[Paper Review] MITS: the Multi-Imaging Transient Spectrograph for SOXS

Adam Rubin, Sagi Ben-Ami+48 more|arXiv (Cornell University)|Sep 5, 2018

Astronomy and Astrophysical Research4 citations

TL;DR

This paper presents the optical design of the Multi-Imaging Transient Spectrograph (MITS) for SOXS, a medium-resolution spectrograph (R ~ 4500) for the ESO 3.6 m NTT. It uses four ion-etched gratings in first order (m=1), split across four quasi-orders via dichroic mirrors, imaged simultaneously by a single three-element catadioptric camera, achieving >60% system throughput and enabling high-efficiency transient follow-up with spectral resolution >3500 across 350–850 nm.

ABSTRACT

The Son Of X-Shooter (SOXS) is a medium resolution spectrograph R~4500 proposed for the ESO 3.6 m NTT. We present the optical design of the UV-VIS arm of SOXS which employs high efficiency ion-etched gratings used in first order (m=1) as the main dispersers. The spectral band is split into four channels which are directed to individual gratings, and imaged simultaneously by a single three-element catadioptric camera. The expected throughput of our design is >60% including contingency. The SOXS collaboration expects first light in early 2021. This paper is one of several papers presented in these proceedings describing the full SOXS instrument.

Motivation & Objective

• Address the growing need for dedicated spectroscopic follow-up of transient sources identified by wide-field surveys such as ZTF, ATLAS, PanSTARRS, and LSST.
• Overcome the limitations of conventional échelle spectrographs by developing a high-throughput, medium-resolution design for transient science.
• Enable efficient, simultaneous observation of multiple spectral bands using a novel polychromatic beam-splitting approach with dichroic mirrors.
• Maximize system throughput by employing high-efficiency ion-etched gratings operating in first order (m=1), avoiding the complexity and losses of higher-order échelle systems.
• Ensure robust performance across the full 350–850 nm range with spectral resolution >3500, peak efficiency >25%, and minimal stray light or ghosting.

Proposed method

• Divide the incoming UV-VIS spectrum (350–850 nm) into four quasi-orders (100–200 nm each) using dichroic mirrors to optimize grating efficiency.
• Use custom-made, high-efficiency ion-etched gratings operating in first order (m=1) to disperse each quasi-order, minimizing losses compared to higher-order échelle systems.
• Image all four dispersed beams simultaneously using a single three-element catadioptric camera based on the MOONS design, consisting of a CaF₂ corrector, a mirror, and a fused silica field flattener.
• Optimize the camera for a fast f/6.5 collimator and a 45 mm beam diameter to maintain high throughput and image quality across the spectral range.
• Implement kinematic mounting systems using Al6061, Invar, and stainless steel to ensure mechanical stability, thermal insensitivity, and precise alignment of optical components.
• Conduct rigorous ghost and stray light analysis using sequential ray-tracing models for three wavelengths per quasi-order, confirming negligible ghost intensity (~6×10⁻⁶ relative to main signal).

Experimental results

Research questions

• RQ1 What is the optimal optical configuration to achieve high throughput (>60%) in a medium-resolution spectrograph for transient follow-up?
• RQ2 How can ion-etched gratings in first order (m=1) be effectively integrated into a multi-channel spectrograph design to outperform conventional échelle systems?
• RQ3 What is the impact of spectral leakage and second-order diffraction on system performance, and how can it be mitigated?
• RQ4 How can mechanical design ensure thermal stability and precise alignment of multiple optical elements under operational conditions?
• RQ5 What level of ghosting and stray light can be tolerated, and what is the actual contribution of optical surface reflections to background noise?

Key findings

• The MITS design achieves a system throughput of >60% across the 350–850 nm range, including contingency, significantly exceeding conventional échelle spectrograph designs.
• Peak spectral resolution exceeds 4700 (λ/Δλ) across the full band, with a minimum of 3500 for a 1'' slit, satisfying the requirement for medium-resolution transient spectroscopy.
• Ghosting from reflections at the detector and field flattener is suppressed to a relative intensity of 6×10⁻⁶ due to defocus and coating losses, posing no operational risk.
• Second-order diffraction from i-band grating is suppressed by up to 0.05² (0.0025) due to dichroic leakage and further reduced by 10–20% via grating optimization, with an order-blocking filter as a backup.
• The mechanical design ensures thermal stability through use of Invar and kinematic mounts, maintaining alignment and distance between the corrector and primary mirror despite temperature fluctuations.
• Ray-tracing simulations confirm that only diffuse reflections from the field flattener’s back surface contribute to ghosting, and their intensity is negligible, validating the system's optical cleanliness.