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The NIST STARS Program – Update 2017

Introduction

Vega and Sirius, are currently considered as fundamental standards. However, there are two main 

problems in applying these standards to WFIRST. The first is that they lack the requisite accuracy. 

An ongoing program at NIST aims to improve the accuracy of Vega and Sirius, and will be 

described in the paragraphs below. The second problem is that, with apparent brightnesses of V= 

0 mag (Vega) and V=-1.2 mag (Sirius), they are 1 000 000 times brighter than the V=14-19 mag 

stars suitable for large (8+ meter-diameter) telescopes and similarly, they are much too bright for 

the surveys planned for WFIRST. This problem has been reviewed separately (Bohlin et al 2014).

Vega (Alpha Lyrae) is an A-type, main sequence star (A0V) whose SI pedigree originates in 

experiments carried out almost 45 years ago relying on thermal standards (black-body furnaces) 

to set the scale in the infrared between 1 and 5 micrometers (Mountain et al 1985, Blackwell et al 

1983, Selby et al 1983), and in the visible between 300 nm and 900 nm (Hayes & Latham 1975, 

Hayes, Latham & Hayes 1975, Oke & Schild 1970). However, the uncertainties in the IR 

measurements were larger than the uncertainties in models of Vega, and the systematic 

uncertainties in the visible are on the order of 5%. 

Sirius (Alpha Canis Majoris) is an A1V star. Its spectral energy distribution was calibrated with 

MSX (Midcourse Space Experiment) using emissive reference spheres that were ejected at various 

times during the mission (Price et al. 2004). The uncertainties at the MSX wavelengths are of 

order 1% to 2%. 

Significant effort has been made to extend these absolute measurements to other wavelength 

ranges and stars (cf. Engelke et al 2010 and Bohlin et al 2011). Historically, Vega is considered 

the primary standard star and undergirds the commonly used photometric systems, despite the 

relatively large uncertainties. However, due to Vega’s (variable) dust disk, Sirius has become the 

primary standard in the mid infrared, but lacks direct SI traceable calibration at wavelengths 

shorter than about 3 micrometer. 

To address these shortcomings, the NIST Stars program aims to determine the top-of-theatmosphere, SI-traceable, absolute spectral irradiance of bright stars to with an uncertainty

of better than 1 % from a ground-based observatory.

Observation Overview: 

NISTStars has developed a novel, fully SI1 lab traceable calibration strategy that will enable 

achieving the desired accuracy. This strategy has two key components. The first is the SI-traceable 

calibration of the entire instrument system, and the second is the repeated spectroscopic 

measurement of the target star throughout the night. 

 1 International System of Weights and Measures
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A) Calibration of the telescope-spectrograph system

This is carried out by measuring the flux of a laboratory traceable calibration light source with a 

transfer standard spectrograph with an SI-traceable spectral irradiance responsivity mounted to a 

10 cm telescope and then observing the source with the telescope-spectrograph system. The light 

source is located approximately 100 m from the telescope. This allows us to determine the absolute 

spectral responsivity scale of our telescope-spectrograph system.

B) Stellar Observations

A bright star is continuously observed at a variety of air masses as it transits the sky. This allows 

us to use a Langley analysis which exploits the Beers-Lambert-Bouger Law to calculate the 

spectral atmospheric extinction and correct out ground based measurements to a top-of-theatmosphere spectral flux. We have conducted successful observations at the Fred Lawrence 

Whipple Observatory on Mt. Hopkins in southern Arizona. Our targets have been Vega and Sirius,

(see Figures 5 and 6) and we would like to add some southern sky targets as well. These 

observations would benefit from the more consistent photometric conditions available at Paranal 

and the atmospheric characterization facilities available at the Observatory.

Beers –Bouger-Lambert Law

This extinction law states that extinction (absorption and scattering out of the line of sight) is linear 

with radiation intensity and with the amount of matter through which the radiation traverses under 

the assumption that the material’s physical state is constant. This relation can be expressed as: 

IIλλ = IIooee−mmooττ(λλ)

where I0 is the incident radiation at wavelength λ, Iλ is the emerging radiation, m0 is the airmass at 

zenith, and τ(λ) is the optical depth. 

Langley Extrapolation (Langley Analysis)

The Langley analysis exploits the Beers-Bouger-Lambert Law to estimate the above the 

atmosphere incident radiation, taking into account that the path length through the atmosphere 

changes with altitude, i.e. 

