Are Hubble Images Real? Part II: A Brief History of Astrophotography

TLDR: Yes, Hubble images are real. This series of posts is dedicated to the scrutiny of Hubble imagery and a broader discussion of the veracity of astronomical imagery. In this post, we’ll take a brief look at the history of astrophotography in order to provide a historical context to Hubble.

The Hubble Space Telescope was launched in 1990 and is now approaching thirty-one years in orbit. Three decades is a long time, it covers the span of an entire generation; a generation whose perception of what the universe looks like is inextricably linked to Hubble. I remember seeing images like the now iconic Eagle Nebula as I was beginning my own journey into astrophysics and wondering how these images could possibly be real. After all, when I step outside at night and look up, I see a mostly dark (or more likely light polluted) sky peppered with pinpoints of light. Even in the darkest skies I’ve ever seen, I’ve clearly seen the Milky Way, and countless stars with just barely discernible colors, but still, I do not see anything close to what Hubble sees. Indeed, for many thousands of years, and even in the humble beginnings of astrophotography, this was how humanity saw and interpreted the night sky. Hubble’s clarity of vision and its vivid and colorful views of the cosmos are a relatively new phenomenon in the world of astrophotography.

Given how far-removed Hubble’s vision is from our own experience of looking up, it is only natural to question the reality of these images. In exploring this question, I find it helpful to gain a historical perspective of the incremental advances in photographic and telescopic technology that led to the development and launch of Hubble. This post is by no means a comprehensive history of astrophotography, nor does it explore the important topics of spectroscopy or photometry which both developed concurrently with astrophotography. My intent here is to provide a timeline of the most important advances and “greatest hits” in the ever-evolving development of astrophotography within the larger context of truth in photography. At this point, it is worth pointing out that you will see two different types of telescopes described going forward: the refractor and the reflector. A refractor uses large glass lenses to focus incoming light to a point, and a reflector uses concave mirrors to bounce and focus light. The refractor was eventually superseded by the reflector due to issues of weight and calibration.

For hundreds of years after Galileo’s invention of the telescope, the only way to record telescopic observations was to make intricate sketches and take detailed notes. Both methods are largely subjective, leaving a lot of room for interpretation–or even error–on the part of the observer. That all changed starting in the 1800s with the development of the ability to permanently capture light in an image. The first attempts to record light onto a photosensitive material were primitive and quickly degraded with time. In 1839 Louis Daguerre, in partnership with Joseph Nicéphore Niépce, patented the daguerreotype. This method uses an iodized silver plate to record the image, taking advantage of the light sensitivity of silver salts with the image preserved by bathing the plate in mercury vapor after the exposure is taken (Tenn 1989). The term “photography,” which means “writing with light,” was coined at this time by John Herschel (Ré 2010).

The First Astrophotos

Armed with this new photographic method, the first astrophotographers naturally aimed their telescopes at the nearest bright celestial objects, the moon and sun, in the hopes of recording their abundant light. The first successful astrophoto was taken by John William Draper in 1840, earning him the recognition of being known as the “First Astrophotographer” (Hughes 2015). Using his 5-inch (13 cm) reflecting telescope, he gathered the light of the moon into a daguerreotype, forever capturing details of the cratered lunar surface (Gendler and Gabany 2015). Here we see for the first-time light captured in a way far removed from that of human vision. Draper’s image of the moon was exposed for twenty minutes, collecting photons to reveal faint details in a way that our eyes cannot. This is an important distinction between photography and human vision, and is one of the key ways in which Hubble extends our ability to perceive the universe.

Fast forward to 1851 and a new method of photography building on the daguerreotype is invented by the English sculptor Frederick Scott Archer. The so-called wet-plate (collodion) method, which involved spreading photo-sensitive chemicals over a sheet of glass, was much cheaper than a daguerreotype and allowed for both higher-resolution photos and more importantly, unlimited reproductions (Ré 2010). Although a huge improvement over the daguerreotype, this method was not without limitations for astrophotography. It required the observer to maintain a laboratory stocked with chemicals within the observatory, and the drying time of the chemical mixture on the plate limited the available exposure time on such plates. Once dry, the image was set and the plate was no longer sensitive to light. This made it impossible for astronomers to take the longer exposures needed to reveal faint details in nebulae and capture faint stars. However, it was particularly well suited for the short exposure times needed when acquiring images of the sun.

During a solar eclipse in 1860, the English amateur astrophotographer Warren de la Rue took exquisitely detailed collodion images of the sun using a specially designed solar telescope. His photographs provided conclusive evidence that solar prominences actually originated from the sun and not the moon. His work inspired the physicist Michael Faraday to proclaim: “Photography could therefore render evident to us phenomena of the sun which the human eye could not discern” (Tenn 1989). As we now clearly know, this sentiment extends beyond just phenomena of the sun and also clearly demonstrates the need to extend human vision in our scientific pursuits. During this time, we also see the first hints of what would become color astrophotography when James Clerk Maxwell suggests that color images could be produced by combining black and white photographs taken through red, green, and blue filters (Ré 2010).  It would take nearly a hundred years before this method would be employed by astrophotographers.

Throughout the 1870s the next evolution in photography took shape in the form of the dry plate process first proposed by Richard Leach Maddox in 1871. The dry plate is a glass plate coated in a dry emulsion of silver bromide dissolved in gelatin. In 1879 George Eastman (founder of Eastman Kodak) perfected a machine that could automate the process of coating glass plates with the dry emulsion and mass produce photographic plates, the direct precursor to film (Tenn 1989). At that time, dry plates proved to be the perfect medium for astrophotography. They were far more sensitive to light than their predecessors, the emulsion was pre-applied (leaving the whole mess of chemicals out of the equation), and they could be exposed for hours.

Using new dry plate technology, amateur astrophotgraphers once again led the way, producing the very first images of faint nebulae. In 1880 Henry Draper, son of John Draper, obtained the first image of the Orion nebula in a 51-minute exposure on a dry plate, revealing only the brightest inner stars of the nebula. