Sky & Telescope - July 2023 - 70
ASTRONOMER'S WORKBENCH by Jerry Oltion
The Telrad Prescription
Ditch the eyeglasses while observing once and for all.
ONE OF THE MOST FRUSTRATING
aspects of wearing eyeglasses while
observing is that you have to take them
off to look through most eyepieces, yet
you have to put them back on to look
through a Telrad-style finder. On-off,
on-off all night until you want to throw
the eyeglasses into the dark and just
scan the sky at random.
If you're nearsighted, you often take
your glasses off to read charts, eyepiece
labels, make sketches, etc. The only
time you need the glasses is when you
look through the finder.
Retired optical research engineer
Bob Schalck reasoned that if he could
put the correction on the Telrad instead
of on his face, he could eliminate the
swapping on and off and enjoy observing
without distraction.
Bob is nearsighted by 3 diopters.
Diopters are how opticians measure
a lens's focal length, and it's a simple
conversion to more astronomer-friendly
units: Divide diopters into
1,000 millimeters (1 meter)
to get the focal length in
millimeters. So 3 diopters
of nearsightedness require
a minus-3-diopter lens to
correct, so that's 1,000/-3, or
-333 mm.
A little hunting on the
Surplus Shed website (surplusshed.com)
found Bob a
50-mm double-concave lens
of -300-mm focal length.
Close enough. In fact, the
extra strength of the lens
(it's -3.3 diopters) provides
an advantage: At night when our pupils
dilate we often become even more
nearsighted, a phenomenon called
night myopia. Having a little extra
power in the corrective lens helps
compensate for that.
Once Bob had his lens in hand, the
next question was how to mount it on
The mount requires holes for the
lens and for the Telrad adjustment
knobs.
the Telrad. These finders have
an angled glass window set
inside an open-backed framework,
so there's no convenient
place to attach a lens. If you
had a small enough lens, you
could set it on top of the Telrad's
lens (which makes the
target look like it's an infinite
distance away if you have
20/20 vision), but that would
leave the sky blurry when you
looked through the window.
Bob wanted the sky to be as sharp as the
target, which meant putting the lens
between his eye and the glass window.
Also, there would undoubtedly be
times when Bob still had his glasses on
when he looked through the finder, so
he wanted the lens to be easily removable.
Ideally it should be easier to
pLeft: In the upright position, the corrective lens functions like an eyeglass lens, allowing you to use a Telrad without glasses. Right: With the lens
tilted down and out of the way, the Telrad works normally. This also gives access to the Telrad's alignment knobs.
70 JULY 2023 * SKY & TELESCOPE
BOB SCHALCK (3)
http://www.sur-plusshed.com
http://www.sur-plusshed.com
Sky & Telescope - July 2023
Table of Contents for the Digital Edition of Sky & Telescope - July 2023
Contents
Sky & Telescope - July 2023 - Cover1
Sky & Telescope - July 2023 - Cover2
Sky & Telescope - July 2023 - 1
Sky & Telescope - July 2023 - Contents
Sky & Telescope - July 2023 - 3
Sky & Telescope - July 2023 - 4
Sky & Telescope - July 2023 - 5
Sky & Telescope - July 2023 - 6
Sky & Telescope - July 2023 - 7
Sky & Telescope - July 2023 - 8
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Sky & Telescope - July 2023 - Cover4
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