Asymmetric Offner Spectrograph

Undergraduate thesis project: Evan Batteas

Spectrographs are a key instrument in astronomical observation. Analyzing the spectra produced by spectrographs can tell us the elemental composition and radial velocity of astronomical targets, which can inform how old an object is, how far away it is, and if an exo-planet could host life.

This long slit spectrograph is designed and assembled utilizing an asymmetric Offner relay. Due to the asymmetry of the Offner relay, an accessible pupil is formed, where a grating is placed to disperse the light. This instrument is designed to utilize 3D-printing and commercially available parts in its construction and is designed for mounting on a Celestron C14 telescope.

When this project is complete, it will be tested and used at the Texas A&M Teaching Observatory on the Celestron C14 telescope.

Concept

Inside instruments, light must reach the detector with as little deformation as possible. One of the ways to do this is with an optical system know as an Offner relay. The Offner relay makes use of the characteristics of spherical mirrors to transport the light with minimal aberrations. Generally, an Offner relay consists of two spherical mirrors, one convex and one concave, with the radius of curvature of the concave one being twice that of the convex. A typical Offner relay is depicted below (diagram A). However, Offner relays can also be asymmetric to allow for an accessible pupil (diagram B).

A: A symmetric Offner relay, where the radius of curvature (RoC) of the concave mirror is twice that of the convex mirror
B: An asymmetric Offner relay. The RoC of the third mirror is determined by the RoC of the first two mirrors in the relay, by the equation r3 = r1r2/(r1 –r2)1

The benefit of an asymmetric Offner relay is that an accessible pupil is formed while maintaining all other properties of a normal Offner relay. An accessible pupil is a place where all the light in a system passes through only once. This is useful as it allows for the placement of a filter or grating in the pupil.

Design

All possible mirror combinations are programmatically scraped from online retailers to choose a combination that would work for an asymmetric Offner relay. The selected mirrors are two concave mirrors with radii of curvature of 300mm and 150mm from Thorlabs and one convex mirror with a radius of curvature of 75mm from Edmund Optics.

The relay is modeled in Zemax Optic Studio, customized for a Celestron C14, and outfitted with a grating from Edmund Optics in the accessible pupil. The Zemax model is imported into SolidWorks.

The optical setup of the spectrograph as seen in Zemax OpticStudio, an optical design software. Light from the telescope enters the slit and reflects off the first mirror and second mirrors. The light is dispersed by passing through the grating, and the dispersed light reflects off the final mirror into the detector.

sw_model — The instrument design in SolidWorks. In red are the three mirrors and their mounts. In blue is the grating mount. In black is the telescope interface and guide camera housing. Inside the housing is an adjustable slit, a 50/50 beam splitter at a 45 degree angle, a mount for a camera and a guiding camera, and threading to attach to the telescope.

The optics are positioned on a machined aluminum baseplate. The optics are enclosed by an easily assembled case, designed to be similar to a computer case, which is 3D-printed and screwed together.

The guide camera housing contains a slit, beamsplitter, camera, and a threaded interface for a C14 telescope. The interface is machined from aluminum, and the main housing is 3D-printed with polyethylene terephthalate glycol (PETG).

printed_parts — The guide camera housing and telescope interface, mirror mounts, and grating mount.

A room temperature vulcanizing (RTV) silicone adhesive is used to adhere the mirrors to the mounts. A point source microscope (PSM) is used to ensure the mirrors are at the same height on each mount. A PSM is an instrument that allows opto-mechanical assemblies to be aligned precisely by referencing both mechanical and optical reference points.

Two of the mirrors adhered to their mounts with RTV adhesive, and the grating installed in its mount. The optics are mounted to a machined aluminum baseplate.

Current Status

The spectrograph is complete, minus the 3D-printed enclosure. It can be installed on a Celestron C14 telescope using the 3D-printed guide camera and eyepiece housing.

The completed spectrograph. The 3D-printed guide camera and Celestron C14 eyepiece mounting holds the blue guide camera. The red SBIG detector records the spectra.

Publications

Asymmetric Offner Spectrograph, Evan Batteas, Texas A&M University Undergraduate Research, May 2025

Long Slit Spectrograph using an Asymmetric Offner Relay, Evan J. Batteas, Luke M. Schmidt, Darren L. DePoy, 2024 AggieSTAAR and Texas A&M LAUNCH poster session

Long Slit Spectrograph using Asymmetric Offner, Evan Batteas, Luke M. Schmidt, D.L. DePoy, 2023 AggieSTAAR and Texas A&M LAUNCH poster session