If you’ve ever done public events on astronomy—giving lectures, or hosting stargazing events—you’ve probably had to warn people or been cautioned: to never look directly at the sun.
It is well known that raw, unfiltered sunlight can burn your retinas, causing blind spots that ophthalmologists strongly discourage, the publication “Scientific American” wrote.
The danger multiplies when using optical aids like binoculars or telescopes, as these devices concentrate sunlight to hazardous levels, potentially damaging not only your eyes but also the equipment itself.
Yet, despite these risks, NASA once used the Hubble Space Telescope to observe the sun. Yes, you read that right.
Why NASA Risked It
This unusual decision dates back to the late 1980s when engineers were developing Hubble’s first camera, the Wide-Field/Planetary Camera (WF/PC). The camera suffered from a problem called quantum efficiency hysteresis (QEH)—a condition where bright objects left behind afterimages that interfered with subsequent observations.
To fix this, engineers needed to flood the camera’s detectors with ultraviolet (UV) light, essentially “resetting” them.
Realizing the sun is a natural UV powerhouse, the team devised a clever plan: instead of building a separate UV light source, they decided to use sunlight—directly from space.
Executing The Plan
Of course, the scientists couldn’t just point Hubble at the sun—doing so would destroy the telescope’s delicate instruments. Instead, they turned Hubble away from the sun and aimed it at a placeholder location called the “antisun,” a point 180 degrees opposite the sun.
They then deployed a small reflector, like a periscope or dentist’s mirror, to catch sunlight and redirect it into WF/PC. Filters ensured that only the right fraction of far-UV light reached the detectors, protecting them from damage.
In December 1990, a few months after Hubble’s launch, NASA executed the plan. The telescope scanned across the sun’s surface using tiny movements to create a mosaic of split-second exposures.
While some scans missed the target, others succeeded, producing a somewhat crude but groundbreaking UV image of the sun.
The primary goal of cleansing the camera’s detectors was achieved, and the images also provided valuable insights into how light reflected inside the telescope—aiding both future observations and calibration.
A First in Solar Observation
Though the resulting images were riddled with defects and artifacts, they marked the first-ever far-UV observations of the sun, predating dedicated solar observatories like the Solar and Heliospheric Observatory (SOHO).
This story stands as a testament to NASA’s ingenuity, when astronomers caution against looking at the sun without proper safeguards.
Using one of the most expensive observatories ever built to clean a camera might sound risky, but it highlights how sunlight, when handled with care, truly is the best disinfectant.