Texas A&M is currently developing a new type of instrument designed specifically for taking transmission spectra of transiting exoplanets. The Exoplanet Transmission Spectroscopy Imager (ETSI) instrument uses a prism and multiband filter to simultaneously image 15 bandpasses from 430-975nm during exoplanet transits (a full list of ETSI parameters is given in Table 1).

Table 1: ETSI Instrument Parameters

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*

*Based on prototype on-sky testing

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 (ETSI will have 7 or 8 bands per detector, but up to 25 bands across the visible is possible, see example 25-band filter in Figure 1) followed by a prism to separate the all spectral bands.

Block Diagram
Figure 1: Layout of Single Image, Multi-band Photometry with Slitless Spectroscopy (SIMPSS) Instrument.

A detector then images the dispersed color bands such that there are up to 8 images of each star in the field of view. 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 allowing for up to 15 distinct spectral measurements of the exoplanet's atmosphere in the visible. The layout of a SIMPSS instrument is shown in Figure 1 including an example on-sky image of HD189733 taken with our prototype SIMPSS instrument which uses a commercially available 5-band Alluxa filter. The optical layout of ETSI is shown in Figure 2.

Optical Layout
Figure 2: ETSI optical layout.

Preliminary Results

A prototype of the ETSI instrument was assembled using all COTS parts and tested on-sky at the McDonald 0.9m in July 2019. The prototype instrument successfully measured the color during a WASP- 3b transit at the σ = 340ppm level (WASP-3 V mag = 10.6). The results from that measurement are shown in Figure 3.

On Sky Data
Figure 3: Measurement of two colors of WASP-3b during transit with a prototype of the ETSI instrument on the McDonald 0.9 mtelescope in July of 2019. An on-sky accuracy of 340ppm with 122 minutes of transit data on Wasp-3 (V = 10.6) at λ=510 nm (FWHM = 16nm).

Current Status

ETSI had a successful first light April 19-23, 2022 at the McDonald Observatory 2.1m telescope. We estimate that during a two year survey, ETSI will collect transmission spectra from approximately 100 exoplanets doubling the number of exoplanets with measured transmission spectra. Figure 4 illustrates simulated exoplanet spectra of T = 600K and T = 1000K “Jupiters” with the ETSI bands overlaid. As is shown in this figure, ETSI is capable of detecting up to six atmospheric features (Na, K, water, methane, TiO and Rayleigh scattering) and has bands tuned specifically to maximize sensitivity to these features.

Simulated Spectra
Figure 4: Simulated spectra of T=600K (left) and 1000K (right) Jupiters with ETSI spectral bands overlaid. Gray bands are transmitted and red bands are reflected by the multi-band filter. Plots illustrate that the 15 tuned ETSI spectral bands shoudl be sensitive to six atmospheric features.
Figure 5: A look inside ETSI. First light occured April 19, 2022 with one channel (transmitted bands) at the McDonald Observatory 2.1m telescope.
Figure 6: ETSI mounted to the McDonald Observatory 2.1m telescope.

The ETSI project is funded by the NSF MRI grant no. 1920312.