The Hubble Space Telescope (HST), just after release from its final service mission, 2009 (

The Hubble Space Telescope (HST), just after release from its final service mission, 2009 (

Hubble Space Telescope

APRIL 24, 2023

There is little doubt that the Hubble Space Telescope, launched 33 years ago today, is the most famous and best loved astronomical instrument ever built. Seriously flawed in its...

Scientist of the Day - Hubble Space Telescope

There is little doubt that the Hubble Space Telescope, launched 33 years ago today, is the most famous and best loved astronomical instrument ever built. Seriously flawed in its construction, its optics were corrected and fixed by Space Shuttle astronauts in a complex series of operations, and for the last 30 years, the HST, as it is commonly called, has taken a raft of stunning images to please the public and performed cutting edge infrared, ultraviolet, and spectrographic imaging for the professional astronomers. It has been condemned to death several times, and yet it is still there, and still functioning, and showing every sign of lasting into the 2030s.

The HST is basically a tube that is over 43 feet long and almost 14 feet across, weighing some 12 tons (first image).  At its heart is a precision-ground 94" diameter glass mirror, called the primary, with a shiny aluminum skin to provide reflection. Most reflecting mirrors have spherical or parabolic surfaces, which are relatively easy to figure; the HST's primary mirror has a hyperbolic surface, which is very difficult to grind accurately, and a secondary mirror that is also hyperbolic. The secondary mirror, near the front, reflects light back from the primary mirror, through the central hole in the primary, to a bevy of instruments, including a wide-field camera, which has recorded most of the visible-light images that have wowed us all. An instrument with this configuration of hyperbolic primary and secondary mirrors is called a Ritchey-Chrétien telescope, and it is the configuration used in most modern large telescopes. But it is difficult to determine when the mirror surfaces have been ground to the proper hyperbolic shape, and this is what created the almost fatal flaw in the HST's primary mirror. The subcontractor who ground the large mirror constructed a special optical device to tell the surface grinder where glass should be removed, and this device was improperly assembled, and it directed the removal of more glass around the outer edge than it should. The result is that the outer part of the mirror brought light to a different focus than the inner part. No one was aware of this until the HST was in orbit. We include here is a view of the primary mirror in the grinding process (second image).

After many delays, the HST was finally launched on Apr. 24, 1990, by the Space Shuttle Discovery, on a mission known as STS-31, and it was deployed by the shuttle astronauts the next day. There was considerable anticipation as the first image was taken, and even more consternation immediately thereafter, since the image was out of focus. It took some time to come to the realization that the problem was in the primary mirror – that it had been incorrectly figured. The scientists were sick; NASA was embarrassed;  the public and Congress were outraged. After the waves of despair and anger subsided, cooler heads eventually ventured to suggest that an optical defect could be optically fixed. Humans had been doing that for centuries, with eyeglasses; why couldn't Hubble be given corrective lenses?

It wasn't easy to figure out how to do this, but there were a lot of clever people involved, and corrective optics were devised. There had to be different corrective lenses for all four of the primary instruments, which meant designing a series of lenses that could be rotated in and out of place, but this was accomplished. On an 11-day mission, Dec. 2-13, 1993, astronauts from Space Shuttle Endeavour (STS-61) put the new optics in place, and also replaced several instruments and failing mechanical parts, such as gyros, and the next image taken by the Wide-Field camera was perfectly in focus. There have been three other service missions, to replace parts and update instruments; we include here a photograph taken during mission 3A in 1999 (third image).  The last servicing was done in May of 2009 (STS-125), and indeed that was the last shuttle mission ever, as the program was terminated. It was said at the time that the gyros on HST would fail by 2014 and that would be it, but the Hubble is still doing its magic, even though it a technically been replaced, by the James Webb Space Telescope, launched in 2021.

There have been many memorable images taken by Hubble's CCD cameras; everyone has their favorite. Some of the simple ones taken of local objects are quite powerful, such as one of Jupiter, with the shadow of the moon Ganymede right in the middle of the eye of the Great Red Spot (fourth image, above), or the first photograph to capture in one frame both Pluto and its moon Charon (fifth image, above). But the images of distant nebular and galactic objects are the ones that really caught the public fancy. One can only marvel at the intricate beauty of the Whirlpool nebula, M51, and at the instrument that captured this image (sixth image, just above).

Two of the most famous and impressive of the HST images are: the “Hubble Deep Field” (seventh image, above), taken over many hours as the HST looked at a tiny bit of empty space up near the Big Dipper and recorded an image in which everything you see is a galaxy – not a single nearby star – each galaxy consisting of billions of stars,  and the famous “Pillars of Creation,” a glimpse of a portion of M16, the Eagle Nebula, where one can see dozens of stars in the process of formation (eighth image, just above).  I acquired large prints of both of these two when they were first released and hung them on my office walls; they hang there still – I have yet to get tired of them. But I would like to add one more to the Wow! list, an image taken in 2012 of an object known as the Horsehead Nebula (ninth image, below).  For many years we were quite happy with photographs by earth-based telescopes of this lovely celestial object, such as this one.  And then Hubble delivered the image below in 2012, of just the horse’s head. One quickly runs out of superlatives.

Hubble keeps providing new images, and the technicians continue to rework the old ones with the latest technology, so that not long ago, they released a new and brighter version of the “Pillars of Creation,” which you can see here. Thank you, HST, and the thousands of devoted people at the Space Telescope Science Institute in Maryland, who over the last 30 years have made all of us excited again about deep space.

William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor emeritus, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to