Canada-France-Hawaiʻi Telescope’s MegaCam Helps Discover 128 New Saturnian Moons

The Canada-France-Hawaiʻi Telescope (CFHT) played an integral role in discovering 128 new moons of Saturn, which were recognized by the International Astronomical Union on March 11, 2025. From 2019 to 2021, CFHT has monitored the sky around Saturn, with its first run yielding 64 moons and even more uncategorized objects. Fueled by the knowledge that there was more to discover, scientists pressed on, leading them to find a total of 128 new “irregular” moons.

Each irregular moon is a fragment of a small number of original moons that broke apart within the last 100 million years, after a collision. CFHT was able to collect this data thanks to MegaCam and a team of dedicated scientists, including Dr. Heather Flewelling, a CFHT instrument scientist. MegaCam is a wide-field-of-view imager that is about the size of a pizza and consists of an array of charge-coupled devices (CCDs). Of the five CFHT instruments Heather works with, MegaCam is the one she spends the most time with, and this allowed her to provide a behind-the-scenes look at the technology and process behind the discovery. 

MegaCam is on the sky for ten consecutive days per month, and one of Heather’s responsibilities is scheduling observations for various CFHT programs. The Saturn program had a high priority and needed six hours or more of observation time a night. When scheduling observations, Heather considers factors like the weather and moon phase, as MegaCam is sensitive to background light and can’t be used during a full moon. Heather describes building these schedules as a big puzzle, and prepares several queues for each night to cover the possible weather conditions on Maunakea. This program required the best image quality possible, and with the right conditions, MegaCam was able to deliver.

MegaCam's wide field of view covers one degree by one degree on the sky, about twice as big as the size of the moon. This is a huge field of view compared to other imagers and makes it ideal for studying a large patch of sky. “MegaCam also has a wide filter that covers many colors at the same time. For many imagers, filters are narrow, capturing, for example, only red or blue light, but MegaCam has a GRI filter that captures green to red, and even a little bit of infrared,” said Heather. “This means that even if it’s a faint object, we have a better chance of catching it.”

The program used MegaCam’s capabilities to observe a series of fields, or patches of sky, multiple times to collect the data needed. A big part of Heather’s job is making sure the data is processed and ready for the principal investigator to use. Heather said, “To get the data ready, we have to clean up the CCD images, which are very sensitive and kept cold because any noise will show up.” Noise introduces a visual distortion in the image. 

To fix this, CFHT takes a series of calibration images, one of which is called a bias frame. “We take images of zero-second exposure time, where you just get noise from the camera,” said Heather. “We take a series of these and then subtract off that noise from the images taken on the sky.” CFHT also takes flat frames by opening the telescope near twilight when there are no stars visible in the sky. “You’ll see a pattern in the images; the center is brighter and the edges are dimmer, but it’s supposed to be blank, or flat,” said Heather. This pattern is uneven illumination, which needs to be fixed. Heather added, “We correct for that with the flat image by subtracting the bias and dividing by the flat. You get a nicely cleaned-up image, and it’s ready for the scientists to use.”

With a series of cleaned-up images from MegaCam, scientists can see objects moving between the stars from one exposure to the next, providing the data required to produce this incredible discovery. Thanks to MegaCam’s unique capabilities and the team members like Heather who keep it operating, astronomers successfully identified 128 new Saturnian moons. 

Researchers believe they’ve found all they can with current technology, so after years of research, this program has come to a close. MegaCam will, however, continue to be of great use to many other CFHT programs, aiding scientists in discovering more of our universe.