Validating the Local Volume Mapper acquisition and guiding hardware

TL;DR

This paper validates the performance of CMOS cameras used for acquisition and guiding in the Local Volume Mapper survey, ensuring they meet the requirements for high-quality interstellar gas mapping.

Contribution

It provides a comprehensive validation of CMOS camera performance, including noise, dark current, thermal behavior, guiding brightness limits, and focal plane geometry measurement methods.

Findings

Cameras have a readout noise of ~5.6e- and dark current of 21e-/s.

Guiding with 5s exposure reaches Gaia gmag 16.5 at SNR>5.

Sufficient guide stars are available for all survey pointings.

Abstract

The Local Volume Mapper (LVM) project is one of three surveys that form the Sloan Digital Sky Survey V. It will map the interstellar gas emission in a large fraction of the southern sky using wide-field integral field spectroscopy. Four 16-cm telescopes in siderostat configuration feed the integral field units (IFUs). A reliable acquisition and guiding (A&G) strategy will help ensure that we meet our science goals. Each of the telescopes hosts commercial CMOS cameras used for A&G. In this work, we present our validation of the camera performance. Our tests show that the cameras have a readout noise of around 5.6e- and a dark current of 21e-/s, when operated at the ideal gain setting and at an ambient temperature of 20{\deg}C. To ensure their performance at a high-altitude observing site, such as the Las Campanas Observatory, we studied the thermal behaviour of the cameras at different ambient pressures and with different passive cooling solutions. Using the measured properties, we calculated the brightness limit for guiding exposures. With a 5 s exposure time, we reach a depth of around 16.5 Gaia gmag with a signal-to-noise ratio (SNR)>5. Using Gaia Early Data Release 3, we verified that there are sufficient guide stars for each of the around 25000 survey pointings. For accurate acquisition, we also need to know the focal plane geometry. We present an approach that combines on-chip astrometry and using a point source microscope to measure the relative positions of the IFU lenslets and the individual CMOS pixels to around 2 $\mu$m accuracy.