Absolute flux calibration of standard stars, traceable to the International System of Units (SI) standards, is essential for 21st century astrophysics. Dark energy investigations that rely on observations of Type Ia supernovae and precise photometric redshifts of weakly lensed galaxies require a minimum uncertainty of 0.5% (k=1) in the absolute color calibration. Other areas of astronomy and astrophysics, e.g. fundamental stellar astrophysics, will also benefit. In the era of large telescopes and all sky surveys, well-calibrated standard stars that do not saturate, are available over the whole sky, and extend to fainter magnitudes are needed. Our collaboration, NIST Stars, has developed a novel, fully SI-traceable laboratory calibration strategy that will enable achieving the demanding 0.5% requirement which we shall describe here. We discuss our results from a pilot study to determine the top-of-the-atmosphere absolute spectral irradiance of bright stars and the next steps.
The calibration hardware system of the Large Synoptic Survey Telescope (LSST) is designed to measure two quantities: a telescope's instrumental response function versus wavelength and atmospheric transmission. First of all, a "collimated beam projector," which projects monochromatic light, monitored with a NIST-traceable photodiode, through a mask and a collimating optic onto the telescope, is designed to measure the instrumental response function. This method does not suffer from stray light effects and the reflections/ghosting present when using a flat-field screen illumination, which has a systematic source of uncertainty from uncontrolled reflections. It allows for an independent measurement of the throughput of the telescope's optical train as well as each filter's transmission as a function of position on the primary mirror. Second, CALSPEC stars can be used as calibrated light sources to illuminate the atmosphere and measure its transmission. To produce spectrophotometry necessary to measure the atmosphere's transfer function, we use the telescope's imager with a Ronchi grating in place of a filter to configure it as a low resolution slitless spectrograph. In this paper, we describe this calibration strategy, focusing on results from this prototype system at the Cerro Tololo Inter-American Observatory (CTIO) 0.9 meter telescope. We compare the instrumental throughput measurements to nominal values from the vendor. We describe measurements of the atmosphere made via CALSPEC standard stars during the same run.
Improving the precision of ground-based astronomical observations is an objective of both current (e.g. PanSTARRS1) and future (e.g. Dark Energy Survey and the Large Synoptic Survey Telescope) sky surveys. An important element of this effort is to determine the optical attenuation imposed by the atmosphere. We have obtained atmospheric extinction observations from narrowband photometry (typically 10 nm bandwidth) at central wavelengths of 380 nm, 488 nm, 500 nm, 585 nm, 656 nm, 675 nm and 840 nm. The passbands were selected to measure the continuum component (predominantly from Rayleigh and aerosol scattering) of atmospheric attenuation, and to avoid molecular absorption features in the atmosphere. We compare these atmospheric extinction observations with predictions from MODTRAN5, a commonly used computer model of atmospheric optical transmission. The MODTRAN5 calculations were informed by a satellite-based determination of atmospheric ozone on the night of observations. We also adjusted the MODTRAN5 predictions of Rayleigh scattering to account for the difference between the default pressure and that measured at the observatory on the night of observations. We find excellent agreement across all passbands between the pressureadjusted MODTRAN5 extinction model and the observations, within our typical extinction uncertainty of 0.013 mag/airmass, but only if we exclude any aerosol scattering component in the MODTRAN5 model. Even though this is a very limited test, with observations of a single star for a single night, the fact that we obtain excellent agreement between extinction measurements and the MODTRAN5 model, with no adjustable fit parameters, bodes well for exploiting MODTRAN5 to increase the precision of ground-based flux measurements.
In this work, development of a fiber-optically coupled, vacuum-compatible, flat plate radiometric source applicable to the characterization and calibration of remote sensing optical sensors in situ in a thermal vacuum chamber is described. The original flat plate radiometric source configuration’s performance was presented at the 2009 Berlin SPIE. Following the original effort, design upgrades were incorporated in order to improve radiometric throughput and uniformity. Results of thermal and radiometric performance, with incorporated upgrades, of a flat plate illumination source in a temperature-controlled vacuum chamber operating at liquid nitrogen temperature are presented. Applications, including use with monochromatic tunable laser sources for the end-to-end system-level testing of large aperture sensors, are briefly discussed.
We describe a new apparatus for measuring the spectral irradiance of the Moon at visible wavelengths. Our effort builds upon the United States Geological Survey’s highly successful Robotic Lunar Observatory (ROLO), which determined a precise model for the time-dependent irradiance of the Moon from six years of observations obtained with an imaging telescope equipped with a set of narrow-band filters. The ROLO Irradiance Model allows the Moon to be used as a radiometric reference for tracking changes in the absolute responsivity of near-infrared to visible satellite sensors as a function of time to better than 1 %. The goal of the present effort is to improve the absolute radiometric accuracy of the ROLO model, presently estimated at 5 % - 10 %, to better than 1 %. Our approach, which uses an integrating sphere at the focal plane of a telescope to direct light from the integrated lunar disk into a stable spectrograph, also eliminates the need to interpolate between the 32 visible and near-infrared bands measured by ROLO. The new measurements will allow weather, climate, land-surface, and defense satellites to use the Moon as an absolute calibration reference, potentially reducing the impact of disruptions in continuous long-term climate data records caused by gaps in satellitesensor coverage.
