ETSI

The Exoplanet Transmission Spectroscopy Imager (ETSI) instrument is designed for taking transmission spectra of transiting exoplanets, but it has also been used to observe supernovae, nebulae, galaxies, and the occultations of Titan and Uranus. Since its commissioning in 2022, ETSI has operated on McDonald Observatory's 82" Otto Struve telescope for hundreds of nights by many different people in the astronomy community.

Anyone in the astronomy community can sign up to use ETSI at McDonald Observatory by following the instructions in the McDonald Observatory Call for Proposals.

The WoodNext Foundation has generously provided funding so we can make significant upgrades to ETSI and support observing campaigns over the next three years.

ETSI amalgamates a low resolution slitless prism spectrometer with custom multiband filters to simultaneously image 15 spectral bandpasses between 430 nm and 975 nm with an average spectral resolution of R = λ/∆λ ≈ 20. This enables a new technique called common-path multi-band imaging (CMI), which is used to observe transmission spectra of exoplanets transiting bright (V<14th mag.) stars. ETSI is capable of near photon-limited observations, with a systematic noise floor on par with the Hubble Space Telescope and below the atmospheric amplitude scintillation noise limit. ETSI requires only moderate telescope apertures (~2 m) and is capable of characterizing the atmospheres of dozens of exoplanets and other objects per year, enabling selection of the most interesting targets for further follow up with other ground and space-based observatories.

Images of a star showing the 8 bandpasses from the transmitted channel (top) and 7 bandpasses from the reflected channel (bottom). Bands are blue to red, left to right. Central wavelength is given in nm above/below each band. ETSI captures almost complete wavelength coverage from 430-975 nm.

Once an image sequence is obtained, either over the course of hours for a transiting planet or just a few minutes each day for a supernova, we use a self-referencing differential photometric technique to eliminate sources of uncertainty from non-common path sources such as atmospheric scintillation, instrumental effects, and telescopic effects. Each spectral band of the science star is referenced to another spectral band of the same science star. This self-referential photometry eliminates nearly all common-path systematics and allows for a theoretical differential photometric precision on the order of 10 − 25 ppm.

ETSI Instrument Parameters

*Based on prototype on-sky testing
ETSI Parameter	Value
Wavelength Coverage	430-975 nm
Resolution	λ/Δλ = 20
Number of Spectral Bands	15
Field of View	6.2' x 6.2'
Plate Scale	0.18"/px
Photometric Accuracy (1σ, 20min Vstar = 7.5)	250ppm*

Concept

ETSI makes use of a novel instrument technique called SIMPSS (single image multi-band photometry with slitless spectroscopy) to collect spectral information during exoplanet transits at low resolutions (R = λ/Δλ ≈ 20). The instrument can do this by using a novel picket fence multi-band interference filter followed by a prism to separate the spectral bands. A detector images the dispersed color bands, and a second detector and prism set can be used to image the bands that are reflected (rather than transmitted) by the multi-band interference filter.

ETSI has 7 and 8 bands on the reflected detector and transmitted detector respectively, but up to 25 bands across the visible spectrum is possible. The layout of a SIMPSS instrument with a 25-band filter is shown below, along with an example on-sky image of HD189733 taken with our prototype SIMPSS instrument, which uses a commercially available 5-band Alluxa filter.

Figure1_blockDiagram — Layout of Single Image, Multi-band Photometry with Slitless Spectroscopy (SIMPSS) Instrument with a 25-band filter

Design

ETSI has a five-element collimator and two identical six-element cameras. The collimator is a reverse telephoto with the five lenses separated into three groups. The multi-chroic is located at the pupil and is responsible for the inital splitting of the light into eight transmitted bands and seven reflected bands. In order to perform photometry on each individual band, prisms (N-SF5 glass type) disperse the light, separating the filter bands into distinct PSFs, which results in the unique dashed-line appearance of ETSI images. Before each camera is a clean-up filter that improves out of band blocking and cut-on/off sharpness for each of the fifteen filter bands.

ETSI section view — Section view of the ETSI optics and detectors. The telescope focal plane is to the left of the image. Light enters the collimator and is split into two channels by the multi-chroic which is located at the pupil. Identical prisms disperse the light which is then further filtered to sharpen the transmission cut on/off transitions and imaged with identical cameras onto sCMOS detectors.

etsi_opticalbench — ETSI optics (with reflected channel detector missing).

The filter bands were chosen to coincide with exoplanet atmospheric features. We modeled dozens of exoplanet atmospheres using Exo-Transmit and aligned the spectral bands with detectable molecular and atomic absorption features. Two small portions of the spectrum (680-697 nm and 797-847 nm) are blocked completely in order to maintain adequate spacing between spectral bands of interest on the detectors.

etsi_channels — The efficiency of both channels including collimator, multi-chroic, prisms, clean up filters, camera optics, and detector quantum efficiency from vendor measurements. The gap in coverage at ∼ 6900 ˚A and ∼ 8250 ˚A is to ensure sufficient separation between bands on the detector when dispersed.

A lightweight carbon fiber frame holds all the elements of ETSI. A mounted computer runs Windows and the ETSI control software, programmed in Python. The optics are sealed inside a specially designed 3D-printed case. An Andor Marana sCMOS detector and Teledyne Kuro sCMOS detector collect the light from the transmitted channel and reflected channel respectively.

