Image Quality of SOLIS/VSM in Helium vs. Nitrogen

TL;DR

This study compares the image quality of the SOLIS/VSM instrument when filled with helium versus nitrogen, revealing a slight degradation in sharpness with nitrogen after 11 years of helium use.

Contribution

It provides the first comparative analysis of SOLIS/VSM performance with nitrogen filling, addressing operational challenges due to helium shortage.

Findings

Nitrogen filling causes slight image sharpness degradation.

Helium remains superior for optimal image quality.

Operational viability of nitrogen as an alternative is demonstrated.

Abstract

The National Solar Observatory (NSO) Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector SpectroMagnetograph (VSM) is sealed and was designed to be filled with helium at slightly above ambient pressure. After 11 years of operation filled with helium, an acute shortage of helium prompted a test using nitrogen as the fill gas. Four months of nitrogen-filled observations in 2014 are compared the same months in 2013 with helium fill. On average, the image sharpness is slightly degraded when using nitrogen.