Mounting the Camera/Telescope

Do you really need to pack a tracking mount?

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Mounting of a camera or camera/telescope combination requires some consideration. A tripod or rigid post capable of holding the equipment is essential when using any sort of long lens. Even a shorter lens will benefit from a mount because during the total solar eclipse you may be too excited to hold a camera steady.

The question is whether you can use a regular tripod or do you need something better suited to astronomical picture taking - a tracking mount.

Any experienced astrophotographer will tell you that an equatorial mount is best when taking pictures of things in the sky. An equatorial mount is one that tracks the sky during the course of time. The most common has one axis pointed at the pole (north or south) allowing it to rotate slowly matching the turn of the Earth.

The Earth rotates about an axis once a day. That means that the stars along the equatorial part of the sky move (relative to an observer on Earth) once a day or 360 degrees per day, 15 degrees per hour, 0.25 degree per minute, 0.004 degree per second. It works out that the stars move at about 15 arc seconds per second of time at the equator.

As the target star is located closer to either pole the speed slows down by a factor of the cosine of the declination angle until you are at the pole; at which point the stars in that region do not appear to move relative to the Earth bound observer (like the North Star which is pretty darn close to the 90 degree mark and thus appears in the same position of the sky every clear night).

Astrophotographers use an equatorial tracking mount when taking pictures of the stars. The mount moves very slowly and follows the star across the sky. To move the mount a small electric motor is often employed. The size and weight of the mount, motor, and counter weights will often be several times that of the telescope and camera.

Note that many commercial tracking mounts do not account for the slightly different speeds that the sun and moon are traveling relative to the back ground stars. To obtain 100% tracking of the sun you will need a variable speed drive that can be set to keep the sun centered. The sun is moving at rate slightly slower than the background stars when viewed from the surface of the Earth (1/365th of a day slower).

The issue is that equatorial mounts tend to be heavy. When packing for a solar eclipse chase weight can be very important. Heavy counter weights and mounts can mean the difference between regular luggage and paying for overweight luggage.

There are two reasons you might elect to bring along an equatorial mount.

  1. To assist in keeping the image centered.
  2. To allow for longer exposures.

Looking at the first situation - A tracking mount will keep the image in the center (when set up right) but if your focal length is not very long then you can get away with out one. For every minute of the eclipse the sun moves about a quarter of a degree in a fixed frame. To see if you can keep the image in the view calculate the field of view and size of the image. The total image will shift a quarter degree for each minute; or one and a half degrees for a long, six minute eclipse.

Determining if you really need to bring an equatorial mount is a function of the focal length of the camera/telescope you are planning to use as well as the types of images you hope to obtain. The situation can be summarized as saying that the longer the exposure and the longer the focal length, the greater the chance you need to haul a heavy equatorial tracking mount (with a small motor) along.

You can calculate the maximum exposure without star trails if you know the detailed size of the imager or film plane.

 

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