If you’ve ever done public outreach for astronomy, given open talks, volunteered at a star show, written about the sky, done it anything—then at some point you had to warn people not to look directly at the sun.
The need for such warnings is at its peak during solar eclipsesof course, when people tend to have recommended solar observations. Unfiltered sunlight is so intense that it can burn tiny holes in the retina, which is a good reason not to look directly at the sun. In general, this cannot cause total permanent blindness, but doing something to stain the burned blind spots on the back of the eyes is not exactly recommended by ophthalmologists.
So you can imagine how much that damage is multiplied if you use some kind of optical aid, like binoculars or, heaven forbid, a real one. the telescope. The purpose of these tools is to collect more light from the sky, like a bucket that collects rainwater, to make faint objects appear brighter.
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Considering that the sun is literally there the brightest object in the whole sky, A gentle push of binoculars or telescopes will turn that bright object into a blowtorch. It’s the same principle as using magnifying glasses 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 look down the barrel of an active phaser bank. (The only exception is if the tool is properly filtered). And remember that increased glare isn’t just bad for your eyes; it can also be devastating to a telescope’s delicate sensors and instruments.
So with that preface, let me tell you astronomers used the Hubble Space Telescope to observe the sun– yes, seriously.
I first heard this story from my friend Glenn SchneiderHe was an operations astronomer at the Space Telescope Science Institute (Hubble’s science operations center) after Hubble’s launch and was responsible for understanding and using the telescope’s first workhorse instrument, the Wide-Field/Planetary Camera. WF/PC (pronounced ‘whiff pick’), as it was called by those in the know, went live with the observatory in the early 1990’s and remained in operation until it was replaced. WFPC2 in 1993
I worked with Schneider on the Space Telescope Imaging Spectrograph, or STISThe camera flew to Hubble on the Space Shuttle The discovery In 1997 it is still running today. One day, he was using STIS to observe a nearby star that he suspected had a planet and asked me to help him analyze the images. I don’t remember now what prompted him to tell the story, but I remember his mischievous look of delight when he mentioned that Hubble had observed the sun.
When he did, about 30 red Klaxon alerts went off in my head. A thought pushed me forward, and I was able to say, “But there’s a 50 degree limit to how far you can observe from the sun; this has only been avoided once to look at Venus! And even then the observations were delicate and dangerous!’
He smiled again and told me the rest of the story.
In the late 1980s, when the WF/PC was being built, the engineers ran into a problem. The digital detectors suffered from a problem called used cameras quantum efficiency hysteresisor QEH – When WF/PC took an image of a bright object, it left an afterimage that would confuse later observations. It’s basically like seeing multi-colored dots in your vision after seeing something bright (although it’s not the sun, never the sun remember?).
The cure for QEH was to flood the camera with ultraviolet light, which “resets” and flushes the detectors. QEH went into the detectors but only had to be fixed once before the WF/PC replaced the WFPC2. Given that the Sun emits UV light, Schneider and his colleagues decided to take advantage of it while Hubble was in orbit.
But how did they get all those cleaning photons into the detector? The trick here, Schneider told me, was that the engineers did not point Hubble at the sun. That would be a bad, like, “let’s destroy a lot of precious multi-billion dollar observatory equipment” bad. Instead, they realized they could turn the telescope’s back to the sun and point it directly outside from our star to a spot on the sky marker sun protection a point in the sky that moves due to the motion of the Earth and Hubble’s orbit, but is always 180 degrees in front of our star.
They will then be able to deploy a small “pickoff” reflector (like a submarine’s periscope or a dentist’s mirror) in the instrument bay on the side of Hubble’s back half to capture sunlight and direct it to WF/PC’s cameras.
So they built the necessary equipment, blasted Hubble into orbit in April 1990, and in December of the same year put their fast-track scheme into action. Hubble warmed up the background with the sun, the selection mirror came out, and WF/PC enjoyed a wash of ultraviolet light generated 150 million kilometers away, which washed out the QEH detectors.
The sun is much larger than the WF/PC field of view, so engineers programmed Hubble to make very small movements to scan the sun, taking a few split-second exposures to map our star into a mosaic. For its part, the WF/PC had filters that blocked visible light and allowed only a portion of the “far” ultraviolet light to pass through, ensuring that just the right amount of sunlight hit the sensors. Hubble’s look at the sun not only did the QEH sweeps, but also allowed engineers and scientists to see how light is reflected inside the telescope, which is useful both for pointing the telescope and for analyzing future scientific images.
The images that the engineers obtained were full of errors and artifacts; such a bright source would reveal a multitude of sins on any camera. A few sweeps missed the sun, too, but generally enough was available to create a rather janky image of our star. It was the first solar image obtained in the far UV and predated space-based solar observatories such as NASA. Solar and Heliophysics Observatory.
Even today, when I talk to people about seeing eclipses or sunspots and I warn them that they never, ever I look at the sun without proper shielding and filters, sometimes with a wry smile on my face because I know that the very smart and dangerous NASA engineers took one of the most expensive observatories ever built and looked at the brightest and brightest. A dangerous light source in the sky basically clears the camera.
Sunlight is truly the best disinfectant.
