High-Resolution Optical Spectroscopy from a CubeSat? It’s more likely than you think!

Title: CubeSpec: optical payload

Authors: Gert Raskin, Jeroen De Maeyer, Philippe Neuville, Maddalena Reggiani, Pierre Royer, Hugues Sana, Andrew Tkachenko, Sibo Van Gool, Wannes Verstraeten, Jorden Windey, Leonardo Peri, Wim De Munter, Tjorven Delabie, Jakob Pember, Dominic Bowman, Dirk Vandepitte, Bart Vandenbussche

First Author’s Institution: Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Status: Published in SPIE Astronomical Telescopes + Instrumentation Proceedings Volume 13092 [closed access]

Featuring additional content from “The CubeSpec Mission: high resolution spectroscopy from a CubeSat” [SPIE; closed access] and “The CubeSpec space mission I. Asteroseismology of massive stars from time-series optical spectroscopy: Science requirements and target list prioritization” [A&A; open access]

NASA flagship missions (or large strategic missions–like Hubble, JWST, and future Roman) are growing increasingly expensive (+$1 billion USD in cost) and taking a longer amount of time to develop, so there is growing interest in developing cost-effective, bite-sized, specific science case focused missions using CubeSats. CubeSats are a class of nano-satellite that range in volume from 1U (10x10x10 cm) to 12U (120x120x120 cm) that can work individually or as part of a “swarm”. Commonly, CubeSats provide an avenue for instrumentalists to demonstrate the effectiveness of new technology for larger NASA missions in a lower-risk/cost environment (see Technology Readiness Level; TRL), but CubeSats can also provide valuable and high quality science data. In today’s bite we are exploring the high-resolution optical spectrograph European Space Agency (ESA) CubeSat concept named CubeSpec!

CubeSpec’s science mission is to produce high cadence, high-resolution (R > 50,000), high signal-to-noise, time series optical spectra of some of the brightest (V < 4 mag) Beta Cephei variables to better understand the internal structures of these massive stars. The pulsations of Beta Cephei variables cause physical deformations in the region near the surface of the star where the spectral lines we can observe form, causing spectral line profile variability (LPV; specifically in the Si III triplet and He I). Figure 1 shows a simulated LPV using the expected spectral resolution and SNR from CubeSpec for Beta Cep over the course of a single pulsation cycle. The engineering goal is to show that the high-quality and high-resolution spectroscopy required to observe LPV in Beta Cephei variables is achievable within 12U footprint, meaning an entire telescope, high-resolution spectroscopy instrument, stellar tracking system, and accompanying power, electronics, and communications systems will have to fit within ~47x47x47 inches–an incredibly daunting task!

Figure 1 (Figure 3 in Bowman+ 2021): The LPV for Beta Cep for one pulsation cycle generated using the BRUCE code assuming a resolving power of R = 55,000, inclination angle of 60 deg, rotation velocity (vsini) of 25 km/s, with a spectral cadence of 15 minutes at SNR of 200. The red line is the average line profile from all the generated spectra and the top most line(s) are the residuals of the line in each simulated spectra after the average line profile is subtracted.

The Telescope

The telescope in CubeSpec is a rectangular take on a classical Cassegrain telescope, with the optics boxed in pink in Figure 2. Both the primary mirror (M1 in Figure 2) and the secondary mirror (M2 in Figure 2) are rectangular in shape to match the standard square shape of the CubeSat form. Since the volume of the CubeSat is very small, the faster the incoming light is focused (the less distance the light travels before reaching the focus), the better. In CubeSpec, M1 and M2 are only about 7 inches (18 cm) apart! This introduces a number of engineering challenges, including manufacturing a strong curvature M1 (f/1) and maintaining a very precise alignment for M2. The alignment of the telescope mirrors is so precise (within 5 microns–14x smaller than the diameter of human hair!) that thermal expansion of the mirror support structure has to be considered! (If you were wondering, it’ll be made out of a ceramic material called Cordierite–the same material as some pizza stones.)

