Attached are some schematics of a little-known design -- the Pfund telescope.
The first one may properly be called a Pfund-Newtonian; the second one may
properly be called a Pfund-Herschelian. The Porter Turret telescope is the
most famous example of a Pfund telescope, except that the flat pivots and
the mount is at the end of the tube, opposite the concave mirror; the
illustrations given here assume a stationary flat and a conventional
alt-azimuth or equatorial mount.
Pfund telescopes have some advantages over the more conventional designs:
- LESS DIFFRACTION. Using curved vanes is the most common way to reduce
diffraction in conventional Newtonians, but this approach smears detail
out of the final image. The Pfund design eliminates the vanes entirely
so there are no diffraction crosses on star images and no loss of
- LESS OBSTRUCTION. The size of the perforation in the flat is determined
by the eyepiece O.D., so fast mirrors and slow mirrors would have the
same obstruction. This means that a 16" f/4 Pfund mirror with a 1-1/4"
eyepiece would suffer a light loss from obstruction of only 0.6%. No
secondary vanes also means no obstruction of binary stars.
However, Pfund telescopes also have some disadvantages:
- MORE COSTLY. Pfund telescopes require two large mirrors instead of one;
in fact, the flat has a major axis 1.414 (the square root of 2) times
larger than the concave mirror.
- MORE COOLING TIME. Both of the large mirrors require temperature adjustment
(cooling fans, heating resistors, or some other method of temperature
- MORE DIFFICULT TO MOUNT. The perforated flat cannot be made as a conical
mirror and mounted by a bolt through the middle, so the problem of a pinched
flat has to be carefully addressed with this design.
Despite the flaws in the Pfund design, the possibility of improved images
suggests that construction of these telescopes should be attempted. Any
reports or comments would be welcomed.