Sky & Telescope - August 2021 - 74

ASTRONOMER'S WORKBENCH by Jerry Oltion
A Better Ronchi Tester
An even light source and ease of use make all the difference.
A better solution is to repurpose the
head of one of those ubiquitous, cheap
9-LED fl ashlights.
You can't just stick the fl ashlight up
against the grid. If you do that, you're
back to staring at individual LEDs;
you'll just have nine to choose from.
You need to put a diffuser between the
LEDs and the Ronchi grid.
I've experimented with various
materials, and I've discovered that the
best diffuser, one that evens out the
light without dimming it very much, is
a two-stage unit that holds two layers
of white plastic bag about half an inch
apart. The fi rst layer goes right over the
fl ashlight head, and the second layer
goes next to the Ronchi screen. I use a
thin bag for the one closest to the light
and a thicker, whiter one for the outer
stage. That combination seems to give
me the most even light.
You could just stick the diffuser on
MANY MIRROR MAKERS use the Ronchi
test to evaluate their mirrors during
the parabolization phase. With the Ronchi
test, over- or under-parabolization,
zonal deviations, and turned edges are
easy to detect and easy to quantify. The
test is simple: Shine a light at the mirror
through a grid of evenly spaced lines
and look at the refl ected image of those
lines through that same grid. The result
will be a series of bars that bend in ways
that are specifi c to the shape of your
mirror. A spherical mirror will display
straight bars, and a parabolized mirror
will show curved bars.
The devil is in the details, and one
of those details is the test apparatus
itself. I've seen many mirror makers
struggling to get a decent image out
of their Ronchi tester, and the problem
is almost always the light source.
Somehow, a relatively large number
of people have become convinced that
they should use an LED or a pinhole or
slit. The problem with that approach
is that it doesn't fully illuminate the
mirror until you're very close to the
center of curvature, and the Ronchi
test is useful at a far greater range of
74 AUGUST 2021 * SKY & TELESCOPE
The LED head from a mini fl ashlight and two
diffusers provide a nice, even light source.
distances than that. At least it is when
you have a good light source.
The ideal light source for a Ronchi
test is a wide, evenly illuminated white
fl at-fi eld about an inch across. I've seen
people put light bulbs inside ping-pong
balls, and that works pretty well except
that the light falls off toward the edges.
a fl ashlight and hold that up against
the screen, but you'll soon fi nd that the
fl ashlight body gets in the way of your
head. You want to get your eye as close
to the light source as possible to minimize
the angle between the outgoing
and incoming light beams.
I take the LED pack out of the fl ashlight.
Some models can be removed
down to the circuit board, like the one
pictured here; others are most easily
used by simply cutting off the barrel of
the fl ashlight just behind the light element.
Either way, what you get is short
Left: The light source is uniform across a wide swath of the Ronchi grid. The grid is interchangeable
for lower- or higher-resolution testing. Right: The potentiometer and on/off switch are simply
glued to the frame.
ALL IMAGES COURTESY OF THE AUTHOR

Sky & Telescope - August 2021

Table of Contents for the Digital Edition of Sky & Telescope - August 2021