The apparent motion of the stars across the image plane of a camera fixed on a tripod and aimed at them that causes blurring of the individual star images. This is often referred to as "trailing" and is the basis for the making of "star trail" photographs.
To make photographs of the stars so that they do not show significant blurring due to the rotation of the earth, it is necessary to counterrotate the camera along an axis parallel to that of Earth and at such a rate that it just matches that of our planet.
Trailing can become evident with even such short exposure times as a few seconds and this effect can be used to great advantage when making images of the starts that actually exploit the Earth's rotation to show how all the stars appear to rotate around a point very close to the North star (Polaris). One simply points the camera, attached to a firm tripod, towards the north and aims up by an angle approximately equal to the latitude of one's location. This will place the North star roughly in the center of the camera's viewfinder. By opening the camera's shutter for a few minutes (or hours if the sky is really dark) one will easily see concentric circles surrounding the image of Polaris. Measuring the angle that any one segment of a circle includes and dividing by 15 degrees will indicate the exact time that the camera's shutter was held open.
It is interesting to note that the density of any given line associated with a particular star will not increase much beyond a certain point. This is due to the fact that the exposure time for the star "trail" is not governed by the time the shutter was open at all but by the rate at which the image of the star moves across the film. This rate actually varies by how far away any given star is located away from Polaris. Thus stars of equal brightness or luminosity will produce less exposure on the film if located near the celestial equator
If one wishes to keep the images of the stars fixed in one spot on the film one needs to overcome the motion induced by the Earth's rotation by panning counter to its rotation. This can not be done easily because the stars follow a circular path across the sky and so the camera needs to be moved simultaneously in both a horizontal and a vertical direction.
To deal with this problem, equatorial mounts were developed. These mounts rotate about an axis that is aligned with the axis of rotation of the Earth. They are equipped with a second mount that allows an attached telesope or a camera to be pointed to almost any location in the sky. As the equatorial shaft makes one revolution per day (or 1/4 degree per minute) the stars appear to stand still within the field of view of the telescope or at the image plane of a camera pointed towards them.
A simple way to make a "star tracker", a device that for all intents and purposes overcomes the Earth's rotation, is illustrated below. It essentially consists of two "pages" or "doors" with a common hinge which is aimed at the North star and which can be separated from each other at a particular rate (nominally 15 degrees per hour!). The "pages" are made out of good, stiff, wood such as 3/4 inch plywood. One of the "pages" of the device is attached to a tripod while a camera-carrying ball-head or pan-tilt head is attached to the other. A plan view of the tracker is included in the illustrations that follow.
The distance between the center of rotation of the hinges and the center of the 1/4 x 20 threaded bolt (which will cause the two leaves to separate as it is turned in the threaded "T" nut) must be very close to 11.46 inches otherwise the tracking rate will not be as accurately as possible equal to the required rate.
Onto the 6" carriage bolt (these have smooth round heads) one attaches a "handle" that one can turn at the rate of one revolution per minute. A 3" wooden disk, with a 1/4 inch hole drilled into center and held onto or "clamped" between nuts threaded above and below onto the bolt's shaft works quite well. If the disc is marked off in 1/12 segments then the time to make each arrive at a fixed reference mark is 5 seconds. If simply divided into quarters, then the time is 15 seconds per quarter.
Make sure that the T nuts are installed as indicated in the illustration. That is, thay are "countersunk" and installed so they do not protrude above the bottom leaf's surface and of such depth that the T nut's tip is flush with the bottom surface of the bottom leaf. This way the "pulling" action of the tripod threads locks the bottom of the tracker onto the tripod head. If attached the "wrong" way you run the risk of the tracker-plus camera landing on the ground because all that is holding the T nut in place is friction and the T nut's prongs or small nails.
A suggestion for the turning knob attached to the carriage bolt is to mark it off in 6 degree increments, ie. divide the circumference into 60 separate markings - one for each second of time.
Make sure hinge is on your left side. Use the tripod's pan-tilt head to point the hinges towards the North or "pole" star. Lacking the North star to point the axis of the hinges towards, orient the hinges so that they are pointing in the north-south direction using a compass. Then raise the angle of the hinges with respect to level ground so that they point as high above the horizon as your locations longitude on the Earth. For Rochester, NY this is about 43.5 degrees. You may find a protractor useful for this purpose.
To use the Tracker, aim the camera using the ball head to find the portion of the sky you want to include, and lock the shutter of the camera open. Use a piece of dark paper to shield the lens from light. This is best done by an assistant!. Then, start turning 1/4x20x6 bolt once per minute, or 6 degrees per second (synchronized with your sweep second hand of your watch or the seconds counter on your digital watch) separates the "leaves" at the required rate of 1/4 degree per minute. While you are "tracking" your assistant uncovers the lens for the required exposure time without jostling the camera. Once the exposure is done the shutter can be closed by unlocking the cable release.
Note that cameras that are dependent on small batteries to operate or keep their shutters open may draw so much power from them that the cameras may fail in short order if you start to make a series of exposures where the shutter is kept open for a long time. This is particularly a problem when photographing in low temperature conditions. It is recommended you use a manual camera or mechanically controlled shutter to avoid such problems. Or, at the least, have spare batteries with you.
As a starting exposure it is suggested you use 400 speed film and an aperture of f/2 or f/2.8 if possible and then make a bracketed series of exposures starting with maybe 1 second without tracking and increasing exposure times doubling in time from one shot to the next (these latter ones done while using your Star Tracker) until you have compiled a series to your satisfaction. Or until you are so cold you can no longer stand it!
Warning: Unless you take some measure of protection inadvertent transportation of the tracker with a camera attached to the ball head could cause the "book" to open up and the camera to hit some unexpected object or location ... such as the ground. Make sure that such accidents do not happen by taking appropriate precautions whatever they may be in your particular situation.
The small photograph on the right is Comet Hyakutake that came by in 1996 and I photographed it with an early version of one of my book-style trackers. The larger photograph on the left is Comet Hale-Bopp and it was taken with a more carefully made home-made tracker in 1997. I used a 200m lens on my camera. Several of the images I made this way were subsequently uploaded to a NASA website that collected images of that comet and these can be seen online at:
Let me know if these instructions are not clear enough, if there are errors in
it or if you need further assistance! If you would like to discuss any aspect of this
process write to me at the Rochester Institute of Technology, 70 Lomb
Memorial Dr., Rochester, NY 14623 or send me e-mail