Measuring Cometary Nuclei Behind Bright Comae: PSF Delta Decomposition with Bicubic Resampling and an Application to Interstellar Comet 3I/ATLAS C/2025 N1

TL;DR

This paper introduces a novel PSF delta decomposition method using bicubic resampling to measure unresolved cometary nuclei embedded in bright comae, demonstrated on interstellar comet 3I/ATLAS with HST data.

Contribution

The paper presents a new instrumental technique combining bicubic resampling and PSF modeling to accurately separate cometary nuclei from bright comae in unresolved images.

Findings

Recovered nucleus radius between 0.16 and 2.8 km for comet 3I/ATLAS.

Method aligns with recent HST upper limits on nucleus size.

Suitable for survey pipelines like Rubin LSST.

Abstract

Measuring cometary nuclei is notoriously difficult because they are usually unresolved and embedded within bright comae, which hampers direct size measurements even with space telescopes. We present a practical, instrumental method that, stabilises the inner core through bicubic resampling, performs forward point-spread function PSF+convolution, and separates the unresolved nucleus from the inner-coma profile via an explicit Dirac Delta function added to a Rho^-1 surface brightness law. The method yields the nucleus flux by fitting an azimuthal averaged profile with two amplitudes only PSF core and convolved coma, with transparent residual diagnostics. As a case study, we apply the workflow to the interstellar comet 3I/ATLAS alias C/2025 N1, incorporating Hubble Space Telescope constraints on the nucleus size. We find that radius solutions consistent with 0.16 <= Rn <= 2.8 km for Pv = 0.04 are naturally recovered, in line with the most recent HST upper limits. The approach is well-suited for survey pipelines Rubin LSST and targeted follow up.