By Andy Tomaswick
May 28, 2025
Ultraviolet astronomical observations have always been hindered by one simple fact – the Earth’s atmosphere blocks most UV photons, especially in the UV-C and UV-B range of 100-315nm wavelengths. So, astronomers must have a collector above the atmosphere if they want to know what is happening in those wavelengths. A consortium from Institut d’Estudis Espacials de Catalunya (IEEC) hopes to provide additional insight into that realm with their PhotSat mission, a CubeSat that will observe the whole sky in UV and visible light once every few days.
PhotSat is designed as a 12U CubeSat in an orbit about 500km above the surface, to provide quick, consistently updated data on almost the whole sky in the UV spectrum once every few days. It consists of a 6U “payload” with two cameras, an X-band antenna, and an “instrument control unit” specially designed for the mission. It will also include a 6U “platform,” which contains standard CubeSat operational kit like an attitude control system, electrical system, and onboard computer.
All those packages will be used to scan the entire sky in UV and visible light (using the two different cameras) once every 2-3 days. That frequency of updates has advantages, such as tracking transits of exoplanets, finding transients such as GRBs or supernovae, and monitoring space weather, which changes constantly throughout the solar cycle. Data from PhotSat can inform more detailed observations from higher-power telescopes, whether on the ground for visible light or up in space for the more advanced UV observatories.
Fraser discusses the advantages / disadvantages of CubeSats and “big missions”.
To enable those observations, PhotSat has some unique technologies that have already been tested on a bench setup. The main one is the siderostat—essentially a mirror control system that allows PhotSat to take a picture of the whole sky simply by moving its mirror to focus on different sections of it for defined amounts of time. A benchtop prototype has undergone testing and is ready to integrate into the overarching system.
That system should allow PhotSat to capture about 14 arcseconds of the sky for every picture, with a stability of around 5 arcseconds. It will be angled 45 degrees from the Sun to ensure the UV rays from our star don’t interfere with its measurements. Over the course of its mission, it expects to monitor about 40 million of the brightest objects in the sky (mostly stars).
As part of that, it is also expected to find about 3,000 transient events a year, which can inform larger telescopes to look for those same events, collecting even more detailed data about these fascinating phenomena. To ensure PhotSat captures all the potential transients, it will use a scanning algorithm to capture as much of the sky as possible. Doing so consistently over the complete mission’s lifetime will also add depth to the measurements, allowing scientists to analyze even relatively slow transient changes in the objects it PhotSat observes.
Fraser discusses why having telescopes out of Earth’s atmosphere is so advantageous.
Currently, the IEEC is looking for partners to develop specific scientific missions for the project and finish covering its costs. According to nanosats.eu, PhotSat has not yet launched and is expected to launch in 2026. If it does get off the ground, it will represent a milestone for the IEEC itself and for the EU as a whole, as it contributes to increasing our knowledge of the realm of the ultraviolet.
Learn More:
J.M. Carrasco et al – THE PHOTSAT MISSION
UT – A CubeSat to Capture a Supernova’s UV Spectrum
UT – Taking a High-Resolution Ultraviolet Image of the Sun’s Corona Will Require VISORS
UT – A Tiny, Inexpensive Satellite Will be Studying the Atmospheres of hot Jupiters