Building an Equatorial Platform
Long exposure photos
The earth's rotation makes the stars show an apparent East-West movement. Such displacement is precisely the cause of our worst difficulty in taking long exposure photos. In order to neutralize this movement, we need a mechanism moving the camera in the same direction, sense and speed of the stars, making the projection of their image always remain in the same point of the film, which render long exposures feasible.
One of the mechanisms employed for this purpose is the follow-up platform. Out of the several existing types, we chose BARN-DOOR, which received its name because its appearance it that of in- and out-going saloon and barn doors.
We will present here the design of a platform accessible to everyone: double-arm Trott type 3, which, due to its quality and simplicity, allows photos of up to two hours, with practically no error depending on the accuracy of the polar alignment. All we need are wooden plates, hinges, bolts, a washing-machine motor provided with timer and some hints . The whole process will be illustrated with photos of the parts, dimensions and assembling. To work.
All of the parts were made in wood-board; all of the measures are given in centimeters.
It is important that the hinges have no clearance. Select a good brand and test them.
The endless screw must be ¼" thick and allow 20 turns per inch.
The motor, provided with timer, is taken from the washing-machine or the dish-washer. It is a synchronous 1 r.p.m motor working on 120V. It may be found in shops repairing domestic electrical appliances or in the authorized representatives of such appliances.
The revolving rack for the photo camera may be bought in stores selling photographic material. This rack allows us to position the camera on a revolving head, for better focus of the object we intend to photograph.
The endless screw holder may be manufactured by a turner, or you may use a nut with two laterally fixed screws. A "butterfly" may also be used - one of those you find in any shop selling iron and similar devices.
The endless screw must be coupled with the motor by means of a small metal coupling-sleeve, fixed by a Hallen bolt.
In order to adapt the tripod we used a piece of threaded metal, but you may use any other device which allows it to be firmly fixed. It is very important to have a holder as stable as possible , and we may use a good tripod, or even a rigid column, bending in accordance with the observer's latitude.
Another important point is the platform orientation: the more precise it is, the smaller will be the error in guiding. We employed a searcher made of the lenses of an old binocular, but we may also use a plastic tube with a cross in its inner part to locate the pole with the utmost precision. The searcher must be positioned on the piece supporting the camera, and should be aligned with the hinges as follows: place the most distant object, put it in the center, move the supporting part to the vertical position, check whether the object suffers a displacement from the center; in case there is such displacement, realign the searcher as often as necessary, so that the object remains in the field center, both in the vertical and in the horizontal position. This allows you to align the optical axis with the platform mechanical axis.
In order to facilitate the platform positioning and its alignment with the pole, we developed a wedge with adjustable feet to render leveling possible.
Here are some photos as an example of what may be accomplished with the platform. Exposures varied from 5 to 15 minutes, using a 400 ISO film.
I hope you will build your platform and thus enjoy the pleasure of taking beautiful photos. The work involved will be compensated by a solidification of your knowledge and the sense of accomplishment it will certainly give you.
Send me your pictures and I´ll publish them with your copyright !
Bibliography
di Cicco, Dennis - A simple camera mouting for astrphotography- S&T outubro 1985 pag 391
Pursell, Wallace G - A portable photographic platform- Astronomy 1987 pag 91
Sinnott, Roger W. - The double-arm barn-door drive - S&T fevereiro 1988 pag 213
Sinnott, Roger W. - Two arms are better than one - S&T abril 1989 pag 436