NEWPORT GLASS WORKS, LTD.    
   8610 CENTRAL AVE, STANTON, CA 90680
   TEL. 714-484-8100 * FAX 714-484-7600 * E-MAIL newportglass.sales@gmail.com

SELECTING YOUR DIAGONAL MIRROR

The selection of a diagonal requires at least six factors to be considered:
  • the diameter of the primary mirror
  • the focal length of the primary mirror
  • the distance between the center of the diagonal and the focal plane
  • the diameter of the fully illuminated field desired
  • the height of the focuser (standard or low-profile)
  • the intended use of the telescope

Most books on telescope making cover this topic in detail.


A rule of thumb, the commercial manufacturers' try to use a diagonal that has a minor axis size that is 0.18 to 0.22 times the primary mirror's diameter, i.e., 6"- f/8 would have a diagonal between 1.08" and 1.32".

Assuming the telescope tube diameter is approxmately 2 inches larger than the related primary mirror's diameter and the focuser is 1.250" in diameter then the following may be used as a general reference.

PRIMARY MIRROR STANDARD FOCUSER LOW PROFILE FOCUSER
6" f/8 1.30" 1.00"
6" f/5 1.52" 1.30"
8" f/6 1.83" 1.52"
10" f/6 2.14" 1.83"
12.5" f/62.60" 2.14"
12.5" f/4.53.10" 2.61"
13.1" f/4.5 3.60" 3.10"
14.5" f/4.5 - 3.16"
16.0" f/4.5 - 3.50"
17.5" f/4.5 - 4.00"



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