Representing the largest objective option in their line of modular spotting scopes, the Swarovski Optik ATX/STX/BTX 115mm Objective Lens Module will accept any of the three available eyepiece modules: straight (STX) or angled-viewing (ATX), or the binocular viewer (BTX). The bayonet mount for the eyepiece modules is backward compatible with the original ATX/STX modules, to make it easy for users to upgrade their objectives without needing a new eyepiece module. This extra-large objective, versus the 65mm, 85mm, and 95mm models, draws in more light and produces a larger exit pupil to improve its low-light capabilities and make it an invaluable companion from the pre-dawn hours to all the way past dusk.