Changing atmospheric transmission accounts for the largest systematic errors limiting photometric measurement
precision and accuracy for ground-based telescopes. While considerable resources have been devoted to correcting the
effects of the atmosphere on image resolution, the effects on precision photometry have largely been ignored. To correct
for the transmission of the atmosphere requires direct measurements of the wavelength-dependent transmission in the
same direction and time that the supported photometric telescope is acquiring its data.
We describe a multi-wavelength lidar, the Facility Lidar for Astronomical Measurement of Extinction (FLAME) that
observes the stable upper stratosphere, and the Astronomical Extinction Spectrophotometer (AESoP), a
spectrophotometer that creates and maintains NIST absolute standard stars. The combination of these two instruments
enables high photometric precision of both the stellar spectra and atmospheric transmission. The throughput of both
FLAME and AESoP are calibrated to NIST radiometric standards.
This inexpensive and replicable instrument suite provides the lidar-determined monochromatic transmission of Earth’s
atmosphere at visible and near-infrared wavelengths to better than 0.25% per airmass and the wavelength-dependent
transparency to better than 1% uncertainty per minute. These atmospheric data are merged to create a metadata stream
that allows throughput corrections from data acquired at the time of the scientific observations to be applied to
broadband and spectrophotometric scientific data. This new technique replaces the classical use of nightly mean
atmospheric extinction coefficients, which invoke a stationary and plane-parallel atmosphere and ultimately limit
ground-based all-sky photometry to 1% - 2% precision.
NIST-calibrated detectors will be used by the ground-based 100mm diameter Astronomical Extinction
Spectrophotometer (AESoP) to calibrate the spectral energy distributions of bright stars to sub-1% per 1nm spectral
resolution element accuracy. AESoP will produce about a hundred spectroradiometrically calibrated stars for use by
ground- and space-based sensors. This will require accurate and near-continuous NIST calibration of AESoP, an
equatorially mounted objective spectrophotometer operating over the wavelength range 350nm – 1050nm using a CCD
detector.
To provide continuous NIST calibration of AESoP in the field a near-identical, removable 100mm diameter transfer
standard telescope (CAL) is mounted physically parallel to AESoP. The CAL transfer standard is calibrated by NIST
end-to-end, wavelength-by-wavelength at ~ 1nm spectral resolution. In the field, CAL is used in a near-field
configuration to calibrate AESoP. Between AESoP science observations, AESoP and CAL simultaneously observe clear
sub-apertures of a 400mm diameter calibration collimator. Monochromatic light measured simultaneously by AESoP and
CAL is dispersed by the objective grating onto the AESoP pixels measuring the same wavelength of starlight, thus
calibrating both wavelength and instrumental throughput, and simultaneously onto a unique low-noise CAL detector
providing the required throughput measurement. System sensitivity variations are measured by vertically translating the
AESoP/CAL pair so that CAL can observe the AESoP sub-aperture.
Details of this system fundamental to the calibration of the spectral energy distributions of stars are discussed and its
operation is described. System performance will be demonstrated, and a plan of action to extend these techniques firstly
into the near infrared, then to fainter stars will be described.
Earth's atmosphere represents a turbulent, turbid refractive element for every ground-based telescope. We describe the
significantly enhanced and optimized operation of observatories supported by the combination of a lidar and
spectrophotometer that allows accurate, provable measurement of and correction for direction-, wavelength- and timedependent
astronomical extinction. The data provided by this instrument suite enables atmospheric extinction correction
leading to "sub-1%" imaging photometric precision, and attaining the fundamental photon noise limit. In addition, this
facility-class instrument suite provides quantitative atmospheric data over the dome of the sky that allows robust realtime
decision-making about the photometric quality of a night, enabling more efficient queue-based, service, and
observer-determined telescope utilization. With operational certainty, marginal photometric time can be redirected to
other programs, allowing useful data to be acquired. Significantly enhanced utility and efficiency in the operation of
telescopes result in improved benefit-to-cost for ground-based observatories.
We propose that this level of decision-making will make large-area imaging photometric surveys, such as Pan-STARRS
and the future LSST both more effective in terms of photometry and in the use of telescopes generally. The atmospheric
data will indicate when angular or temporal changes in atmospheric transmission could have significant effect across the
rather wide fields-of-view of these telescopes.
We further propose that implementation of this type of instrument suite for direct measurement of Earth's atmosphere
will enable observing programs complementary to those currently requiring space-based observations to achieve the
required measurement precision, such as ground-based versions of the Kepler Survey or the Joint Dark Energy Mission.
Ground-based telescopes supported by lidar and spectrophotometric auxiliary instrumentation can attain space-based
precision for all-sky photometry, with uncertainties dominated by fundamental photon counting statistics. Earth's
atmosphere is a wavelength-, directionally- and time-dependent turbid refractive element for every ground-based
telescope, and is the primary factor limiting photometric measurement precision. To correct accurately for the
transmission of the atmosphere requires direct measurements of the wavelength-dependent transmission in the direction
and at the time that the supported photometric telescope is acquiring its data. While considerable resources have been
devoted to correcting the effects of the atmosphere on angular resolution, the effects on precision photometry have
largely been ignored.
We describe the facility-class lidar that observes the stable stratosphere, and a spectrophotometer that observes NIST
absolutely calibrated standard stars, the combination of which enables fundamentally statistically limited photometric
precision. This inexpensive and replicable instrument suite provides the lidar-determined monochromatic absolute
transmission of Earth's atmosphere at visible and near-infrared wavelengths to 0.25% per airmass and the wavelengthdependent
transparency to less than 1% uncertainty per minute. The atmospheric data are merged to create a metadata
stream that allows throughput corrections from data acquired at the time of the scientific observations to be applied to
broadband and spectrophotometric scientific data. This new technique replaces the classical use of nightly mean
atmospheric extinction coefficients, which invoke a stationary and plane-parallel atmosphere. We demonstrate
application of this instrument suite to stellar photometry, and discuss the enhanced value of routinely provably precise
photometry obtained with existing and future ground-based telescopes.
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