[Paper Review] A Near-Infrared Spectroscopic Survey at the SDSS 2.5-meter Telescope?

Michael F. Skrutskie, John C. Wilson|arXiv (Cornell University)|Mar 31, 2015

Astronomy and Astrophysical Research3 citations

TL;DR

This paper proposes adapting the SDSS 2.5-meter telescope for near-infrared spectroscopy using existing fiber and plug-plate infrastructure, enabling high-resolution (R ~ 24,000) and low-resolution (R ~ 3,000) surveys of Milky Way stars. It demonstrates that with fiber and corrector modifications, the telescope could achieve SNR=10 in 30 minutes for H~14 (R=3000) and H~12 (R=24,000), enabling kinematic and abundance studies of galactic giants via molecular CNO lines in the H-band.

ABSTRACT

We are posting this 10-year-old white paper to support an upcoming survey description paper for the SDSS-III Apache Point Galactic Evolution Experiment (APOGEE) led by PI Dr. Steven Majewski. The white paper presented here was a contribution to a 2005 "futures" planning process for the Astrophysical Research Consortium led by Dr. Donald York that examined both prospects for extending the work of SDSS and SDSS-II as well as enhancing the capabilities of the Apache Point 3.5-meter telescope and the overall scientific reach of the Consortium. This particular white paper describes the potential for using the Sloan 2.5-meter telescope and its fiber optic infrastructure to conduct a galactic plane chemical abundance survey in the low-extinction 1.6um H-band. The survey would target >1000 red giant stars per night selected from the Two Micron All Sky Survey using a >200 fiber near-infrared spectrograph operating at spectral resolution of R~24,000 with a magnitude limit of H~12 - very close to the final APOGEE implementation. A number of features suggested in the white paper did not survive to the actual survey including an R~3000 low-resolution spectral mode emphasizing kinematics to a fainter magnitude limit of K~14, a tunable high-resolution grating (retired by using three HAWAII-2RG arrays to cover most of the H-band all at once), a refrigerated optical train (the actual APOGEE is LN2 cooled), and the use of InGaAs arrays (HgCdTe remained the most mature technology at the time of construction). The white paper also suggests making the APOGEE instrument accessible to the 3.5-meter telescope at Apache Point, a project now underway.

Motivation & Objective

• To enable near-infrared spectroscopic surveys at the SDSS 2.5-meter telescope using existing multi-object fiber infrastructure.
• To overcome the lack of a dedicated near-infrared spectrograph despite the abundance of visible-wavelength spectrographs.
• To study the kinematics and chemical evolution of the Milky Way using high-resolution (R~24,000) and low-resolution (R~3,000) spectroscopy of red giants.
• To assess technical feasibility of using existing fibers and corrector plates with modifications for improved near-infrared transmission.
• To evaluate InGaAs detector arrays as a cost-effective, low-noise alternative to HgCdTe for near-infrared spectroscopy.

Proposed method

• Adapt the existing SDSS fiber and plug-plate system for near-infrared use, retaining the 2-meter fiber path and mechanical configuration.
• Use two interchangeable gratings to provide R~3,000 (faint-object) and R~24,000 (high-resolution) spectral modes.
• Optimize for spectral coverage from 0.9–1.8 μm (R=3000) and 1.52–1.66 μm (R=24,000), focusing on the H-band where fiber transmission exceeds 90%.
• Replace high-OH fibers with low-OH fibers to improve long-wavelength transmission beyond 1.85 μm, where current fibers drop to 40%.
• Recoat or replace the fused silica common corrector (Corning 7940/7980) to extend transmission beyond 2.0 μm and reduce visible performance compromise.
• Use 2K x 2K InGaAs arrays as detectors, leveraging recent advances in low dark current and noise performance for astronomical use.

Experimental results

Research questions

• RQ1 Can the SDSS 2.5-meter telescope be effectively repurposed for near-infrared spectroscopy using existing infrastructure?
• RQ2 What are the limiting transmission factors in the current fiber and corrector system for near-infrared wavelengths?
• RQ3 Is it feasible to achieve SNR=10 in 30 minutes for H~14 (R=3000) and H~12 (R=24,000) with modified components?
• RQ4 Can InGaAs detector arrays provide sufficient performance for high-resolution near-infrared spectroscopy at a lower cost than HgCdTe?
• RQ5 What are the mechanical and thermal constraints for mounting a high-resolution (R~24,000) spectrograph on the telescope’s back end?

Key findings

• The SDSS 2.5-meter telescope can support a near-infrared spectroscopic survey with R~3,000 and R~24,000 modes using existing fiber and plug-plate infrastructure.
• Fiber transmission drops to ~55% in the long-wavelength J band due to OH absorption, but remains >90% in the H band (1.5–1.7 μm), making this region optimal for high-resolution work.
• Beyond 1.85 μm, current high-OH fibers transmit only ~40% of light, necessitating replacement with low-OH fibers for K-band observations.
• The common corrector’s visible-optimized coating limits near-infrared transmission beyond 2.0 μm, but replacement or recoating is feasible with minimal impact on visible performance.
• InGaAs arrays with 2K x 2K format are viable for the spectrograph, offering low dark current and good noise performance, and are expected to be available in time for implementation.
• A high-resolution mode (R~24,000) covering 1.52–1.66 μm can achieve SNR=10 in 30 minutes for H~12, enabling precise abundance measurements of CNO lines in red giants.