I mentioned my old telescope's secondary mirror in this answer (stiffness) but I'm not certain I am remembering correctly. My first telescope was a Edmund Scientific 4¼ inch Newtonian reflector. This was in the 1970's. I seem to remember that the secondary mirror was a right angle prism rather than an elliptical front-surface mirror.

Am I dreaming or mis-remembering, or did Edmund use prisms on at least some 4¼ inch Newtonians?

Either way, any discussion on the tradeoffs between front-surface elliptical mirrors and 90° prism secondaries would be appreciated.

example of Newtonian with prism secondary

Source: John M. Pierce's HobbyGraph Articles HobbyGraph #15; Diagonals and Diagonal Supports.


2 Answers 2


My Edmund F/10 4 1/4" was bought in 1975, and had a rectangular front-surface mirror as the diagonal, with a single arm support attached to the tube near the eyepiece holder.

I believe the Astroscan 2001 (or at least some models of it) used a prism for the diagonal, supported by the flat glass plate at the front of the tube; haven't found a reference though.

The advantage of a prism is that it is more maintenance-free then a mirror which will eventually need to be re-aluminized. The disadvantages are that there are two optical surfaces instead of one in the light path that could reflect or scatter light, you can get color dispersion, and a rectangular prism will block more light than an elliptical mirror (especially important in a fast scope like the AstroScan). The star diagonal Wikipedia page has a pretty good discussion of the trade-offs.


Supplemental to @antlersoft's answer:

I was poking around a bit further after I posted and ran across this "1960's Edmund Scientific 'Space Conqueror', 3" reflector" with a prism diagonal at http://www.retrotechnology.com/glass/edmund.html

Not exactly 1970's but at least they were using them.

"Edmund Prism"

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    $\begingroup$ From the picture, it doesn't look like it could be a prism diagonal. It's a hunk of glass, but since it is attached to the support on one of the right angle sides, it seems like the diagonal face must be silvered for it to work. $\endgroup$
    – antlersoft
    Commented Jul 19, 2019 at 14:49
  • $\begingroup$ @antlersoft I see what you mean. Maybe you can post that as an "it's not a prism" answer to Does anybody recognize this kind of prism? $\endgroup$
    – uhoh
    Commented Jul 19, 2019 at 14:55

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