Pointing the Telescope

CubeSats come with an altitude determination and control system (ADCS; largely improved in recent years thanks to the ASTERIA CubeSat) which helps the CubeSat point at targets of interest. However, since the spectrograph on CubeSpec has a slit (2.6×6.5 arcsec), the pointing requirements for CubeSpec are more precise than what the ADCS can currently provide. To reach the required pointing precision, the CubeSpec team added a high-precision pointing platform (HPPP). A HPPP is comprised of a “steering mirror”, a mirror mounted on several actuators that can finely adjust the position of the beam traveling into the spectrograph slit, as well as a fine guidance sensor, which actively controls the actuators on the steering mirror to keep the target of interest on the slit (see Figure 3). A dichroic is used to split the incoming light from the telescope so that only wavelengths longer than 650 nm are used by the fine guidance sensor–preserving all of the precious science photons for the spectrograph!

Incoming light from the telescope comes from the top and hits a mirror that is mounted on three actuators to finely control the position of the incoming beam. The mirror is placed close to a 45 degree angle sending the incoming beam off to the right hand side and through a beam splitter. The beam splitter is placed 45 degrees with respect to the beam so that light passing through, anything longer than 650 nm, continues straight through the beam splitter to hit the fine guidance sensor. Light less than 650 nm reflects perpendicular to the beam splitter and travels through the spectrograph slit. — Figure 3 (Adapted from Figure 4 in today’s paper): An outline of the HPPP components. The steering mirror is on the left hand side of the figure, supported by three actuators. The incoming telescope light directed by the steering mirror then passes through a dichroic where wavelengths longer than 650 nm transmit, while wavelengths less than 650 nm reflect! The fine guidance sensor (FGS) on the right hand side can then use the longer wavelengths to adjust the actuators so the reflected wavelengths from the target can pass through the spectrograph slit.

The Spectrograph

The spectrograph takes the incoming light from the telescope (wavelengths < 650 nm that are reflected off of the beam splitter/dichroic from the HPPP) and disperses it using an echelle diffraction grating. Diffraction gratings are surfaces ruled with little grooves spaced a specific distance apart. When incoming light hits the grooves, the light is dispersed (reflected or transmitted depending on the grating type) at different angles according to wavelength. Orders are groupings of wavelengths that are diffracted at the same angle, and the angles for each order are determined by the groove spacing!

Echelle diffraction gratings are special because they offer much higher resolution than typical diffraction gratings. When the incoming light from the telescope hits the echelle grating at a high angle of incidence, the echelle’s lower groove density produces higher dispersion. Each order from an echelle grating spans a smaller range of wavelengths, but the wavelengths are spread out more than a typical diffraction grating, producing higher resolution! Figure 4 shows the expected footprint of the cross-dispersed echelle spectrum (2D echellogram) on the CubeSpec detector.

Each order appears on an angle, the left hand size of each order is higher on the left hand side of the detector than on the right. The redder orders overfill the length of the detector, so the edges of the orders fall off the detector. The redder orders near the top of the detector have less vertical separation between them than orders that are bluer toward the bottom of the detector. The order number is highest at the shortest wavelengths and highest at the reddest wavelengths. — Figure 4 (Adapted from Figure 6 in today’s paper): The footprint of the cross-dispersed echelle spectrum on the CubeSpec detector. The most relevant stellar lines for the CubeSpec science case are labeled. *Note: physically longer orders on the detector correspond to redder wavelengths, but these orders have a lower order number than the physically shorter, bluer orders.*

Looking to the Future

The optics in CubeSpec can be reconfigured to support many other science cases that benefit from time-series and/or high-resolution space-based spectroscopy such as: characterizing the stellar activity of exoplanet host stars, exoplanet spectroscopy, absolute flux calibration of stellar models, and observing of diffuse interstellar bands–so, it’s possible we could see a whole “swarm” of CubeSpec-like CubeSats in the not too distant future. Additionally, CubeSpec is just one of an increasing number of CubeSats that might be small in footprint but have the potential to pack a large scientific punch! If you’re interested in more of the technology/engineering innovation or science goals from CubeSats, you can check out all of the past, ongoing, and planned CubeSat missions supported by the Jet Propulsion Laboratory (JPL) and the ESA.

Astrobite edited by Kat Lee

Featured image credit: Adapted from Figure 1 in Vandenbussche+ 2024 by Alexa Morales (UT Austin)

About the Author

About Erica Sawczynec

I am a fourth year graduate student at the University of Texas at Austin working on NIR spectroscopy instrumentation. When I'm not in the lab I manage the archive for IGRINS (RRISA) and use the data products to study molecular hydrogen emission in circumstellar disks. Outside of work you can find me reading sci-fi and fantasy novels, baking bread, hanging out with my cat, or over on Twitter @EricaSawczynec.

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