[Paper Review] ACCESS: Enabling an Improved Flux Scale for Astrophysics

M. E. Kaiser, J. W. Kruk+21 more|arXiv (Cornell University)|Jan 22, 2010

Calibration and Measurement Techniques28 references3 citations

TL;DR

ACCESS proposes a sub-orbital rocket mission to achieve 1% absolute spectrophotometric calibration of standard stars across 0.35–1.7 μm using NIST-traceable detector standards. By observing stars above Earth's atmosphere with a single optical path and rigorous error budgeting, it enables precise flux calibration critical for dark energy and stellar physics research.

ABSTRACT

Improvements in the precision of the astrophysical flux scale are needed to answer fundamental scientific questions ranging from cosmology to stellar physics. The unexpected discovery that the expansion of the universe is accelerating was based upon the measurement of astrophysical standard candles that appeared fainter than expected. To characterize the underlying physical mechanism of the "Dark Energy" responsible for this phenomenon requires an improvement in the visible-NIR flux calibration of astrophysical sources to 1% precision. These improvements will also enable large surveys of white dwarf stars, e.g. GAIA, to advance stellar astrophysics by testing and providing constraints for the mass-radius relationship of these stars. ACCESS (Absolute Color Calibration Experiment for Standard Stars) is a rocket-borne payload that will enable the transfer of absolute laboratory detector standards from NIST to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of R = 500 across the 0.35-1.7 micron bandpass. Among the strategies being employed to minimize calibration uncertainties are: (1) judicious selection of standard stars (previous calibration heritage, minimal spectral features, robust stellar atmosphere models), (2) execution of observations above the Earth's atmosphere (eliminates atmospheric contamination of the stellar spectrum), (3) a single optical path and detector (to minimize visible to NIR cross-calibration uncertainties), (4) establishment of an a priori error budget, (5) on-board monitoring of instrument performance, and (6) fitting stellar atmosphere models to the data to search for discrepancies and confirm performance.

Motivation & Objective

• Address the need for 1% precision in the astrophysical flux scale to resolve fundamental questions in cosmology and stellar physics.
• Overcome systematic uncertainties in current flux calibration, which exceed 1% across the visible to near-infrared (NIR) range.
• Establish a direct, absolute calibration of standard stars using NIST-traceable irradiance standards to eliminate reliance on Vega as a sole reference.
• Enable accurate cross-color calibration across 0.35–1.7 μm for Type Ia supernovae studies to distinguish dark energy models.
• Validate high-resolution stellar atmosphere models by fitting them to high-precision spectral data, improving flux predictions beyond observed bands.

Proposed method

• Deploy a rocket-borne payload to observe standard stars above Earth's atmosphere, eliminating atmospheric contamination.
• Use a single optical path and detector system to minimize visible-to-NIR cross-calibration uncertainties.
• Perform repeated observations of each standard star (at least twice) to verify measurement repeatability and reduce statistical uncertainty.
• Implement an a priori error budget with systematic uncertainty contributions quantified from literature, specifications, and prior instrument experience.
• Monitor instrument performance in-flight to detect and correct for drifts or anomalies during observations.
• Fit stellar atmosphere models to observed spectra to identify discrepancies and validate model accuracy across the 0.35–1.7 μm bandpass.

Experimental results

Research questions

• RQ1 Can a sub-orbital rocket mission achieve 1% absolute flux calibration of standard stars across 0.35–1.7 μm using NIST-traceable standards?
• RQ2 What are the dominant sources of systematic uncertainty in visible and NIR spectrophotometry, and can they be reduced to below 0.86%?
• RQ3 How well can stellar atmosphere models fit high-resolution, medium-resolution spectroscopic data to extend flux predictions beyond observed wavelengths?
• RQ4 To what extent does observing above the atmosphere eliminate atmospheric contamination in flux calibration of standard stars?
• RQ5 Can repeated observations of the same stars on separate flights verify calibration repeatability at the 1% level?

Key findings

• The identified systematic uncertainties in the visible range are estimated at 0.59%, and in the NIR at 0.73%, well below the 0.86% threshold for a 1% total error budget.
• A margin of 0.47% remains for unidentified systematic errors or calibration of future dark energy missions, indicating feasibility of the 1% goal.
• The statistical uncertainty per resolution element is 0.5%, which allows for a systematic error budget of 0.86% when combined quadratically.
• Model fitting to observed spectra improves confidence in flux predictions beyond the measured bandpass, especially when compared to models fit only to broadband photometry.
• Repeatability of observations across two separate rocket flights is expected to be better than 1%, confirming the reliability of the calibration method.
• The use of a single optical path and detector system significantly reduces cross-calibration uncertainties between visible and NIR bands.