Early Commissioning Results of Slit Mask Integral Field Units on the Southern African Large Telescope

TL;DR

This paper reports the development, laboratory testing, and initial on-sky commissioning results of the first Slit Mask Integral Field Unit (SMI-200) for the Robert Stobie Spectrograph on SALT, demonstrating its capabilities for high-resolution spectroscopy.

Contribution

It introduces the design, fabrication, and commissioning of the SMI-200 IFU, a new fiber-fed instrument for SALT's RSS, with detailed performance metrics and future upgrade plans.

Findings

SMI-200 provides a median spectral resolution of 2400 at Hα wavelengths.

The IFU covers an 18 x 23 arcsec footprint with 309 spatial elements.

On-sky commissioning demonstrates successful operation of the SMI-200.

Abstract

Three fibre feed integral field units (IFUs), called Slit Mask IFUs (SMI), are being developed in the SAAO fibre-lab for the Robert Stobie Spectrograph (RSS). The smaller, 200 micron fibre IFU (SMI-200) has 309 x 0.9 arcsec diameter spatial elements covering an elongated hexagonal footprint of 18 $\times$ 23 arcsec is now being commissioned. The larger, 300 (400) micron fibre IFU, SMI-300 (SMI-400), has 221 $\times$ 1.35 arcsec (178 $\times$ 1.8 arcsec) diameter spatial elements covering an on-sky area of 18 $\times$ 29 sq. arcsec (21 $\times$ 44 sq. arcsec). In all SMI units there are two groups of 13 fibres offset by roughly 50 arcsec on either side of the primary array to sample sky. SMI-200 provides a median spectral resolution of 2400 at H$\upalpha$ wavelengths in a low resolution mode simultaneously covering 370 to 740 nm. At a higher grating angles the SMI-200 delivers spectral resolution up to 10,000. A future red spectrograph arm for RSS will extend the simultaneous wavelength coverage up to 900 nm at a median resolution of 6000 for the same IFU. With this upcoming red arm and with the fibre-fed, near-infrared spectrograph NIRWALS on SALT, SMI-300 enables wavelength coverage from blue to NIR wavelengths at the same spatial resolution and footprint. The SMI units are inserted in the same fashion as the existing long-slit cassettes at the SALT focal plane. Prismatic fold mirrors direct the focal plane into the fibre IFU and then back into the RSS collimator after the fibres are routed 180 deg within the cassette and formatted into a pseudo-slit. Fold-prisms ensure that the spectrograph collimator continues to see the same focal plane. In this paper we report the laboratory characterization and on-sky commissioning-performance of the first Slit Mask IFU, SMI-200.