Himalayan Faint Object Spectrograph (HFOSC)

• Himalayan Faint Object Spectrograph is a versatile optical instrument designed for low-to-medium resolution spectroscopy and imaging across 3500–10300 Å.
• It enables detailed studies in protostellar evolution and exoplanet transit spectroscopy, providing precise measurements like Hα equivalent widths and Rayleigh scattering slopes.
• The instrument supports extragalactic spectroscopy and atmospheric dispersion tests, informing calibration pipelines and next-generation ADC designs on moderate-aperture telescopes.

The Himalayan Faint Object Spectrograph (often referenced as HFOSC in recent literature) is a multi-purpose optical instrument mounted on the 2-m Himalayan Chandra Telescope (HCT) at Hanle, India, designed to enable low- to medium-resolution spectroscopy and imaging of faint astronomical sources. HFOSC serves as a principal spectrograph for studies ranging from protostellar evolution and exoplanet atmospheric characterization to large-scale radio galaxy surveys and atmospheric site testing. Its technical flexibility and positional context at Hanle's high-altitude site underpin its critical role in advancing observational astrophysics on moderate-aperture telescopes.

1. Instrument Design and Operational Modes

HFOSC is engineered for faint-object spectroscopy and imaging between approximately 3500 Å and 10300 Å. Grism selection (e.g., Grism 5: 0.38–0.7 μm; Grism 7: 0.35–0.8 μm; Grism 8: 0.5–0.91 μm) determines both wavelength coverage and resolving power, typically ranging from R ≈ 870–2500. Slit widths, adjustable—common values are 1.92″ and 1340 μm—enable tailored observational trade-offs between spectral resolution and throughput.

Instrument calibration protocols include:

• Bias subtraction and flat-field correction
• Wavelength calibration using Fe–Ne or Fe–Ar arc lamps
• Cosmic ray removal
• Aperture extraction via multidisciplinary pipelines (e.g., IRAF with PYRAF scripts, or dedicated Python codes leveraging PyKosmos and CCDProc).

Such reduction pipelines are often modified to accommodate HFOSC’s data format and faint-object requirements, notably in studies processing low SNR spectra from protostars or exoplanet transit time series.

2. Applications in Protostellar Spectroscopy

HFOSC enables the detection and analysis of faint protostars, particularly those moderately embedded, through optical spectroscopy in the 5000–9100 Å band. In recent campaigns, Grism8 and a 1.92″ slit yielded spectra with R ≈ 1100–2500, encompassing both accretion tracers (e.g., Hα) and molecular photospheric features (e.g., TiO bands at ~7140 Å). Data reduction utilized the HAPILI pipeline—a Python-based automated workflow incorporating bias subtraction, flat-fielding, aperture extraction, and wavelength calibration via IRAF routines.

This instrument was pivotal in the identification of optical photospheric features in flat-spectrum and Class I protostars from the HOPS catalog. Detection of strong TiO absorption bands in four objects demonstrated early formation of photospheres, analogous to late M-type stars. Accretion rates were inferred by extracting Hα equivalent widths (EW) and applying extinction-corrected continuum fluxes, then translating Hα luminosities to mass accretion rates using empirical scaling relations: $$\log_{10}(L_{acc}/L_\odot) = (1.13 \pm 0.05) \times \log_{10}(L_{H\alpha}) + (1.74 \pm 0.19)$$ and,

$$\dot{M}_{acc} \approx 1.25 \frac{L_{acc} R_*}{G M_*}$$

Estimated rates for observed protostars fell within 10⁻⁷–10⁻⁸ M₋ₒ/yr, comparable to values measured for T-Tauri stars, demonstrating continued accretion despite evolutionary advancement (Narang et al., 2023).

Name	Instrument	EW(Hα) (Å)	log₁₀(𝑀̇ₐcc)
HOPS 58	DOT/ADFOSC	39 ± 2	–7.53 ± 0.70
HOPS 235	HCT/HFOSC	65 ± 2	–7.17 ± 0.64

Observation targets generally excluded the most deeply-embedded Class 0 sources due to high extinction.

3. Exoplanet Transit Spectroscopy

HFOSC’s capabilities have extended to low-resolution transit spectroscopy of exoplanets, as first demonstrated in observations of HAT-P-1b, WASP-127b, and KELT-18b (Unni et al., 24 Oct 2024). The methodology employed spectral extraction in wavelength bins tuned for SNR ≈ 1000, using carefully-selected reference stars (including binary companions for HAT–P–1b and WASP–127b) to mitigate slit losses and atmospheric effects.

Analysis utilized the Common Mode Correction (CMC) technique: $$L_{\lambda,\text{corrected}}(t) = \frac{L_{\lambda,\text{observed}}(t)}{R_{\text{white}}(t)}$$ where $R_{\text{white}}(t)$ represents the residuals of the white lightcurve. This approach minimized time-dependent systematics and preserved planetary transit signatures.

