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 occultation of Titan. It was designed, prototyped, built, and operated by lab faculty, staff, graduate students, and undergraduate students. It has operated on McDonald Observatory's 82" Otto Struve telescope for over 100 nights 2022-2024, and we have continuously improved its design and construction.

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 an F-type star showing the 8 bandpasses from the transmitted channel (top) and 7 bandpasses from the reflected channel (bottom), offset to show that when combined, ETSI captures almost complete wavelength coverage from 430-975 nm. Bands are red to blue, left to right. Central wavelength is given in nm above/below each band.

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).

Preliminary Results

Light curves have been investigated for several objects using data obtained by the instrument during April 2022 commissioning. One such object, 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

Further observations of dozens of objects have been conducted over the course of 2022 to 2024 by our faculty, graduate students, and undergraduate students, and the results are still in the works. Stay tuned!

Publications

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)