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WWW.ASTRONOMY.COM 57
and collecting more photons
over more pixels for a longer
time without overexposing
the image. When the data
are represented in a graph,
they appear as a point-spread
function (PSF) that is bell-
shaped (see “Out of focus”
on page 56).
My team and I have also
studied, at the Mark Slade
Remote Observatory (MSRO)
in Wilderness, Virginia, a
new technique that I discov-
ered in April 2018. This tech-
nique, called the diffuser
method, is based on a
technique first studied by
astronomers at Penn State in
a paper published in October
2017 entitled “Towards
Space-Like Photometric
Precision from the Ground
with Beam-Shaping
Diffusers.”
The diffuser method uses
an instrument called an
Engineered Diffuser, pro-
duced by RPC Photonics Inc.,
from Rochester, New York.
The diffuser spreads the light
out over more pixels, like the
defocus method. Placed in the
image train like a filter, it
serves as an optical beam-
shaping element that creates a
“top-hat”-shaped PSF (see
“Data spread” above).
Diffuser results
With the traditional defocus
method, even though the light
is spread out to increase preci-
sion, its bell-shaped PSF does
nothing to mitigate the effects
of scintillation or reduce the
need for precise tracking to
eliminate RCE. At the MSRO,
I used a 0.5° divergence dif-
fuser and analyzed the data
with AstroImageJ, a freely
available light-curve analysis
tool.
I found that using the dif-
fuser resulted in a very stable
PSF. It also significantly
reduced the short-term
scintillation.
I also found that, even
with a significant amount of
drift in the image over several
hours, RCE was virtually
eliminated. This is because
the light is spread out among
many pixels and the RCE is
“averaged out.”
The benefits of using the
diffuser method are shown in
a paper describing our work,
published recently in the
Proceedings of the Society for
Astronomical Sciences 2019
Symposium, entitled “A
Comparison of the Diffuser
Method Versus the Defocus
Method for Performing High-
Precision Photometry with
Small Telescope Systems,”
Hubbell et al.
The diffuser method typi-
cally reduces the shot noise to
less than 0.001 magnitude.
For three-minute exposures,
the short-term scintillation
error could be reduced to less
than 0.002 magnitude for a
magnitude 10.8 star.
The results for one exo-
planet I imaged, HAT-P-16 b,
proved to be of high quality.
The long-term scintillation
performance was the same as
the defocus method, but the
diffuser method provided
high-precision results even
with significant amounts of
haze and the Moon high in
the sky. The defocus method
did not reduce the short-term
scintillation in these same
conditions.
By using the diffuser
method, you’ll have more
opportunities to observe exo-
planet transits and make
high-precision measurements
with your equipment. All you
need to add is a diffuser. This
inexpensive method will help
you contribute to science by
making follow-up observa-
tions of exoplanets discovered
by NASA’s TESS mission or
archived Kepler data.
Jerry Hubbell is assistant
director of the Mark Slade
Remote Observatory and vice
president of engineering for
Explore Scientific.
The author and his team collect data at the Mark Slade Remote Observatory
in Wilderness, Virginia. JERRY HUBBELL
When light passes through the 0.5° Engineered Diffuser, the
collected data forms a “top-hat” point-spread function. The X and
Y axes are positions on the detector, and the Z axis is the count
The Engineered Diffuser developed by RPC Photonics Inc. is installed in the filter (photons). The star is shown at upper right. JERRY HUBBELL
wheel of a CCD camera at the Mark Slade Remote Observatory. JERRY HUBBELL
DATA
SPREAD