HFOSC optical spectra revealed Rayleigh-scattering slopes in WASP-127b and HAT-P-1b, consistent with high-altitude atmospheric hazes. Chi-squared mapping using combined HFOSC and HST/Spitzer IR datasets enabled tighter constraints on equilibrium temperature, haze factor, and metallicity. Noise levels after CMC processing approached the photon noise limit (residual RMS ~0.002 normalized flux units), underscoring the instrument’s competitiveness with larger telescopes (such as Gemini/GMOS and GTC/OSIRIS).

A plausible implication is that HFOSC, and by extension similar 2 m-class telescopes, may be broadly applicable to cost-effective exoplanet atmospheric characterization when paired with optimized data collection and reduction pipelines.

4. Spectroscopic Surveys of Giant Radio Galaxies

HFOSC has facilitated precision spectroscopic redshift measurements for host galaxies of giant radio galaxies (GRGs), essential for confirming projected linear sizes above the 700 kpc threshold. Observational protocols included long-slit spectroscopy across multiple grisms, enabling coverage of emission lines such as Hβ, Hα, [O III], [N II], and [S II].

Redshift computation followed: $$z = \frac{\lambda_{obs} - \lambda_{rest}}{\lambda_{rest}}$$ and projected linear size determination used: $D = \theta \times d_A(z)$ where θ is the measured angular size and $d_A(z)$ the cosmological angular diameter distance.

Notably, in GRG J0151–1112, extended [O III] emission to ≈12 kpc along the radio jet axis indicated the presence of an AGN jet-driven ionized outflow—a rare and significant diagnostic of AGN feedback. The spectra—scored for equivalent width (e.g., [O III] λ5007 above ~5 Å for HERGs)—supported detailed AGN state classification, crucial for population studies and radio galaxy evolution analyses (Sethi et al., 9 Feb 2025).

5. Site Testing and Atmospheric Dispersion Measurement

For the first time, atmospheric dispersion was directly measured at Hanle using HFOSC (Bestha et al., 22 Sep 2025), employing observations of KELT-20 over zenith angles from 6° to 50°. The slit was aligned along the parallactic angle, and Grism 7 provided wavelength coverage over 350–800 nm. Trace shifts due to dispersion were quantified: $\Delta S(\lambda) = S(\lambda) - S(\lambda_0)$ where λ₀ = 0.7 μm served as the reference wavelength. Corrections for instrumental effects were derived from near-zenith exposures and subtracted using model results (Cassini atmospheric model via AstroAtmosphere).

Conversion from pixel to arcseconds used an empirically determined image scale from star separations in the field. The measured atmospheric dispersion at 50.61° zenith angle was 0.6082 arcsec, compared to the model’s 0.7429 arcsec—a discrepancy of 134.6 mas. This suggests that real atmospheric conditions and instrumental profile must be considered alongside theoretical models when designing atmospheric dispersion correctors (ADCs) for future observatories like the NLOT.

Improved dispersion correction enhances spectroscopic throughput and reduces systematic radial velocity errors, directly benefiting studies in exoplanet detection and atmospheric characterization.

6. Significance for Observational Astrophysics

The Himalayan Faint Object Spectrograph exemplifies the scientific potential of versatile, moderate-aperture instrumentation in frontier astrophysical research. Its applications span:

• Early stellar evolution via optical spectroscopy of protostars
• Precise atmospheric retrievals via exoplanet transit spectroscopy
• Confirmation and physical characterization of large-scale extragalactic radio sources
• Empirical site testing and enabling technical design improvements for new observatories

This breadth demonstrates the strategic importance of HFOSC for expanding the observational toolkit, enabling high-quality science across stellar, planetary, galactic, and instrumental domains on the HCT platform.

7. Future Directions

Ongoing and planned studies suggest continued expansion of HFOSC’s role. Further refinements in data reduction pipelines, increased automation (e.g., with HAPILI and custom Python modules), and collaborative usage with archival multi-wavelength datasets (e.g., HST, SPITZER, LOFAR, Pan-STARRS, SDSS) will strengthen the tool’s capacity for complex analyses. The measurement of atmospheric dispersion informs next-generation ADC designs for the NLOT, promoting enhanced precision in all future slit-based spectroscopic work conducted at Hanle. A plausible implication is that more comprehensive empirical calibration and integration of real-time site monitoring will emerge as standard practice for spectroscopic observatories at high-altitude sites.

In summary, the Himalayan Faint Object Spectrograph, as implemented on the Himalayan Chandra Telescope, is manifestly a core scientific resource—delivering advanced optical and spectroscopic data for a wide range of faint targets and advancing key methodologies in astronomical instrumentation and data analysis.