UVic researchers at the leading edge of a new field of astronomy
by Nicole Crozier
When we look up at the sky on a clear night, we can see the light coming from far away stars and galaxies. But just how bright are the stars in the sky? It’s a question that has always been difficult to answer precisely, but new satellite technology piloted by a team of researchers at the University of Victoria (UVic) is getting us a little closer.
The Optical Reference Calibration Satellite (ORCASat) was designed and built by researchers and students at UVic and was deployed from the International Space Station into a low-Earth orbit near the end of 2022. It aims to assist with the calibration of Earth-based telescopes by providing an artificial light source in orbit. This method of calibration will allow for more precise measurements.
“We’ve never before tried to calibrate our telescopes using artificial light sources above the Earth’s atmosphere. ORCASat is one of the earliest practical tests of how this will work, and it’s exciting to be leading work that could have major implications for many different aspects of astronomy and astrophysics.”
- UVic professor Justin Albert, the lead astrophysicist on the project.
Photometry, the measurement of light in terms of its perceived brightness to the human eye, is one of the basic types of astronomical measurements. However, it can be difficult to very precisely and accurately measure apparent magnitudes, i.e. the amount of light coming from astronomical objects. Before being measured, the light must first pass through the optics of the telescope, and this results in attenuation or loss of light, with the amount of light lost varying with the light paths taken through the optics. For ground-based telescopes, varying amounts of light are lost depending on the light paths taken through the Earth’s atmosphere. Historically, telescopes can best be calibrated to a precision of 1%, and while atmospheric models can be used to predict how much light is lost, the rapidly changing composition of the Earth’s atmosphere due to aerosols can make light attenuation difficult to predict.
The ORCASat project aims to calibrate ground-based telescopes for this undesirable loss of light by providing a reference light source in orbit that can be viewed by a telescope on Earth. The satellite itself carries two laser light sources and can measure, while in orbit, the amount of light that these light sources are emitting. Astronomers can measure, from the ground, how bright ORCASat appears, just as they would an astronomical object. Using the data from ORCASat’s onboard photodiodes, astronomers can determine how bright ORCASat should have appeared to the telescope on Earth if there were no attenuation. The difference between how bright ORCASat appeared to the telescope and how bright it would have appeared without attenuation is the amount of light that was attenuated by the atmosphere and/or the telescope optics.
“Calibrating our telescopes using an artificial light source in orbit allows us to increase the precision of our calibration. ORCASat is proof of concept that this is possible and is the first dedicated satellite for hybrid space and ground observatories, a whole new field of astronomy,” says Albert.
The absolute brightness of astronomical objects is important to many different areas of research including learning more about supernovae, understanding dark energy, and measuring the properties of exoplanets. Albert himself is most interested in reducing the uncertainties involved in measuring the accelerating expansion rate of the universe and the properties of dark energy.
Since observations began in late January 2023, Albert and his team have attempted over 50 observations of the satellite from the Blanco telescope in Chile and the Canada-France-Hawaii telescope in Hawaii. So far, they’ve had one successful observation, on January 31, using the Blanco telescope and ORCASat’s 660nm onboard light source. Other observation attempts have been hindered by weather, telescope access, and human error. Albert is greatly hoping for at least one more successful observation, ideally from ORCASat’s 840nm onboard light source, before the satellite deorbits in mid-June.
Albert will be presenting about the ORCASat project at the 242nd meeting of the American Astronomical Society in Albuquerque on June 6 at 10:30 a.m. MDT.
The ORCASat project is funded by the Canadian Space Agency.
More about the ORCASat project