Necessity of a TDI optical corrector for ILMT observations

TL;DR

This paper demonstrates that a TDI optical corrector is essential for the ILMT to accurately straighten stellar trajectories, enabling precise astrometric calibration during TDI mode observations.

Contribution

It provides the first empirical evidence of the TDI corrector’s effectiveness in correcting star-trail curvature in ILMT data.

Findings

TDI corrector successfully straightens stellar trajectories

Astrometric calibration shows linear relations between CCD coordinates and celestial coordinates

Confirms the necessity of the TDI corrector for accurate ILMT observations

Abstract

The International Liquid Mirror Telescope (ILMT) has recently become operational at the Devasthal Observatory of ARIES, Nainital, India. The ILMT observes in the Time delay integration (TDI) mode where the images are formed by electronically stepping the charges over the pixels of the CCD, along a column. Observations near the zenith impose certain constraints dependent on the latitude such as image deformation due to the star-trail curvature and differential speed. These effects make the stellar trajectories in the focal plane of the ILMT to be hyperbolic, which are corrected for by the introduction of a TDI optical corrector, designed specifically for the ILMT. Here, we report the first results on the effect of this corrector on the trajectories followed by the stars in the ILMT focal plane. Astrometrically calibrating nine nights of data recorded with the ILMT during its first commissioning phase, we find simple (nearly linear) relations between the CCD-y coordinate and the right ascension (RA) of stars and between the CCD-x coordinate and their declination (DEC), respectively, which confirms that the TDI corrector works very fine in converting the stellar trajectories into straight lines.