Equatorial Table

How to make an equatorial table to take pictures of the sky with a simple camera, without a telescope.


1.  Explanation of the problem

To take pictures of the night sky, you need to expose the film or the CCD chip during a several minutes. During that time, the Earth rotates and stars seem to move.


2.  Using a tripod

If you do not compensate for the rotating movement of the Earth, stars leave a streak on the film. With a 50 mm lens on a 24 x 36 mm film, you start seeing a streak after 15 to 20 s exposure. You can let the sky flow for more than an hour to simulate a star rain over a photogenic foreground. The aesthetic effect is unquestionable.

3.  Interest of an equatorial table

If you want stars to appear as points and see nebulae, you need to compensate for the Earth's movement. Our planet rotates 15° per hour, or 15' per minute, or 15" per second. The equatorial table rotates the camera at that speed, with a precision that is good enough in the conditions we are going to explain below.


4.  The camera

It is better to have a mechanical camera with a B exposure setting. An electronic camera is acceptable as long as temperature conditions let it work and as long as its batteries let it run the B exposure. In B exposure, batteries may only last a few minutes.

Contents

  1. Explanation of the problem
  2. Using a tripod
  3. Interest of an equatorial table
  4. Camera

  5. Exposure time
  6. Sensitivity loss
  7. Film
  8. How to make an equatorial table
  9. How to use the equatorial table
  10. Results
  11. Last advice

15' stands for 15 minutes of angle
15" stands for 15 seconds of angle

No feet, no inches here.

Using a 400 ISO sensitivity film with a f/1.7 aperture, you start seeing the brightest stars after 15 s. To see the main sky objects, large open clusters, the brightest large nebulae, 5 min are enough. Beyond 10 min, you may start seeing stars flowing because of siting errors. With a 50 mm lens, aligning the Pole Star as best as you can by eye, you can push up to 20 min. If you are lucky that day. I obtained excellent results with a 1600 ISO film exposed 10 min.

CCD chips do not have this problem. The sensitivity of a film drops as soon as the first second of exposure. As it is easy to make a difference between two day pictures exposed 1/125 s and 1/250 s, a night picture exposed 5 min will not appear very much darker than a night picture exposed 10 min. The difference is even less visible between 10 and 20 min, but the risk of having stars flowing is a lot higher. That is why I limit my exposure time to 10 min.

Black and white negatives and slides have a finer grain than color negatives. It is better not to let an untrained professional print your pictures because colors and light may appear whimsical and let you believe your pictures are bad when they can be used. Several films give good results in terms of grain and color balance:

8.1  Technical choices and drawings

The axis of the hinge must be placed parallel to the Earth's axis and stay there. The hinge on the black and white picture is a little flexible and its axis changes direction when you install the camera. 

  More technical drawings

Here the axis of the hinge is replaced by a rigid steel pipe that allows you to point the Pole Star. This strong pipe does not bend when you install the camera.

8.2  Calculation of the distance between the threaded rod and the axis of the hinge

The table turns 15°/h or 15'/min (0.25°/min). Turn the rod one turn a minute in order to be able to check easily every 15 s that you are turning at the right speed. By doing so, we suppose that the rod stays perpendicular to the table, which is true enough for short exposure times. In 10 min, the table is turning 2.5°, which is an angle small enough to keep the error acceptable.

This formula is valid with metric (international) and imperial (American) units. With a 1 mm thread (metric thread, 6 mm in diameter rod) the distance between the tip of the rod and the axis of the hinge is 228 mm. 

Stay away from any source of light pollution (lamp post,  window... ) Point the rotation axis to the Pole Star. You just need to look through the pipe. It is convenient to stand the equatorial table on a stable stool or a little table.

Adjust the star field you want to shoot and block the rotule. Start the exposure and the chronometer and start turning the rod immediately, clockwise at a speed of 1 rev/min.

Turn at a constant speed and check every 15 seconds if you are turning at the right speed. Every 15 s, you control the position of the knob, not its speed. Slow down or speed up to have the knob in the right position on time. You can install a small motor to make this more comfortable. See how to use a stepper motor.

Some exposures will be shifted, blurred or under-exposed. To be able to correct those defects you will need to know in what conditions these pictures were taken. For each photo, write down the following information:

Using a map, learn how to shoot the areas that show the greatest number of nebulae, galaxies or clusters.

Before shooting at night, repeat all operations by day, without a film, several times. Repeat closing your eyes because you will be in the dark and you must make sure that no operation requires a lot of light since you will not have any light during exposures.

Even if it is not very cold, +10°C for example, the temperature is going to go down during the night, +5°C first, then maybe -5°C because the night is clear. You are not going to move for 10 minutes, then 10 minutes again, several times and thus during hours. Dress as if you were going to the Pole.