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Getting it in Focus

by Ralph Paramor


It is surprisingly difficult to achieve the focusing accuracy required to produce satisfactory results when using prime focus or eyepiece projection astrophotography. Comments and suggestions on the following paragraphs apply when these methods are employed. One phenomenon which should be recognised is that once the correct focus is achieved, it cannot be assumed that it will last indefinitely. Small changes in viewing conditions or temperature of the instrument may degrade the sharpness of focus during a session and it is good practice to check it from time to time.

Camera Focusing Screens

Sadly, split image focusing screens do not work for f ratios slower than about f/4, so the best method is to achieve focus by using a matte screen. Unfortunately the standard screen on most cameras is too coarse to achieve good results. Some cameras have interchangeable screens and one with a very fine matte would be best for astrophotography. If the object being photographed is suitable, focus should be attempted on fine, low contrast detail which disapppears if the focus is less than perfect. Often it is more practical to focus on a nearby star and then move to the object. Some cameras can accomodate a magnifier which is focused on the camera screen and greatly assist in achieving overall focus. They are usually right-angled viewer types which make the photographer's job much more comfortable.

Knife-edge Method

This is an extremely accurate, if somewhat fiddly method of focusing, and some photographers use it in preference to other methods. It works on the fact that, unless they are interrupted, the rays of light from a point source (a star is used in this method) will leave the telescope in a converging cone, cross over at the apex, and continue on in a diverging cone. The precise focus is at the point where the rays cross over, ie at the apexes of the cones. A tool with a straight sharp edge is located in the film plane and is moved across at right angles to the axis of the cones. When it cuts through the light path either in front of or behind focus, the edge can be seen to be extinguishing the image slowly from left or right. When it cuts through at the apexes, the image will be extinguished instantaneously, indicating precise focus. I have a small piece of perspex cut so that it can be held and slid along the film guide while the camera back is open. A small piece of film with one straight and bevelled is glued to it. The "knife edge" is the edge of the film which is located precisely where the film will be in the camera. One problem with this system is that the camera back must be open to set the focus initially. Once the film is loaded, it cannot be opened for checking the focus for later shots. A solution to the problem is to make a device which has an edge set permanently at precisely the same distance from the telescope as the film plane when it is in focus. With the camera removed, this is held against the telescope, focusing is carried out and then the loaded camera is placed in position.

The Split-Image Method

This involves placing a solid diaphragm with two diametrically opposed holes cut in it in front of the objective. The idea is that the two images which can be seen would merge when true focus is achieved. I have not used this type, though I believe they perform better on bright objects because of the inevitable light loss. Devices such as this can be obtained from commercial sources or made by DIY enthusiasts.

Use of a Calibrator

One firm with which I am familiar produces an astrophotography system which incorporates a fairly unusual method of focusing. It involves setting a 10mm eyepiece in a frame on a calibrator remote from the telescope and adjusting its focus onto a preset target within the calibrator. The frame with the eyepiece held in it is then placed behind an open box fitted to the telescope eyepiece holder and the telescope focus adjusted onto the desired object through the 10mm eyepiece. The frame is then removed and the system camera placed in its position. The film plane is then at the precise focus position. This sounds rather complicated but is relatively simple in practice.

Whatever method is used the attainment of precise focus is an essential ingredient for the production of the quality photgraphs.



Ralph Paramor's interest in astronomy started when he was required to complete a unit which included astronomical observations in surveying as part of an engineering course. His interest remained dormant until he joined ASWA about 16 years ago, and since that time he has made several presentations at Society meetings and written various articles for The Sidereal Times. He is interested in all aspects of astronomy but is currently wrestling with the art of astrophotography.