LAT Project Optical Layout


Jack Kellythorn


The proposed optical layout is a first attempt to quantify the physical parameters of the LAT. The choice of mirror size, focal length as measured, size of available secondary, and Naysmith tertiary ultimatly drive the mount design. The mount design is subservient to the required optical layout. The drawing is simply a means to help me understand how the mount should be designed.

The following assumptions were used to generate the drawing. If these assumptions are in error, please feel free to let me know. At this stage corrections are cheap (free).

  1. The usable mirror diameter is 70 inches. This is based on a guess as to edge dings and difficulty in figuring the bitter edge. If a larger diameter is to be used, please let me know.
  2. The focal length of 198.625 inches drives the the Newtonian focus. This number will likely change as final figuring proceeds.
  3. A desired field of view of 2 degrees. I used this value when running the Scope.bas program found in the back of Texearau's book "How to make a Telescope".
    This value helps determine the diameter of the secondary for the off -axis rays.
  4. A desired f ratio of 10. I set the f ratio to ten and the program found the location and radius of the secondary. The magnification ratio is determined by the ratios of the secondary to Newtonian distance compared to the Newtonian to primary distance.
  5. An available 24" secondary blank. I played with the geometry until I got a value of just over 23" for the secondary. This gives a 1/2" band around the perimeter for mounting and yet maximizes the available area for a 2 degree field of view. For a zero degree field of view, a 17.9" secondary is all that is required to intercept the on-axis cone of light.
  6. The distance of 66" from the optical axis to the focus of the Naysmith is a completely adjustable number. I choose it as a reasonable distance to keep clear of the aperture and still give some relief for the cameras and eyepieces outboard of the telescope.

    I hope the above explanation helps in understanding how the optics and geometry drive the choices in the layout. I've already learned a little, with a long way to go.

Please look over the layout and offer your comments to:
Jack Kellythorne


page marked up by Rick Powell