Sky & Telescope - December 2023 - 72

ASTRONOMER'S WORKBENCH by Jerry Oltion
Build a Solar Projector
Here's a solar viewer everyone can enjoy at once.
[glasscages.com]. Get the low-iron
glass; it's clearer.) Grind some channels
in the tool to prevent seizing onto
the lens and then get to work. Remember,
you want the tool to be concave
and the lens to be convex, so work with
the tool on top.
Window glass probably isn't flat
Ed Jones's 7-inch solar
projector focuses its image
125 feet away, where
it spans 14 inches.
DURING THE TOTAL SOLAR ECLIPSE
next April, just about every place in
Mexico and North America will see
at least a partial phase. Wouldn't it be
neat to share the view with a big group
of your friends? Easy-peasy. Professional
optician Ed Jones says, " Nothing shows
an eclipse's partial phases better than a
solar projector. "
A solar projector is nothing more
than a very-long-focal-length, singleobjective
lens with a mirror bouncing
sunlight through the lens onto a white
screen. Ed has built several of them
in sizes ranging from 4 to 7 inches
of aperture, and if you're the least bit
comfortable grinding mirrors, you can
do the same.
Only you won't be grinding a mirror
this time unless you want to make
your own flat. (You can use an ordinary
Newtonian secondary for that.) For this
project, you'll be grinding a lens.
Most people cringe at the thought
of grinding a lens, but a solar-projector
lens is probably the easiest lens you'll
ever make. For one thing, the amount
of curvature required is minuscule.
72 DECEMBER 2023 * SKY & TELESCOPE
We're talking a sagitta of only 0.002″
for a 4″-diameter lens. That's about
half the thickness of a sheet of paper.
It's so minuscule a curve that you can
start your grinding with 40-micron
grit, or even 20 if you don't mind hogging
it out for a few minutes. (That's
right: minutes.)
What should you use for a blank and
a tool? Ordinary window glass will do.
A piece with ¼″ thickness will work,
but 3
/8″ thickness would be ideal if you
can find it. (And you can at Glass Cages
enough to make a plano-convex mirror,
so grind both sides. The thing about this
lens is that you really don't need to hit
any tight marks on focal length. Whatever
you get will work fine so long as it's
greater than about f/100 or so.
No kidding! The longer the focal
length, the larger the focused image.
Because the Sun subtends 32 arcminutes
in the sky, the image size is simply the
sine of 32 arcminutes (0.0093) times
the focal length of the lens, so a 72-foot
focal length (f/216 for a 4″ lens) would
give you an 8″-diameter image.
At that focal length, chromatic aberration
isn't an issue, and the figure of
your lens can remain spherical. You can
use a simple spherometer to get close
enough to the right curve, then move
on to your fine grits and pitch polishing.
To test your lens, you can simply
assemble the projector and look at the
solar image. Is it good enough to show
Left: The image of the Sun reveals details within sunspots and will be great for watching the partial
phases of an eclipse. Right: The tracking mechanism prevents having to adjust the aim every
couple of minutes. The mount isn't polar-aligned, but rather is aligned with its base along the path
of solar motion across the sky.
ED JONES (3)

http://www.glasscages.com

Sky & Telescope - December 2023

Table of Contents for the Digital Edition of Sky & Telescope - December 2023