure times will lengthen dramatically. Since telescopes magnify motion as well as size, sturdy mounts and clock drives are now a must. Tracking at the lunar rate is ideal but not absolutely necessary. Photographs taken by standard eyepiece projection, with the telescope running at the sidereal rate, will be sharp at a focal length of 7500mm if the exposure is kept below one second. Exposures by the afocal method should always be kept shorter than a second because the telescope and camera are not moving in unison. The motion of the telescope will cause the image to sweep across the film, blurring fine detail.
Precise focusing becomes more critical as magnification increases and the importance of good focus cannot be overemphasized. Take the time to focus as precisely as you can; the photograph you take will last a lot longer than the time spent taking it.
And, finally, remember that the same rules apply to lunar photography as to visual observing. Use the highest magnification that provides a sharp image, but no more. If the image of the Moon doesn't look sharp in the viewfinder of your camera it can not be made to look sharp.
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