Strategies for spectroscopy on Extremely Large Telescopes. I - Image Slicing

TL;DR

This paper explores the use of image slicing in spectrograph design for Extremely Large Telescopes to reduce size and cost, demonstrating potential savings and strategic design considerations through modeling.

Contribution

It introduces a toy model to estimate spectrograph size and cost for ELTs, analyzing the impact of image slicing and design strategies on efficiency and scalability.

Findings

Image slicing can reduce spectrograph costs by 2-100x.

Single spectrograph with multiple slices is more cost-effective than multiple smaller units.

Model predicts significant cost savings and guides design choices for ELT spectrographs.

Abstract

One of the problems of producing spectrographs for Extremely Large Telescopes (ELTs) is that the beam size is required to scale with telescope aperture if all other parameters are held constant, leading to enormous size and implied cost. This is a particular problem for image sizes much larger than the diffraction limit, as is likely to be the case if Adaptive Optics systems are not initially able to deliver highly corrected images over the full field of the instrument or if signal/noise considerations require large spatial samples. In this case, there is a potential advantage in image slicing to reduce the effective slitwidth and hence the beam size. However, this implies larger detectors and oversizing of the optics which may cancel out the advantage. By the means of a toy model of a spectrograph whose dimensions are calibrated using existing instruments, the size and relative cost of spectrographs for ELTs have been estimated. Using a range of scaling laws derived from the reference instruments, it is possible to estimate the uncertainties in the predictions and to explore the consequences of different design strategies. The model predicts major cost savings (2 - 100x) by slicing with factors of 5-20 depending on the type of spectrograph. The predictions suggest that it is better to accommodate the multiplicity of slices within a single spectrograph rather than distribute them among smaller, cheaper replicas in a parallel architecture, but the replication option provides an attractive upgrade path to integral field spectroscopy (IFS) as the input image quality is improved... [Full abstract in text]