IIλλ = IIooee−mm (λλ)

where m is the airmass, i.e. m= m0 sec(z) where z is the angle from the zenith. Taking the natural 

logarithm, the above equation becomes

ln II λλ = ln II0 − mm ττ(λλ)

Ideally, a plot of ln Iλ versus m is a straight line with a slope equal to τ(λ). I0 is obtained by linear 

extrapolation to m = 0, on the assumption that τ(λ) is constant during the observations. 

Langley analyses have been successfully applied to solar observations. 
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Calibration Chain:

Figure 1 shows a schematic of the calibration chain used for these measurements. At the heart of 

the calibration is a commercial compact array spectrograph (CAS) with an irradiance head that has 

been well characterized and calibrated in the NIST laboratory. To calibrate the spectrograph we 

use a type FEL lamp (FEL is not an acronym – it is a designation for the lamp design), which is a 

quartz halogen lamp with an SI-traceable irradiance through a standard NIST calibration. NIST 

has tested the repeatability of the spectrograph calibration and finds that when the spectrograph is 

maintained in the laboratory the calibration varies less than 0.05 % over months of observation as 

seen in Figure 2. NIST has also recalibrated the spectrograph over a series of field campaigns and 

finds that the calibration holds to ~0.2 % between deployments to mountaintop locations as seen 

in Figure 3. NIST has also used a tunable laser system in the lab to calibrate the wavelength scale 

of the spectrograph and periodically verify this with a HgNe pen lamp. NIST also uses the tunable 

laser system (part of SIRCUS, described separately) to perform stray light correction (Zong 2006). 

This reduces errors associated with calibrating to a 3000 K lamp and observing a star with different 

spectral profile.

In the field the irradiance head of the CAS is placed on a translation stage and aligned to the optical 

axis of the source. Irradiance measurements of the source are made at different distances and fit to 

the inverse square law. This fit both verifies the expected radiometric behavior of the system and 

provides the distance between the source and irradiance head. The CAS is then used to measure 

the spectral irradiance of the source.

The observing telescope/spectrograph then observes the source. The distance from the telescope 

to the source is measured with a laser range finder. Using the inverse square law, and the measured 

distances, we know the expected irradiance at the telescope, if there is no extinction along the light 

path. The correction for the extinction along the ~100 m horizontal path between the source and 

telescope is described below. This gives us a telescope/spectrograph system with an SI-traceable 

spectral irradiance responsivity.

The final step is to observe a star with the telescope. We again need to correct for the extinction of 

the star light along the path through the atmosphere. To calculate a top-of-the-atmosphere spectral 

flux, we observe the star through the night at a number of airmasses and use a Langley analysis to 

extrapolate the measurements to zero airmass.
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Figure 1. A schematic representation of the calibration chain. An SI-traceable FEL lamp is used to 

calibrate the spectral irradiance responsivity [counts per irradiance in each wavelength bin] of a 

spectrograph in the lab. The spectrograph is then used to measure the spectral irradiance [energy 

per second per unit area per unit wavelength] of a source in the field. The source is then observed 

by the telescope. Thus, through this sequence, the spectral response of the telescope is traceable 

to the original FEL lamp, and one considers the telescope to be calibrated. When that telescope 

measures the stellar spectral flux, the latter is directly comparable to the FEL lamp. Calibration 

of the telescope to the field source and the determination of the true stellar require a correction 

for atmospheric extinction along the line of sight.
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Figure 2. The stability of the transfer standard spectrograph measure the FEL lamp calibration 

source demonstrated by weekly measurements over two months. The calibration varies by less 

than 0.05 % outside of the water absorption bands.
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Figure 3. The transfer standard calibration over the course of multiple field deployments to Mt. 

Hopkins, AZ. Calibration values were typically within 0.2 % of the mean.
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NIST Stars Telescope, Instrument, and Field Calibration

The NIST Stars observing telescope is a 106 mm, f/5 refractor (Takahashi FSQ 106) with a 600-

micrometer diameter optical fiber at the focal plane to couple the light into a spectrograph. A 

90/10 beam splitter cube placed before the optical fiber allows for an optical cross where the 10 % 

beam allows a guide camera to image the target star and a second camera with a small lens focused 

on the optical fiber can view the back reflection of the star off the fiber. This second camera allows 

the star to be reproducibly centered in the fiber. The fiber carries the light to a commercial CzernyTurner spectrograph with a 1024 pixel, cooled CCD array detector (Instrument Systems, CAS 

140CT-153). The spectrometer has a 3 nm spectral resolution with a 0.65 nm pixel spacing 

covering a wavelength range from 380 nm to 1040 nm. The telescope is positioned with a 

computerized alt-az mount that is controlled by a PC running The Sky X software. The 

spectrograph is controlled with a separate PC running Labview. A picture of the system on Mt. 

