There are several ways for the amateur astronomer to photograph constellations or star-fields, to record comets, meteors, variable stars or just for pleasure. For meteors, normally we just mount the camera on a tripod, point it at the sky, set it at about f2.8 and open the shutter for about 10 minutes. The result is, of course, trailed stars, but it is the meteors which matter. If you guide the camera correctly you will get point-image stars. How is this done? The camera has to move at the "sidereal" rate, i.e. at the true rate of the Earth's rotation with respect to the stars. One sidereal day is 23h 56m 4s.
One way is to mount the camera "piggy-back" on a telescope with an accurate clock drive, preferably one with controls in RA and Dec. The telescope itself is used as the guider, and an out-of-focus star is kept centrally on cross-hairs in the eyepiece. A counterweight is usually necessary as the camera upsets the balance of the telescope tube. You can get an equatorial head which can carry one or more cameras and drive them at sidereal rate, it even has a sighting device through the polar axis for aligning it on the Pole Star, and it can be driven off a car battery. Jamie Shepherd and John Reid have these, and their wonderful pictures of Comets Hyakutake and Hale-Bopp show what can be done in the hands of real experts.
A: Angle set at observer's latitude; B: Cable release; C: Clock barrel
Fig 1: The Clockwork "Leerie" with camera pointing near the celestial pole
Some of the Oldies among us remember when the lamplighter or "Leerie" used to come round the streets at dusk, lighting up the gas lamps with a puff of flame from the top of his magic pole (we, the bairns, followed him, always hoping for a "shottie" but never got one lest we broke the fragile mantle). Then in the Fifties wee clockwork Leeries were installed in the lamps. They switched the gas on and off and ignited it, like modern electric timers. When they, in turn, were declared redundant in the Sixties as the streets went all electric, amateur astronomers and model makers were able to pick them up very cheaply, or even free, from the lighting depots. I've still got mine! The Leerie is mounted on a simple stand (fig. 1) so that its spindle, which rotates once a day, is pointing at the celestial pole, and upon which spindle is clamped some Meccano or other device for holding a camera. It gives pleasing star-field results with exposures of up to 5 minutes, but any longer and you will get slightly oval stars because the clock is geared to solar, not sidereal time. Also it is quite hard to align accurately on the Pole. Its advantage is that you can leave it running and do something else.
A: Hinge-line points to Pole Star; B: Base board;
C: Stable platform or table; D: Levelling screw; E: Stabilising block
Fig 2: The Haig Mount
Now we come to the Scotch Mount or Haig Mount in honour of the man who championed it and other simple astronomical devices, Mr George Haig, who is now retired from his lecturing post in Physics at Paisley Technical College (now University) where he installed his own home-made planetarium. This consists of two panels of wood hinged together at one end and mounted firmly on a wooden block so that the hinge-line is aligned on the Pole Star. The upper panel carries the camera, and it is made to move at the sidereal rate by turning a screw. (fig. 2).
The "magic number" for a Haig Mount is 229. That is, the distance from the centre of the hinge to the centre of the screw is 229 times the pitch of the screw. The pitch is found accurately by measuring the length of the threaded part of the bolt and dividing by the number of pitches in this length. (fig 3).
A: Ball & socket head; B: Sighting staples; C: Hinge; D: Metal plate set into wood; E: Nut set into wood of lower board; F: Handle to turn screw; G: Rubber band; H: Pitch of screw; I: Distance = 229 x pitch
Fig 3: The Haig Mount
Why 229? Let the screw turn once in one minute. This lifts the upper, camera-carrying panel by one pitch of the screw and turns it through a very small angle given by
arcsin 1/229 = 0.2502°
Multiply this by the number of minutes in a sidereal day, 1436, and you get 359.3 degrees which is as near the full circle of the sky as you need. However, the screw is straight, not an arc of a circle, so it is best to limit your exposures to about 5 minutes because the rate of angular increase gradually changes and again you will get oval stars.
Set up the machine on a firm base, not a tripod. Mine is mounted on a wooden board with levelling screws and two white-painted staples along the hinge-line through which, with the aid of a small mirror, I line it up on the Pole Star. Then it is secured by elasticated straps. The camera, fitted with a lens-hood and a cable-release, is set on the desired star-field. Cover the lens with a black hat, open the shutter, start the exposure and carefully turn the screw, with the aid of a handle, synchronously with the second hand of a watch. Give it about 4 or 5 minutes then cover the lens with the hat and close the shutter. This reduces tremor.
It is best to use a 50 mm or a wide-angle lens as a telephoto will show up the small errors in the system. Use f2.8 or even f4 because with wider apertures you may get coma at the edges of the field and pick up the sodium glow if, like me, you live in or near a big town. A Neodymium filter cuts off the sodium glare from low-pressure lamps but is ineffective with the high-pressure type. A pale blue filter might help as a substitute. Use transparency film such as Ektachrome 200 or 400, indeed, almost any film will give pleasing results. If you want extra accuracy in driving, don't line the machine up on the Pole Star itself but at the true Pole which is about one degree (2 moon-diameters) towards Eta UMaj. And when you begin driving have the two wooden plates as close together as possible. Tension them with a small rubber band.
There are small disadvantages. You have to stand and freeze while turning the handle. I tried using a small electric motor to turn the handle, one rated at one r.p.m., but with the cold and the torque required to drive a heavy camera it never worked properly. In Joppa I now have the misfortune to lie under a frequently-used flight path into Edinburgh Airport. Guess what happens whenever I start an exposure!
Still, I recommend the Haig Mount to all budding photographers. For further information on this and many other topics see Astrophotography : An Introduction by H. J. P. Arnold, (Philip's Observer's Handbook, ISBN 0-540-06086-0.)