SKYWATCH PROJECT: PHOTOGRAPHING THE SKY
EQUIPMENT REQUIRED: Camera with a controllable shutter and (preferably) cable release, tripod (optional); telescope (optional).
TIME REQUIRED: A few hours on one night or more.
WHAT TO DO: You can try any number of things. The important points are that you must get away from as much of the city light pollution as possible (you want a dark background) and that you actually use your picture(s) to make some sort of quantitative measurement. If you are successful and can make extra copies of your best effort(s), we would like to have them. Here are a few suggestions. If you want further information, books on astrophotography are available in the Physics/Astronomy Library. Here are the two main options:
OPTION 1: STAR TRAILS WITH A CAMERA -- Pointing at the North Celestial Pole will give you short circular trails. Pointing elsewhere will give you longer, less-curved trails. With each long exposure you make, it is good to make a short (10-30 seconds) exposure of that same region of the sky. This will enable you to recognize the constellation patterns more easily. The longer the exposure, the longer the trails. We recommend at least an hour, and you can go much longer if you like. The wider open your f-stop is, the more and thicker trails you will see. But you should close down the f-stop if near city lights or otherwise the background will "fog up". Be sure to record the direction and exact length of time of your exposure. To minimize vibration during the exposure, the camera should be securely mounted on a tripod or blocked up in some sturdy manner. Color film will produce fewer trails than black and white, but will produce a more spectacular picture; either is fine. Dew forming on the camera lens can be a problem; if so, you should fashion a shield such as a cardboard tube.
Once you have a decent picture, you can do the following. If you need help with the math or other details, your T.A. is available.
Identify the brightest stars in your photograph. Measure the length of the arcs in degrees with (a) a protractor, or (b) your known photo scale (see page 2), and then use this information, as well as the exposure time, to determine the length of the sidereal day, i.e., the time it takes the stars to make one complete circle in the sky. For method (a), measure the angle within the "pie slice" formed by the arc. For method (b), remember that a star away from the celestial equator moves much less (by a factor of cosine (declination) ) than 360 true degrees every day. Compare the value you get with the correct value and discuss any difference you find.
OPTION 2: PHOTOGRAPHY WITH A TELESCOPE -- If you have access to a telescope with a clock drive, you can take trail-free photographs of the sky, either by mounting the camera on the side of the telescope tube, or by taking pictures through the telescope itself. Your photographs can be of anything, but the moon, planets, and bright nebulae work out best. For each of your photographs, determine its field (in degrees) and scale (degrees per centimeter). Also determine the "limit" of the photograph, i.e., the brightness of the faintest star that can be seen in the picture, by comparing your photograph with a star atlas (which your T.A. can provide).
MEASURING THE SCALE OF YOUR PHOTO (must be done in all cases):
(1) The easiest method to find the number of degrees per cm on your negative or print is to take a picture with the same camera and lens (and same “zoom" setting) of a known object at a known distance. For instance, if a one-foot ruler is 57 ft 4 inches away, it will subtend 1.0 degrees. A one meter stick at 10 meters subtends 5.7 degrees, etc. Your T.A. can help with this, but it's the same principle as for your cross-staff.
(2) You can use a chart from a star atlas if the chart has coordinates marked on it. Identify two of the objects from your photo on the chart, and use the chart scale to determine how many degrees apart they are.
(3) If you can get a photo of the moon, use its diameter - the angular diameter of the moon is very close to 0.5 degrees.
(a) In general there is no need to use film faster than ISO 64. Films faster than ISO 200 should only be used in very dark skies, i.e., with little moonlight and well away from city lights. The result otherwise will be a very "foggy" photograph.
(b) Use a reliable photo shop for your developing and printing; the cheapie places can lead to heartbreak. Also, be sure to tell them that your photographs are "unusual" in that they all look very dark and unexposed compared to the average snapshop of your Aunt Millie. Tell them to develop and print them no matter what they look like, and not to cut the negatives.
(c) Allow enough time for this Skywatch so that if your first roll comes back and nothing is satisfactory, you can go out and try again. If time does become very short, use a type of film which can be quickly processed into prints.
(d) Wide-angle shots are usually more interesting than telephoto because they capture more star trails, but you may want to try for yourself several different lenses or "zoom" settings.
(e) On some automatic cameras, long time exposures can quickly wear out the batteries, especially when it's cold. It's best if your camera can take a time exposure "manually", that is, without using battery power. If this is not possible, bring along a couple spare batteries.