If you’ve ever done any public outreach work for astronomy—if you’ve given open lectures, volunteered at a stargazing event, written about the sky, done anything—then at some point you’ve had to warn people not to look directly at the sun.
The need for such warnings peaks around the times of solar eclipses, of course, when people have a tendency for inadvisable sun observations. Raw, unfiltered sunlight is so intense that it can actually burn little holes in your retinas, which is a pretty good reason not to look directly at the sun. In general, this cannot cause total, permanent blindness, but doing something to cause burned-out blind spots to pockmark the backs of your eyes isn’t exactly recommended by ophthalmologists.
So you can imagine how much that damage is multiplied if you use some sort of optical aid such as binoculars or, heavens forbid, an actual telescope. The whole point of these instruments is to gather more light from the sky, like a bucket collecting rainwater, to make faint objects appear brighter.
On supporting science journalism
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Given that the sun is literally the brightest object in the entire sky, even a modest boost from binoculars or telescopes will turn this bright object into a blowtorch. This is the same principle behind using magnifying lenses or mirrors to start fires. Never, ever look at the sun using an astronomical instrument unless you want to know what it feels like to stare down the barrel of an active phaser bank. (The only exception is if the instrument is properly filtered.) And remember that the amped-up brightness isn’t just bad for your eyes—it can be devastating for a telescope’s delicate sensors and instruments as well.
So, with that preamble, let me tell you about the time astronomers used the Hubble Space Telescope to look at the sun—yes, seriously.
I first heard this story from my friend Glenn Schneider, who was an operations astronomer at the Space Telescope Science Institute (Hubble’s science operations center) after Hubble was launched and tasked with understanding and using the telescope’s first workhorse instrument, called the Wide-Field/Planetary Camera. WF/PC (pronounced “whiff pick”), as those in the know called it, launched with the observatory in early 1990 and stayed in operation until it was replaced by WFPC2 in 1993.
I worked with Schneider on the Space Telescope Imaging Spectrograph, or STIS, a camera that flew up to Hubble on the Space Shuttle Discovery in 1997 and is still operational today. One day, he was using STIS to observe a nearby star that he suspected harbored a planet and asked me to help him analyze the images. I don’t remember now what triggered him to tell me the story, but I do remember his look of mischievous glee as he offhandedly mentioned that Hubble had observed the sun.
When he did, about 30 red alert Klaxons went off in my head. One thought pushed its way to the front, and I was able to remark, “But there’s a 50-degree limit on how close to the sun you can observe; that’s only been circumvented once to look at Venus! And even then the observations were tricky and dangerous!”
He smiled again and told me the rest of the story.
In the late 1980s, when WF/PC was being built, the engineers had a problem. The digital detectors used by the camera suffered from an issue called quantum efficiency hysteresis, or QEH—when WF/PC took an image of a bright object, there was an afterimage left behind that would mess up later observations. In essence, it’s similar to when you see multicolored dots in your vision after looking at something bright (though not the sun, never the sun, remember?).
The cure for QEH was to flood the camera with ultraviolet light, which “reset” the detectors and flushed them out. The QEH was built into the detectors but only needed to be remediated once before WF/PC was replaced by WFPC2. Given that the sun blasts out UV light all the time, Schneider and his peers decided to take advantage of that once Hubble was in orbit.
But how did they get all those cleansing photons into the detector? The trick here, Schneider told me, was that the engineers didn’t actually point Hubble at the sun. That would be bad—like, “let’s destroy a lot of the precious multibillion-dollar observatory’s equipment” bad. Instead they realized they could turn the telescope’s back to the sun and aim it directly away from our star at a placeholder spot on the sky called the antisun, a point that moves in the sky because of Earth’s and Hubble’s orbital motion but is always 180 degrees opposite our star.
They could then deploy a small “pickoff” reflector (a bit like a submarine’s periscope or a dentist’s mirror) out the instrument bay on the side of Hubble’s back half to catch the sunlight and direct it down into WF/PC’s cameras.
So they built the necessary equipment, blasted Hubble into orbit in April 1990 and executed their clever scheme in December of that same year. Hubble warmed its backside with the sun, the pickoff mirror popped out, and WF/PC enjoyed a cleansing blast of ultraviolet light originating from 150 million kilometers away that scrubbed its detectors of QEH.
The sun is far larger than the WF/PC field of view, so the engineers programmed Hubble to execute a series of very small movements to scan across the sun, taking a series of split-second exposures to map out our star in a mosaic. For its part, WF/PC had filters that blocked out visible light and let only a fraction of “far”-ultraviolet light through, ensuring that the proper amount of sunlight hit the sensors. Hubble’s stare at the sun not only enabled the QEH scrub but also allowed the engineers and scientists to see exactly how light reflected inside the telescope—which is useful for both pointing the telescope and for analyzing future scientific images.
The images the engineers wound up getting were loaded with defects and artifacts—such a bright source would reveal a multitude of sins in any camera. Some of the scans missed the sun, as well, but all in all, enough were available to create a somewhat janky image of our star. It was the first ever sun image obtained in far-UV and predated NASA’s dedicated space-based solar observatories, such as the Solar and Heliophysics Observatory.
Even today, when I talk to people about eclipses or sunspot viewing and I caution them to never, ever look at the sun without proper safeguards and filters, I sometimes get a wry smile on my face—because I know that the supercautious and risk-averse engineers at NASA once took one of the most expensive observatories ever built and looked at the brightest and most dangerous source of light in the sky to essentially scrub the camera clean.
Sunlight, it turns out, really is the best disinfectant.