20230924_155010b — A look inside ETSI. On the left of the instrument is the mounted computer that controls the cameras and communicates with the telescope. Inside from top to bottom is the collimator, optics housing, and camera lenses. On the right of the instrument is the reflected channel camera (Teledyne Kuro). On the bottom of the instrument is the transmitted channel camera (Andor Marana).

Scientific Results

Light curves have been investigated for several objects using data obtained by the instrument 2022-2025 by Texas A&M researchers, graduate students, and undergraduate student, as well as researchers from outside of Texas A&M.

HD 94883

HD 94883 is an A2 star observed at a cadence of 0.2 s over a baseline of ~1.5 hours. The change in magnitude for each bandpass was found to be consistent with a flat line (R^2~0), and the mean change in color of HD 94883 was found to be 0.05% over 90 minutes of observations, which is consistent with expectations and is expected to improve as the data reduction methods are finalized.

etsi_prelim_results — Light curve for the 8 transmitted bands over ~1.5 hours for HD 94883

Transit of WASP-33b

Using ETSI's common path multi-band imaging (CMI) method allows astronomers to use the star itself to remove the systematics, which is more reliable than the traditional methods of using comparison star as described in Oelkers, et al. 2025, AJ, 169, 134.

The achieved dispersion in the light curve of WASP-33 b observed on 2022 September 8 when using the CMI method (black; σ = 3 × 10−4) and when using a traditional comparison star method (red; σ = 3 × 10−3). We find a factor of 10 improvement in out-of-transit precision quality when using the CMI method for this light curve. Oelkers, et al. 2025, AJ, 169, 134

Occultations of Titan and Uranus

In 1944, McDonald's 82" Otto Struve telescope was used to detect methane in the atmosphere of Titan. On July 8, 2022, we were able to observe the occultation of Titan in front of HIP 107569 to also study Titan's atmosphere.

Titan moves across HIP 107569, moving from top right to bottom left.

Data from Titan's occultation. Light from HIP 107569 is reduced as Titan crosses in front of the star, as seen in all 15 of ETSI's bands in the image on the left. On the right, Titan's ETSI transmission spectrum shows a smooth increase in transit depth for bluer and bluer wavelengths. Several bumps likely indicate methane absorption.

On April 7 2025, a team from NASA used ETSI to study Uranus during its occultation. ETSI's high framerate allowed the team to capture the moments when Uranus's rings moved in front of the star.

The star is the tiny blob just to the left of Uranus. Each image has an exposure time of 250ms. The star disappears for less than a second, indicating some of Uranus's rings blocking out the star.

Further observations of dozens of objects have been conducted over the course of 2022 to 2025, and the results are still in the works. Stay tuned!

ETSI in the News

NASA team uses ETSI to study Uranus
during its occultation in April 2025

ETSI is featured in: Alluxa Develops
Innovative 15-Band Optical Filters for
ETSI Astronomy Project

Publications

Ground-based reconnaissance observations of 21 exoplanet atmospheres with the Exoplanet Transmission Spectroscopy Imager
The Astronomical Journal, Volume 169, Issue 3, article id. 134, 22 pp. (2025).
Ryan J. Oelkers, Luke M. Schmidt, Erika Cook, Mary Anne Limbach, D. L. DePoy, J. L. Marshall, Jimmy Ardoin, Mitchell Barry, Evan Batteas, Alexandra Boone, Brant Conway, Silvana Delgado Adrande, John D. Dixon, Enrique Gonzalez-Vega, Alexandra Guajardo, Landon Holcomb, Christian Lambert, Shravan Menon, Divya Mishra, Jacob Purcell, Zachary Reed, Nathan Sala, Noah Siebersma, Nhu Ngoc Ton, Raenessa M. L. Walker, Z. Franklin Wang, and Kaitlin Webber, https://iopscience.iop.org/article/10.3847/1538-3881/ada557
(PDF)

Rapid characterization of exoplanet atmospheres with the Exoplanet Transmission Spectroscopy Imager
Luke M. Schmidt, Ryan J. Oelkers, Erika Cook, Mary Anne Limbach, Darren L. DePoy, Jennifer L. Marshall, Landon Holcomb, Willians Pena, Jacob Purcell, and Enrique Gonzalez Vega, "," Proc. SPIE J. Astron. Telesc. Instrum. Syst. 10(4) 045005 (9 December 2024); https://doi.org/10.1117/1.jatis.10.4.045005
(PDF)

The Exoplanet Transmission Spectroscopy Imager (ETSI), a new instrument for rapid characterization of exoplanet atmospheres
Luke M. Schmidt, Mary Anne Limbach, Erika Cook, D. L. DePoy, Ryan J. Oelkers, J. L. Marshall, Landon Holcomb, Willians Pena, Jacob Purcell, Enrique Gonzalez Vega, "," Proc. SPIE 12184, Ground-based and Airborne Instrumentation for Astronomy IX, 1218486 (29 August 2022); https://doi.org/10.1117/12.2630196
(PDF) (Poster)

The Exoplanet Transmission Spectroscopy Imager (ETSI)
Mary Anne Limbach, Luke M. Schmidt, D. L. DePoy, Jeffrey C. Mason, Mike Scobey, Pat Brown, Chelsea Taylor, Jennifer L. Marshall, "," Proc. SPIE 11447, Ground-based and Airborne Instrumentation for Astronomy VIII, 114477D (13 December 2020); https://doi.org/10.1117/12.2562371
(PDF) (Poster)