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Abstract Interstellar objects provide a direct window into the environmental conditions around stars other than the Sun. The recent discovery of 3I/ATLAS, a new interstellar comet, offers a unique opportunity to investigate the physical and chemical properties of interstellar objects and to compare them with those of comets in our own solar system. In this Letter we present the results of a 10 night spectroscopic and photometric monitoring campaign with the 2.4 m Hiltner and 1.3 m McGraw–Hill telescopes at the MDM Observatory. The campaign was conducted between August 8 and 17 while 3I/ATLAS was inbound at heliocentric distances of 3.2–2.9 au. Our observations captured the onset of optical gas activity. Nightly spectra reveal a weak CN emission feature in the coma of 3I/ATLAS, absent during the first nights but steadily strengthening thereafter. We measure a CN production rate ofQ(CN) ∼ 6 × 10²⁴s⁻¹, toward the lower end of activity observed in solar system comets. Simultaneous photometry also indicates a small but measurable increase in the coma’s radial profile and increasingr-bandAfρwith values in the order of ∼300 cm. We derived a gas-to-dust production ratio of $\log Q\left(\mathrm{CN}\right)/Af\rho \sim 22.4$. Our upper limit on the C₂-to-CN ratio ( $\log Q\left({\mathrm{C}}_2\right)/Q\left(\mathrm{CN}\right)\lesssim -0.8$) indicates that 3I/ATLAS is a strongly carbon-chain-depleted comet. Further observations of 3I/ATLAS are required to verify the apparent carbon-chain depletion and to explore whether such composition represents a recurring trait of the interstellar comet population. 

Abstract The boundary of solar system object discovery lies in detecting its faintest members. However, their discovery in detection catalogs from imaging surveys is fundamentally limited by the practice of thresholding detections at signal-to-noise (SNR) ≥ 5 to maintain catalog purity. Faint moving objects can be recovered from survey images using the shift-and-stack algorithm, which coadds pixels from multi-epoch images along a candidate trajectory. Trajectories matching real objects accumulate signal coherently, enabling high-confidence detections of very faint moving objects. Applying shift-and-stack comes with high computational cost, which scales with target object velocity, typically limiting its use to searches for slow-moving objects in the outer solar system. This work introduces a modified shift-and-stack algorithm that trades sensitivity for speedup. Our algorithm stacks low-SNR detection catalogs instead of pixels, the sparsity of which enables approximations that reduce the number of stacks required. Our algorithm achieves real-world speedups of 10–10³× over image-based shift-and-stack while retaining the ability to find faint objects. We validate its performance by recovering synthetic inner and outer solar system objects injected into images from the DECam Ecliptic Exploration Project. Exploring the sensitivity–compute time trade-off of this algorithm, we find that our method achieves a speedup of ∼30× with 88% of the memory usage while sacrificing 0.25 mag in depth compared to image-based shift-and-stack. These speedups enable the broad application of shift-and-stack to large-scale imaging surveys and searches for faint inner solar system objects. We provide a reference implementation via thefind-asteroidsPython package and this URL:https://github.com/stevenstetzler/find-asteroids.