Bird Photography with a Reflecting Telescope

Abstract

Reflecting telescopes have been used for photographing celestial objects for a great many years, but so far as the author is aware, have not previously been used for photographing birds or animals. An interest in both bird study and telescopes led to attempts to use a reflector as a substitute for a telephoto camera. A small Newtonian type reflector was constructed, according to suggestions found in a handbook devoted to the subject of telescope making.1 The concave mirror has a diameter of 4 inches, and a focal length of 20.75 inches. This would seem to give an aperture ratio of F:5.2, but if allowance is made for the area in the center of the mirror which is obscured by the diagonal flat, the figure is F:5.4. This was arrived at as follows: The area of the mirror is 12.57 square inches, but approximately one square inch is obscured, so that the effective area is 11.57 square inches. This is equivalent to a circle with a diameter of 3.84 inches. Then the effective ratio is 20.75 divided by 3.84, which is F:5.4. This is a measure of the photographic speed of the telescope. The aperture may be changed to a smaller value by covering the mirror with a card which has a hole in it of the desired size. The telescope with mount is shown in figure 1. Motion about a vertical axis is secured by rotation on the tripod. Motion about a horizontal axis is made possible by a hinge at one end of the base board, and is controlled by a screw which extends through the base board near the end opposite to the hinge. Coarse adjustments may also be made by extending or retracting the legs of the tripod. The image produced by the concave mirror is reflected at right angles by a diagonal flat mirror (or prism) located near the front end of the telescope tube, and may be viewed by an eyepiece or photographed by a camera (without lens) directed perpendicularly to the tube. If a Ramsden or Huygens type eyepiece is used, the image appears inverted. An image which is erect but reversed right and left may be secured by aid of a small flat mirror or prism mounted in the right angled elbow of a jointed tube which carries the eyepiece. Such an eyepiece and prism assembly is shown in figure 1. If the telescope is directed horizontally, the observer faces the side of the telescope and looks downward into the eyepiece. A camera (without lens) may be fastened to a metal tube, and this tube may be inserted in place of the eyepiece. If a simple plate camera is used, focusing may be done by aid of the ground glass back of the camera. The chief objection to this is the lapse of time before the plate or film holder can be inserted, and the photograph taken. Of course a reflex camera would obviate this difficulty. A roll film camera may be used, if a separate focusing tube with eyepiece is used. Both this tube and the camera tube should be provided with similar scales marked on the metal. The eyepiece must be carefully adjusted in its tube so that it is accurately focused for the same scale reading that is required