Stage-V Spectroscopic Instrument (Spec-S5)

• Stage-V Spectroscopic Instrument (Spec-S5) is a next-generation, optical/near-infrared wide-field spectrograph featuring high multiplexing to map galaxies, quasars, and stars across 0<z<4.5.
• It employs advanced robotic fiber positioners, a dual-hemisphere large-aperture telescope, and precise calibration techniques to achieve record mapping speed and survey grasp.
• The instrument is designed to deliver sub-percent BAO and RSD measurements along with tight constraints on primordial non-Gaussianity, dark energy, and dark matter microphysics.

The Stage-V Spectroscopic Instrument (Spec-S5) is a next-generation, high-multiplex, wide-field, optical/near-infrared spectroscopic facility conceived to address the critical measurement goals of cosmic inflation, dark energy, and dark-matter microphysics across the full extragalactic sky ($0DESI, DESI-II, and SDSS-V, scaling up telescope aperture, fiber count, mapping speed, and spectral coverage to enable precision cosmological and astrophysical constraints that are inaccessible to current facilities (Besuner et al., 10 Mar 2025, Schlegel et al., 2022, Schlegel et al., 2022).

1. Science Objectives and Survey Rationale

The underlying motivation for Spec-S5 is the need for a spectroscopic facility capable of mapping galaxies, quasars, and stars in three dimensions to an unprecedented depth and density, for four principal science drivers:

1. Dark Energy and Expansion History: Sub-percent measurement of the baryon acoustic oscillation (BAO) scale in multiple redshift bins ($z\lesssim4.5$), enabling stringent constraints on the dark-energy equation of state parameters $(w_0, w_a)$ via the CPL model $w(a) = w_0 + w_a(1-a)$ (Besuner et al., 10 Mar 2025).
2. Structure Growth and Gravity: Direct measurement of the growth rate $f\sigma_8(z)$ via redshift-space distortion (RSD) analyses reaching $\sim0.55\%$ precision in $2.1
3. Primordial Physics: Ultra-large-volume mapping of high-redshift galaxies, yielding forecast $\sigma(f_{\rm NL}^{\rm local})\approx 1.0$ (MegaMapper case), sensitivity to $\sigma(N_{\rm eff})\sim0.03$ and $\sigma(\Sigma m_\nu)\sim20$ meV, and enabling tests of the power spectrum and non-Gaussianity of inflation beyond CMB cosmic variance (Besuner et al., 10 Mar 2025, Schlegel et al., 2022).
4. Milky Way Dark Matter and Archaeology: Stellar kinematic and chemical abundance mapping (≈150 M spectra) for detailed study of Galactic halo structure, streams, and subhalos down to masses $\sim10^6\,M_\odot$.

This survey design uniquely provides complementary cross-validation with weak lensing, CMB lensing, line intensity mapping, gravitational wave event host identification, and photometric redshift calibration (Schlegel et al., 2022).

2. Instrument Architecture and Technical Specifications

Spec-S5’s architecture is defined by its dual-hemisphere wide-field, large-aperture telescope system, coupled to an ultra-high multiplex focal plane and a distributed spectrograph array:

Subsystem	Parameter	Value
Telescope Optics	Primary mirror diameter	6.0–6.5 m (ULE/Zerodur)
	Field of view (FOV)	2.2–3.0 deg (diameter)
	Focal ratio	$f/3.6$–2.1
Fiber System	Number of science fibers	12 852 (Spec-S5 baseline) / 26 100 (MegaMapper concept)
	Pitch/patrol radius	6.2 mm, $\sim$1.7′–27″ on sky
	Position accuracy	$\le$10 μm (spec), $\le$2 μm (MegaMapper)
Spectrographs	Number per site	23 (Spec-S5 baseline) / 40–45 (MegaMapper)
	Fibers per spectrograph	567 / 600–675
	Channels	3-arm (blue, red, NIR optional)
Wavelength Range	Coverage	360–980 nm (optical), 0.98–1.2 μm (NIR opt.)
Spectral Resolution	$R=\lambda/\Delta\lambda$	$R\sim2000$ (blue)–5500 (red)
Throughput	Peak fiber–detector	0.4–0.5 (Spec-S5)/0.7–0.9 (MegaMapper) at 600 nm
Detector	Format	4k×4k CCD / 2k×2k HgCdTe arrays
Data Rate	Survey fiber-hour rate	%%%%17$R=\lambda/\Delta\lambda$18%%%% hr/year

The combination of $A_{\rm tel}$ ($\gtrsim 25$ m$^2$), $\Omega_{\rm FOV}$ ($\sim7$ deg$^2$), and $N_{\rm fib}$ yields an étendue $A\times\Omega\times N_{\rm fib}$ of $>$233 m$^2$ deg$^2$ ($>14\times$ DESI) (Schlegel et al., 2022).

3. Multiplexing, Focal Plane, and Calibration

Spec-S5 advances multiplexing via robotic fiber positioners with pitch $\sim6$ mm, patrol radius up to $1.5\times$ pitch, and closed-loop metrology delivering repeatability to $\leq5$ μm over $2\times2$ m$^2$ focal plane. Positioners are grouped in triangular "rafts" (Editor's term), each serving 63–75 fibers for high-density assignment. Sky and guide fibers ($\sim5\%$) are interspersed for real-time calibration (Schlegel et al., 2022, Schlegel et al., 2022).

Wavelength calibration is provided by nightly arc exposures and stabilized etalons for NIR channels. Flat-fielding uses quartz-tungsten and twilight sky. Sky subtraction relies on principal-component analysis from dedicated sky fibers (Kollmeier et al., 9 Jul 2025). Radial velocity precision scales as $\sigma_v = c/(R\times\mathrm{S/N})$, reaching $\sim1.4$ km s$^{-1}$ for $R=22,000$, S/N=100.