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.
Improving the precision of observational astronomy requires not only new telescopes and instrumentation, but
also advances in observing protocols, calibrations and data analysis. The Laser Applications Group at the National
Institute of Standards and Technology in Gaithersburg, Maryland has been applying advances in detector
metrology and tunable laser calibrations to problems in astronomy since 2007. Using similar measurement techniques,
we have addressed a number of seemingly disparate issues: precision flux calibration for broad-band
imaging, precision wavelength calibration for high-resolution spectroscopy, and precision PSF mapping for fiber
spectrographs of any resolution. In each case, we rely on robust, commercially-available laboratory technology
that is readily adapted to use at an observatory. In this paper, we give an overview of these techniques.
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.
It is standard practice at many telescopes to take a series of flat field images prior to an observation run. Typically the
flat field consists of a screen mounted inside the telescope dome that is uniformly illuminated with a broadband light
source. These flat field images are useful for characterizing the relative response of CCD pixels to light passing through
the telescope optics and filters, but carry limited spectral information and are not calibrated for absolute flux.
We present the results of performing in situ, spectroradiometric calibrations of a 1.2 m telescope at the Fred Lawrence
Whipple Observatory, Mt. Hopkins, AZ. To perform a spectroradiometric calibration, a laser, tunable through the
visible to near infrared, was coupled into an optical fiber and used to illuminate the flat field screen in situ at the
telescope facility. A NIST traceable, calibrated photodiode was mounted on the telescope to measure the spectral flux
reaching the aperture. For a particular filter, images of the screen were then captured for each laser wavelength as the
wavelength was tuned over the filter bandpass. Knowledge of the incident flux then allows the relative responsivity of
each CCD pixel at each wavelength to be calculated.
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.
In this work, development of a fiber-optically coupled, vacuum-compatible, flat plate radiometric source applicable to
the characterization and calibration of remote sensing optical sensors in situ in a thermal vacuum chamber is described.
Results of thermal and radiometric performance of a flat plate illumination source in a temperature-controlled vacuum
chamber operating at liquid nitrogen temperature are presented. Applications, including use with monochromatic tunable
laser sources for the end-to-end system-level testing of large aperture sensors, are briefly discussed.
The feasibility of developing a network of telescopes to monitor the composition of the nighttime atmosphere using
stellar spectrophotometry is explored. Spectral measurements of the extinction of starlight by the atmosphere would
allow, for instance, quantification of aerosol, cloud, water-vapor, and ozone levels over the full range of elevation and
azimuth. These measurements, when combined with data from solar spectrophotometry derived from other instruments,
would provide continuous day/night monitoring of the atmospheric composition from the ground. The foundation for
such an effort would be a set of stable standard stars with known top-of-the-atmosphere spectral irradiances traceable to
international standards based on the SI system of units. Fully automated, reliable, easily maintained and highly costeffective
replicas of the spectrophotometric telescope used to calibrate the standard stars can be deployed worldwide at
sites such as atmospheric and astronomical observatories.
Current extreme ultraviolet (EUV) photoresist materials do not yet meet performance requirements on exposure-dose sensitivity, line-width roughness, and resolution. In order to quantify how these trade-offs are related to the materials properties of the resist and processing conditions, advanced measurements and fundamental studies are required that consider EUV-resist specific problems. In this paper, we focus on the correlations between the latent image and developed image in EUV exposed line/space features. The latent images of isolated lines produced by EUV lithography are characterized by atomic force microscopy through the change in topology caused by change in film thickness that occurs upon deprotection. The resulting latent-image deprotection gradient (DGL), based on line cross-sections, and latent-image line-width roughness (LWRL) provide metrics and insight into ways to optimize the lithographic process. The results from a model poly(hydroxystyrene-co-tert-butylacrylate) resist and a model calix[4]resorcinarene molecular glass type resist show the general applicability of the metric before development.
Current extreme ultraviolet (EUV) photoresist materials do not yet meet requirements on exposure-dose sensitivity,
line-width roughness (LWR), and resolution. Fundamental studies are required to quantify the trade-offs in materials
properties and processing steps for EUV photoresist specific problems such as high photoacid generator (PAG) loadings
and the use of very thin films. Furthermore, new processing strategies such as changes in the developer strength and
composition may enable increased resolution. In this work, model photoresists are used to investigate the influence of
photoacid generator loading and developer strength on EUV lithographically printed images. Measurements of line
width roughness and developed line-space patterns were performed and highlight a combined PAG loading and
developer strength dependence that reduce LWR in a non-optimized photoresist.
We have applied chemical force microscopy (CFM) to probe the surface roughness of partially developed model resist
materials in order to understand the fundamental materials properties of the resists leading to line edge roughness
(LER). CFM is capable of providing simultaneous information about surface topography and chemical heterogeneity of
partially developed resist films. We have used CFM to study ESCAP type resists that are used in 248 nm and extreme
ultraviolet (EUV) lithography. We observe changes in both the innate material roughness and chemical heterogeneity
of the resist with the introduction of photoacid generator (PAG) and with exposure and post exposure bake (PEB). We
find several mechanisms by which chemical heterogeneity can contribute to increasing the innate material roughness of
the resist.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.