AI Insight
This study evaluates the feasibility of up-the-ramp (UTR) sampling for ground-based near-infrared spectrographs under variable sky conditions, using the GIRMOS Data Simulator with empirical sky variation data from Mauna Kea. The results show that UTR sampling provides 3-4% time savings for H-band observations in read-noise-limited conditions and achieves excellent cosmic ray rejection (>98%) despite sky variability. However, performance degrades in K-band sky emission line regions due to thermal noise dominance and increased false positive rates in cosmic ray detection under highly variable sky conditions.
Why it matters
These findings provide practical guidance for implementing UTR sampling in ground-based near-infrared instruments, helping optimize observation strategies and detector readout modes. The work directly impacts the design and operation of future ground-based spectrographs, particularly for determining when UTR sampling offers genuine advantages over traditional Fowler sampling methods.
Understand the Science
arXiv:2606.13600v3 Announce Type: replace
Abstract: Many modern near-infrared instruments employ HAWAII-2RG (H2RG) detectors with integration times that can reach 300-600s. Up-the-ramp (UTR) sampling offers advantages over Fowler sampling, including superior cosmic ray rejection and noise reduction, but requires fitting linear ramps from 30-60 reads. Ground-based K-band sky brightness has been reported to vary by 3-10% on timescales of minutes, potentially introducing systematic errors and compromising photometric accuracy. Additionally, UTR data formats involve higher-dimensional FITS files with larger file sizes impacting observatory operations.
We present a feasibility study using the GIRMOS Data Simulator with high-fidelity flux budgets and empirical K-band sky variations estimated, for Mauna Kea, from Gemini-NIRI at 10-20s cadence. Using a Monte Carlo approach we assess whether linear ramp fitting remains viable under variable sky conditions, quantify SNRs and systematic biases, and report nightly data volume estimates. Our results show that, in the H-band, the advantages of the UTR readout hold for read-noise-limited targets placed in the inter-line regions, translating into 3-4% savings in observing time. The K-band inter-line regions do not show significant SNR improvement and can even degrade it due to the dominance of shot-noise generated by the thermal emission of the instrument+telescope system. In these regions, cosmic ray rejection recovers $>$ 98% of events with false positive rates below 0.1%, even under high sky variability. Over the sky emission lines, UTR fitting remains possible but its performance is compromised, both by a degradation in SNR and by a high rate of pixels falsely flagged by the cosmic ray rejection algorithm under highly variable sky. These findings address how ground-based conditions affect UTR implementation in near-infrared spectrographs, with GIRMOS as a concrete case of study.
Source: Feasibility of up-the-ramp sampling under variable sky for ground-based spectrographs