Star Ware: The Amateur Astronomer's Guide to Choosing, Buying ...

16 Star Ware
an upright image, but its optical curves proved difficult for Gregory and his
contemporaries to fabricate.
A second design was later conceived by Sir Isaac Newton in 1672 (Figure
2.6). Like Gregory, Newton realized that a concave mirror would reflect and
focus light back along the optical axis to a point called the prime focus. Here an
observer could view a magnified image through an eyepiece. Quickly realizing
that his head got in the way, Newton inserted a flat mirror at a 45° angle some
distance in front of the primary. The secondary, or diagonal, mirror acted to
bounce the light 90° out through a hole in the side of the telescope’s tube. This
arrangement has since become known as the Newtonian reflector (Figure 2.4c).
The Newtonian became the most popular design among amateur astronomers
in the 1930s, when Vermonter Russell Porter wrote a series of articles
for Scientific American magazine that popularized the idea of making your
own telescope. The Newtonian is relatively easy (and, therefore, inexpensive)
to make, giving amateurs the most bang for their buck. Although chromatic
aberration is completely absent (as it is in all reflecting telescopes), the Newtonian
is not without its faults. Coma, which turns pinpoint stars away from
the center of view into tiny “comets,” with their “tails” aimed outward from the
center, is the biggest problem, and is exacerbated as the telescope’s focal ratio
drops. Optical alignment is also critical, especially in fast optical systems, and
must be checked often.
Figure 2.6 Newton’s first reflecting telescope. From Great Astronomers by Sir Robert S.
Ball, London, 1912.