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Method

camera mounted onto a telescope equipped with an equatorial guiding device

The camera is mounted onto a telescope equipped with an equatorial guiding device. Through the device, the camera can follow the movement of the stars and starlight will accumulate on the film after long exposure. The polar axis should point to the celestial pole around which all stars seem to rotate and the elevation of the axis should be the same as the latitude of the location. (In Hong Kong, the polar axis should point north and the elevation should be 22.5°)

 

Applications

Star Fields and Milky Way

 

The choice of lens depends on the area to be photographed. When the aperture is fully opened, the image at the corners may be distorted. To obtain better results, decrease the aperture by one to two stops. Accordingly, the exposure time has to be increased. Usually high speed films are desirable. However, for most films, the maximum exposure is around 45 minutes after which the gain will be minimal.

Nebulae, Star Cluster and Galaxies

 

Taking photographs of these small deep sky objects require a lens of longer focal length, sometimes a telescope is used instead. The alignment of the polar axis and the monitoring of the guiding process require absolute accuracy. Usually, a large aperture lens will give better results.

Comets

 

Owing to the comet's own movement, the head (or the nucleus) of the comet should be followed as the target for best result. In doing so, the stars behind the comet will become star trails.

To monitor the tracking, fit the guiding telescope with a cross-hair eyepiece. Choose a bright star as the guide star and adjust the slow motion devices accordingly.

  1. 1.Observe the motion of the guide star in the field.
  2. 2.Rotate the eyepiece until the guide star moves along one of the wire.
  3. 3.Defocus the guide star until it shows a disc. Adjust the telescope so that the disc is always divided equally into 4 parts