Ken Hough's Website
Astrophotography

Focusing

Focusing:
When looking directly via the eyepiece of a telescope, it is relatively easy to focus so as to see a clear image. Provided the focuser is adjusted be near optimum position, the human eye is able to adjust to see the subject clearly.

This is not the case when attempting to do astrophotography!

A clearly focused image must be placed EXACTLY onto the surface of the sensor in the camera. If operating via afocal projection, then it may be possible to use the autofocus facility within a camera/lens, but this would require a brightly lit subject such as the Moon.

In most cases ACCURATE focusing must be achieved via the focusing mechanism of the telescope. No matter how good the camera and telescope are, unless ACCURATE focusing is achieved, results will not be good. How easy this is depends on the focusing device used and on the APERTURE NUMBER of the telescope.

A large aperture number of F10 or greater (eg a long focus refractor, SCT, or Maksutov telescope) makes it fairly easy to obtain accurate focusing. Conversely, "fast" Newonian telescopes of F5 or less can be very difficult to focus accurately.

My own 250mm diameter/1200mm focal length Newtonian telescope (ie F4.8) was delivered with a good quality and robust rack and pinion focuser. This focuser quite satisfactory for visual work, but was inadequate for astrophotography. I replaced it with a top quality two speed Crayford focuser. This was expensive, but very worth while.

Note: Aperture number = effective focal length of telescope / diameter of objective lens or mirror. This corresponds with the f number of a normal photographic lens.

I cannot emphasise enough the importance of having a good quality focuser for astrophotographic work.

Assessing sharpness of focus:
It can be difficult to judge when accurate focus has been achieved, especially when viewing poorly lit subjects. If an SLR camera is being used, then an eyepiece magnifier can help. If “live view” is available on the camera, then this might help. These two options are of use only if the subject is relatively bright and can be easily seen.

In the case of poorly lit subjects, the best solution will probably be to focus accurately on a nearby object (preferably a star) and then to CAREFULLY move the telescope onto the item to be photographed.

Remember that sharpness of focus IS CRITICAL!


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22/02/13