Hopkins is shown in Figure 4.

Figure 4. The observing telescope (right) and collimated laser for horizontal extinction (left) on 

the ridge at Mt. Hopkins. The calibration source was located in the support building in the 

background.

The field calibration source consists of a 50 mm diameter integrating sphere with a 1 mm diameter 

aperture. The sphere is illuminated by a 10 W quartz tungsten halogen (QTH) lamp. A second 

spectrograph, of the same design as that used on the observing telescope but with an irradiance 

head fiber-coupled into the spectrograph, is used to measure the output of the calibration source. 

The calibration of this spectrograph is described below. The irradiance head is mounted on a 300 
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mm linear translation stage, which is attached to an optical rail on which the calibration source in 

mounted. A set of removable irises and a laser pointer are used to align the irradiance head and 

translation stage to the optical axis. The calibration source/optical rail assembly is mounted on a 

telescope mount and the same irises are used to align the calibration source to the observing 

telescope. 

Data Collection

The target star is acquired in the telescope and the camera looking at the fiber is used to position 

the image of the star in the center of the optical fiber. The auto-guiding camera is then used to 

control the mount to keep the starlight at that position on the fiber. With the telescope tracking 

the star, the spectrometer is set to acquire data in a 60 s time cycle consisting of a 5 s dark frame, 

followed by 9 spectra with 5 s exposures. The remaining time is used for data transfer. This cycle 

repeats for several hours as the star traverses through a range of air masses.

At least once per night, the telescope is turned to observe the calibration source. The same data 

acquisition protocol is used as for the stellar flux observations. Typically, 20 min to 30 min of 

calibration data is collected.

Figure 5 shows ‘out of the box’ results for two nights of data for Sirius obtained on Mt. Hopkins 

in November 2016. Each night’s observation were fit, wavelength by wavelength, using the 

Bouger-Beers-Langley formulation to compute the ‘top of the atmosphere’ (TOA) flux. The TOA 

spectrum is shown in black, and the HST/STIS spectrum of Sirius is in blue. Although the two 

nights’ spectra were acquired under different conditions, key is that the shape of the two TOA 

spectra match that of the HST/STIS over most of the wavelength range of the ground system, with 

the exception of the water vapour features at about 900 nm. Figure 6 shows the same type of 

results, but for the one night of Vega data.

To establish confidence in this method it is imperative to obtain repeated measurements of each 

star, over many nights, so as to build a statistically significant sample. Additionally, independent 

measurements of atmospheric extinction and characteristics (such as water content) along the lines 

of sight are needed to further reduce the systematic effects due to the Earth’s atmosphere. 

Horizontal Atmospheric Extinction

The light path between the source and telescope is long enough that there is a significant 

attenuation of the light along the path. To measure the attenuation, we use a collimated, stabilized 

HeNe laser mounted near the telescope and directed toward the calibration source. To measure 

the collimated light, we use a 15 cm diameter integrating sphere with a 2.5 cm diameter aperture 

and a photodiode mounted on the side wall. A current-to-voltage amplifier and voltage logger is 

used to measure to output of the photodiode. The sphere is held in front of the collimated laser 

beam at the telescope and then transferred to a position several meters to the side of the source. 

The ratio of these measurements provides a measure of the transmittance along the path at 633 nm. 

MODTRAN is then used to model the transmittance at different wavelengths with this as a tie 

point.
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Figure 5. Preliminary results from the November 2016 MT. Hopkins campaign. Ground 

observations of Sirius compared to the HST/STIS spectra of Sirius. In the good, photometric night

(top figure), the difference between the HST and ground derived top of the atmosphere spectrum 

is small;l, of order 3%
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Figure 6. Plotted are the STIS spectrum and the NIST top of the atmosphere spectrum of Vega 

(Alpha Lyrae), obtained on a good night from Mt. Hopkins (Preliminary results). 

