# Spectral multiplexing using stacked VPHGs - Part I

## TL;DR

This paper proposes an innovative stacked multiplexed VPHG design using photopolymeric holographic films to enhance spectrograph resolution and spectral range efficiently, demonstrated through potential application to the GTC OSIRIS instrument.

## Contribution

It introduces a novel stacked multiplexed VPHG solution leveraging photopolymeric holographic films for improved spectrograph performance with minimal structural changes.

## Key findings

- Demonstrates the feasibility of the stacked multiplexed VPHG design.
- Shows potential for increased resolution and spectral range.
- Applicable to upgrades of existing telescopic spectrographs.

## Abstract

Many focal-reducer spectrographs, currently available at state-of-the art telescopes facilities, would benefit from a simple refurbishing that could increase both the resolution and spectral range in order to cope with the progressively challenging scientific requirements but, in order to make this update appealing, it should minimize the changes in the existing structure of the instrument. In the past, many authors proposed solutions based on stacking subsequently layers of dispersive elements and record multiple spectra in one shot (multiplexing). Although this idea is promising, it brings several drawbacks and complexities that prevent the straightforward integration of a such device in a spectrograph. Fortunately nowadays, the situation has changed dramatically thanks to the successful experience achieved through photopolymeric holographic films, used to fabricate common Volume Phase Holographic Gratings (VPHGs). Thanks to the various advantages made available by these materials in this context, we propose an innovative solution to design a stacked multiplexed VPHGs that is able to secure efficiently different spectra in a single shot. This allows to increase resolution and spectral range enabling astronomers to greatly economize their awarded time at the telescope. In this paper, we demonstrate the applicability of our solution, both in terms of expected performance and feasibility, supposing the upgrade of the Gran Telescopio CANARIAS (GTC) Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS).