A 20 inch aperture Shafer-Maksutov telescope in Wales, UK is described. It is very simple with just spherical surfaces and no full aperture lenses

1. A 20” aperture Shafer-Maksutov telescope in Swansea,Wales

2. In the 1980’s both Field and Shafer
independently published descriptions
of a new kind of telescope. It is
ideally suited for amateur telescope
makers because it is very simple and
inexpensive to make. There are two
spherical mirrors and a single thick
meniscus lens. My version has been
called by others the Shafer-Maksutov.
It uses the lens thickness as a more
important design parameter than
Ralph Field’s design version.
For a large aperture telescope the lens
thickness gets too large and expensive. Then a
better solution is to split it into two thin lenses.

3. A retired doctor, a member of
the Swansea (Wales, UK)
Astronomical Society, read
about my design and offered to
fund the building of a 20” (500
mm) aperture telescope.
The less than ideal
observatory site is
right on the beach.
The telescope is
used mostly for
public education. It
has recently been
relocated to a
better site inland.

4. Around 1990 I did the
design, with a f/2.5
spherical primary
mirror and about an
f/18 system.
The two lenses are
both flat on one side
and have the same
convex/concave radius
on the other side. BK7
glass was used, the
cheapest optical glass.
The design is diffraction – limited over the visual spectrum over a small field size.

5. *LENS DATA
Shafer-Maksutov telecope
SRF RADIUS THICKNESS APERTURE RADIUS GLASS
1 -2500 -1000 250 REFL Primary mirror
2 229.75 -12.000 56.0 BK7
3 flat -44.430 56.0 AIR
4 flat -20.000 53.0 BK7
5 229.75 -5.000 53.0 AIR
6 -494.064 5.000 48.0 REFL_Secondary mirror
7 229.75 20.000 53.0 BK7
8 flat 44.430 53.0 AIR
9 flat 12.000 56.0 BK7
10 229.75 1300.0 56.0 AIR
Image
Here is the design data in millimeters, for
a 500 mm (20”) aperture f/18 design.
There is very little performance benefit to
having unequal radii on the two plano
convex and concave lenses, or departing
from flat sides on them. There is
essentially no benefit from using glasses
that are not the same for the two lenses.
The obscuration is about 22.5% diameter,
before allowing for lens mounts. A faster
speed design results with either a faster
speed primary mirror or some more
obscuration.
The clear aperture diameter of the first lens, the plano-concave one, is 100 mm but here I give
aperture sizes that are over-sized a little to allow for a turned-down edge on the lenses.

7. The main off-axis
aberrations of the
design are image
curvature and lateral
color. Both can be
fixed by adding a
small negative power
field lens down near
the image. That
then allows much
larger field sizes to
be covered with
diffraction-limited
performance.
No change is required of the other design parameters if this field lens is added to the design

8. It is the largest telescope in Wales