PHOTOGRAPHING THE SUN AND PARTIAL SOLAR ECLIPSES:

THE CAMERA MOUNT

By Birger Andresen, Trondheim Astronomiske Forening

An unstable camera mount is the one single factor ruining the most photos. Commercial tripods are usually too small, and especially vibrations due to wind is a big problem. Camera tripods and other mounts can usually become more stable by suspending some weight under the tripod.

The problems with the tripod will increase with the magnification. You will also magnify the vibrations, and a large magnification demands longer exposure time since the light is distributed over a larger area on the film. The difficulties will also increase with the weight and the size of the equipment the tripod supports, since a large mass high up on the tripod will be unbalanced. Large lenses/telescopes and large solar disks will therefore require a more stable tripod than smaller equipment and magnification.

Be extra careful about controlling that the tripod stays stable when the lens/telescope is directed in the direction the Sun will be at when you’re taking the pictures. It can easily become unstable if the Sun is high in the sky during the eclipse.

Photo of a digital camera mounted to a telescope. Click on the picture for full resolution. Photo: Magnar Fjørtoft

The tripod or telescope mount must be easy to use. A camera tripod which is difficult to adjust is not suitable for a large magnification, because the Earth is moving. When the Earth is rotating around its axis, the starry sky will appear to rotate in the opposite direction. The starry sky rotates with a speed equalling 1 solar diameter every 2 minutes, so the Sun will also move 1 solar diameter every 2 minutes. This makes it necessary to adjust the direction if the solar disk fills a large part of the camera view.

This will not be a problem for those fortunate enough to have the lens/telescope mounted on a so-called equatorial mount, where the rotational axis points towards the celestial pole and a clock drive balances out the rotation of the Earth. As a rule of thumb an equatorial mount is required if the lens has a focal length larger than around 1200 mm (around 500 mm on most digital cameras).

The following equation can be used to calculate the maximal exposure times for 35 mm film if you don’t have an equatorial mount:

Maximal exposure time (seconds) = 340/focal length (mm)

So a 1000 mm lens can take exposure times up till 340/1000 = 0,34 seconds before you will have problems with blurry photos due to the Sun’s movement. Realistically this should not be a problem for photographing partial solar eclipses with 400 ISO film, when the exposure times are usually well beyond 0,34 seconds.

OTHER EQUIPMENT

You need a cable release or a self timer when photographing through a telephoto lens or a telescope so high frequent vibrations won’t occur in the tripod when the photo is taken. Some SLR cameras are equipped with a mirror which can be locked in exposure position before the picture is taken. This will further reduce vibrations.