# Design for the First Narrowband Filter for the Dark Energy Camera: Optimizing the LAGER Survey for z ~ 7 Galaxies

## TL;DR

This paper details the design, manufacturing, and testing of the NB964 narrowband filter for DECam, optimized for detecting z~7 Lyman alpha emitting galaxies during the epoch of reionization, enabling efficient high-redshift galaxy surveys.

## Contribution

It introduces the first narrowband filter NB964 for DECam, optimized for high-redshift galaxy detection, and reports its successful laboratory and on-site performance tests.

## Key findings

- Filter has a central wavelength of 964.2 nm and FWHM of 9.2 nm.
- The filter's performance enables high-efficiency detection of z~7 LAEs.
- The LAGER survey has been successfully using this filter since 2015.

## Abstract

We present the design for the first narrowband filter NB964 for the Dark Energy Camera (DECam), which is operated on the 4m Blanco Telescope at the Cerro Tololo Inter-American Observatory. The NB964 filter profile is essentially defined by maximizing the power of searching for Lyman alpha emitting galaxies (LAEs) in the epoch of reionization, with the consideration of the night sky background in the near-infrared and the DECam quantum efficiency. The NB964 filter was manufactured by Materion in 2015. It has a central wavelength of 964.2 nm and a full width at half maximum (FWHM) of 9.2 nm. An NB964 survey named LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) has been ongoing since December 2015. Here we report results of lab tests, on-site tests and observations with the NB964 filter. The excellent performances of this filter ensure that the LAGER project is able to detect LAEs at z~7 with a high efficiency.