Introduction
The Solar Tower Atmospheric
Cherenkov Effect Experiment (STACEE) is an experiment dedicated to the study of
high energy light (gamma rays) produced in astrophysical sources. We study
gamma rays to learn how Nature's powerful accelerators work and to learn about
possible new physics outside of our current theories. Astrophysical sources of
gamma rays include powerful objects such as neutron stars, supernovae, and
supermassive black holes.
STACEE uses a large field of
solar mirrors (heliostats) at the National Solar Thermal Test Facility near
Albquerque, NM. These mirrors were built for solar energy research conducted
during the daytime. STACEE uses the mirrors at night for astronomy. The mirrors
collect the short flashes of blue Cherenkov light that result from gamma-ray
interactions in the atmosphere. The Cherenkov light is then detected and
recorded by the STACEE equipment.
More information about the STACEE project can be found on the
STACEE Main Page at UCLA.
STACEE at CASE
Interesting Science
STACEE is a background-limited experiment, that is, the number of
events triggered by gamma rays from the source we are interested in is far
outweighed by the number of events triggered by background sources like cosmic
rays. There are several possibilities for quantites which STACEE measures -
such as the shape of the shower and the relative timing of the shower reaching
each mirror - which could be used to distinguish "signal" events (gamma rays)
from background events. We are currently working towards such as system which
would allow us to cut a larger fraction of our background which would improve
our sensitivity to gamma-ray events.
STACEE Atmospheric Monitor (SAM)
A daytime image of the SAM apparatus.
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The sensitivity of the STACEE experiment is greatly
dependent on the transparency of the atmosphere. The "SAM" component of the
STACEE experiment is designed to gather data to assist in assesing the
the quality of the "seeing." This includes a weather station to collect basic
atmospheric data such as the temperature and dewpoint, infrared cloud detectors,
and an 8" Celestron telescope. The telescope is used to monitor a "standard"
star from the
HIPPARCOS
catalog which is near to our target gamma-ray source.
In the past, we have used an
Electrim ENC2000N imaging camera with a
3-degree field of view to center that star manually and then perform photometry
using a phototube and PMT stopped down to about 12 arcminutes. The Case
group has recently upgraded the SAM system to a dual-CCD
SBIG ST-7, which allows us to simultaneously
autoguide the telescope using our
Losmandy Gemini controller and perform
CCD-imaging photometry on our reference star.
The autoguider takes a 32x32 pixel frame centered on our
reference star approximately once a second. The movement of the star is used
as feedback to the controller, and allows for accurate tracking over several
hours of observing time. This small frame is also saved to disk and
time-stamped for later use in measuring atmospheric clarity. Averaging over
two-minute periods, we get a measure of the relative intensity of the star
which should be inversely proportional to the column density of the atmosphere
above.
A sample sequence (32 seconds) of autoguider images.
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The dual SBIG CCD also takes a large 765x610 pixel array
adjacent to the autoguiding frame. We take a deeper image with the larger CCD,
with integration times on the order of a minute. The HIPPARCOS guiding star is
not in this frame, but the integration time is long enough that there are
always several stars visible. This allows us to have a measure of the relative
clarity of the sky over a 28-minute run - even when we don't have the guiding
star visible. This is particularly useful for times when we don't have a
pre-selected guide star, such as when we are observing a gamma-ray burst.
A 7.68 second exposure of M42 (The Great Orion Nebula)
taken with the SBIG ST-7 imager CCD on the SAM telescope.
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One of the main reponsibilities of the Case group has been
in leading the analysis of the SAM data. Some of basic data from the weather
station is simple and well-understood. For example, the SAM data gives us the
frost index (the difference between the temperature and the dewpoint), which
indicates when frost on the mirrors can degrade the optics chain. Importantly,
the presence of frost on the mirrors has been visually verified to correlate
with the frost index.
It has been far more difficult to use the weather data as an
indicator of atmospheric transmission, which should correlate with the energy
threshold for detection of gamma-rays. Theoretically, the CCD image intesity
or the IR cloud detectors, for example, should be a good indicator of sky
clarity. Any atmospheric modelling, however, is difficult since so many
variables can effect the "seeing." In addition, we don't have a reliable
means of testing our methods without a priori knowing whether or not
the viewing conditions were already good.
Sunspot Analysis
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