Future Plans

One shortcoming of the current approach is that the Langley analysis assumes that the atmospheric 

extinction is constant over the time that the measurements are made. NIST currently does not have 

any independent monitor of the atmosphere at Mt. Hopkins, and there is significant water vapor 

and aerosols. NIST is proposing to the European Southern Observatory (ESO) for permission to 

take the NIST Stars equipment to a site nearby the ESO Paranal Observatory. ESO operates a 

unique array of instruments in support of Science Operations that can provide NIST with the 

relevant information that can help to independently assess the quality and stability of the

atmosphere on a given night. NIST also plan to acquire a duplicate of the 183 GHz water vapor 

profiling radiometer that exists at ESO but just looks zenith. The intent would be to coordinate an 

intercomparison with the ESO existing system and then co-locate the NIST water vapor profiling 

radiometer with the line of site of the NIST Stars telescope to measure along the same line of site 

when observing stars. NIST also has access to two LIDAR systems, in development, which could 

be used for the aerosol determination along the line of site, if needed.
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After obtaining statistically significant data (i.e. 20 or more nights per star) at Paranal on a set of 

bright stars and showing that the ‘proof of concept’ is successful – i.e systematic effects are 

controlled, the next steps are to: 

1) Apply the same calibration method to a larger telescope and obtain accurate 

spectrophotometry of fainter (e.g. V~14 mag) stars in the 0.3 to 1.0 micrometer range, 

which benefits the entire astronomical community. This step is straightforward, and could 

be done with existing telescopes. 

2) Extend the method into the near infrared (to 2 micrometers) which would then provide SI 

traceable absolute flux calibration for a crucial wavelength range.. The infrared extension 

will require additional R&D to verify the precision of the calibration chain. 

In both cases, just as with the current hardware suite, systematic monitoring of the 

atmosphere with additional instrumentation, e.g water vapor monitors, LIDAR systems, 

are necessary.

References:

Blackwell, D.E. , Leggett, S.K.,Petford, A.D., Mountain, C.M. and Selby, M.J., Absolute 

calibration of the infrared flux from VEGA at 1.24, 2.20, 3.76 and 4.6 microns by comparison with 

a standard furnace, 1983, MNRAS, 205, 897.

Selby, M.J., Mountain, C.M., Blackwell, D.E., Petford, A.D. and Leggett, S.K., Measurement of 

the absolute monochromatic flux from VEGA at 2.20 and 3.80 microns by comparison with a 

furnace, 1983, MNRAS, 203, 795. 

Bohlin, R.C. Gordon, K.D., and Tremblay, P.-E., Techniques and absolute flux calibration from 

the ultraviolet to the mid-infrared, 2014, PASP 12,711-732.

Bohlin, R.C. et al, Absolute Flux Calibration of the IRAC Instrument on the Spitzer Space 

Telescope Using Hubble Space Telescope Flux Standards, 2011, AJ, 141,173.

Engelke, C.W., Price, S.D. and Kraemer, K.E. Spectral Irradiance Calibration in the Infrared. 

XVII. Zero-magnitude Broadband Flux Reference for Visible-to-infrared Photometry, 2010, AJ, 

140, 1919.

Engelke, C.W., Price, S.D. and Kraemer, K.E., Spectral Irradiance Calibration in the Infrared. 

XVI. Improved Accuracy in the Infrared Spectra of the Secondary and Tertiary Standard 

Calibration Stars, 2006, AJ, 132, 1445. 

Hayes, D.S. and Latham, D.W. and Hayes, S.H., Measurements of the monochromatic flux from 

VEGA in the near-infrared, 1975, ApJ, 197, 587.

Hayes, D.S. and Latham, D.W., A rediscussion of the atmospheric extinction and the absolute 

spectral-energy distribution of VEGA, 1975, ApJ, 197, 593.

LeBohec, Stephan et al. Stellar intensity interferometry: experimental steps toward long-baseline 

observations, 2010, SPIE, 7734

Mountain, C.M., Selby, M.J., Leggett, S.K., Blackwell, D.E. and Petford, A.D., Measurement of 

the absolute flux from VEGA at 4.92 microns, 1985, A&A, 151, 399. 
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Oke, J.B. and Schild, R.E., The Absolute Spectral Energy Distribution of Alpha Lyrae, 1970, ApJ, 

161, 1015. 

Price, S.D., Paxson, C., Engelke, C. and Murdock, T.L., Spectral Irradiance Calibration in the 

Infrared. XV. Absolute Calibration of Standard Stars by Experiments on the Midcourse Space 

Experiment, 2004, AJ, 128, 889. 

Stubbs, C.W., and Brown, Y.J., Precise Astronomical flux calibration and its impact on studying 

the nature of dark energy., 2015, Mod. Phys. Lett. A, v30, n40 (2015.30). 

Zong, Y., Brown, S.W., Johnson, B.C., Lykke, K.R., Ohno, Y., Simple spectral stray light 

correction method for array spectrographs, 2006, Appl. Opt., 45, 1111.