4. Survey Design, Target Samples, and Observing Cadence

The Stage-V survey design favors all-sky tiling:

Sample	$z$-Range	Area (deg$^2$)	Density (deg$^{-2}$)	Total N	$t_{\rm exp}$ (s)	Fiber-hr (M)
LRG	0.4–1.0	25,000	1,400	35 M	450	4.4
ELG	0.6–1.6	25,000	1,400	35 M	450	4.4
QSOs	$<2.1$	25,000	250	6.2 M	450	0.8
Ly$\alpha$ QSOs	$>2.1$	25,000	80	2.0 M	3,600	2.0
LBG	2.0–4.5	11,000	2,500	27.5 M	5,400	41.0
LAE	2.1–3.5	11,000	3,000	33.0 M	2,700	25.0

A plausible implication is that Spec-S5 can acquire $\sim$138.7 M extragalactic spectra in dark time over 6 years (Besuner et al., 10 Mar 2025, Schlegel et al., 2022). Stellar and photometric calibration samples comprise >150 M bright-time spectra for Milky Way science.

Exposure time, S/N, and sample selection are optimized for cosmology, with spectroscopic success rates ranging from 60–100% per object class. Operationally, the survey revisits each field typically 2–6 times, both for high S/N accumulation and multi-epoch astrophysics.

5. Cosmological Performance and Science Forecasts

BAO errors are forecast to be $<$0.23% at all $z<4.5$, with RSD-derived $f\sigma_8$ reaching $0.55\%$ precision in the $2.1Besuner et al., 10 Mar 2025). Fisher-matrix combinations with CMB data improve $w_0$ and $w_a$ constraints to $\sim0.02$ and $\sim0.1$, respectively, and can reach $\sigma(N_{\rm eff})\sim0.03$ and $\sigma(\Sigma m_\nu)\sim20$ meV (Schlegel et al., 2022).

For primordial non-Gaussianity, scale-dependent bias enables constraints of $\sigma(f_{\rm NL}^{\rm local})\sim 1$, surpassing the Planck CMB limit by an order of magnitude. The mapping speed metric yields >12$\times$ fiber-hours/year compared to DESI, meeting the Stage-5 leap requirement in survey grasp, multiplex, and cosmological figure of merit.

Medium-band imaging preceding spectroscopic targeting (e.g., IBIS specifications: 7–12 filters, $\sim$11,000 deg$^2$ coverage, $n_3D \sim 2\times 10^{-4}$ h/Mpc$^3$ for LAEs) achieves σ(α) ∼0.7% (combined bins at $z_{\rm eff}=2.8$) (Feder et al., 6 Dec 2025). This scouting strategy optimizes target selection for Spec-S5’s 3D BAO analyses.

6. Instrument Evolution, Technical Challenges, and Timeline

Stage-V development draws heavily on DESI, SDSS-V, and MegaMapper R&D. Key technology risks being retired include closed-loop 6 mm-pitch positioners, Skipper CCD blue-arm detectors (<0.2 e$^-$ read noise), and mass production of low-vignetted wide-field correctors. Engineering pathfinders and prototypes are underway (MegaMapper: 2022–2024). Spec-S5 leverages existing 4 m mounts (Mayall, Blanco) with upgrades, minimizing cost and schedule risk (Besuner et al., 10 Mar 2025, Schlegel et al., 2022).

Projected timeline for completion is first light $\sim$2028 (MegaMapper) to $\sim$2036 (Spec-S5), full science operations by $\sim$2030–2042. This satisfies the Astro2020 priority for “sustaining activity” and is aligned with P5 recommendations (Besuner et al., 10 Mar 2025).

7. Comparative Analysis and Scientific Context

Relative to existing Stage-IV facilities (DESI, SDSS-V), Spec-S5 and MegaMapper present transformative gains in mapping speed, spectral resolution, multiplex, and survey volume:

Metric	DESI	SDSS-V	Spec-S5/MegaMapper
Aperture (m)	4	2.5	6–6.5
N_fibers	5,000	500	12,852–26,100
Field of View (deg$^2$)	3.2	3	7
Survey Grasp (m$^2$deg$^2$)	$\sim$69	$\sim$11	160–233
Mapping Speed Metric	42,750	—	590,000
BAO FoM	—	—	$>2.5\times$ improvement over DESI+Planck

The scientific impact encompasses percent-level $\Omega_\mathrm{DE}(z)$ constraints to $z\sim4$–5, $\sigma(f_{\rm NL}^{\rm local})<1$, improved curvature bounds, and detailed mapping of Milky Way halo dark matter microphysics. Such performance enables synergy with CMB-S4, LSST, LIM, and multi-messenger facilities (Schlegel et al., 2022).

• (Besuner et al., 10 Mar 2025) "The Spectroscopic Stage-5 Experiment"
• (Schlegel et al., 2022) "A Spectroscopic Road Map for Cosmic Frontier: DESI, DESI-II, Stage-5"
• (Schlegel et al., 2022) "The MegaMapper: A Stage-5 Spectroscopic Instrument Concept for the Study of Inflation and Dark Energy"
• (Kollmeier et al., 9 Jul 2025) "Sloan Digital Sky Survey-V: Pioneering Panoptic Spectroscopy"
• (Feder et al., 6 Dec 2025) "Angular BAO Forecasts for the IBIS Medium-Band Survey"

This synthesis defines the Stage-V Spectroscopic Instrument as the foundational spectroscopic capability for the cosmological frontier in the 2030s.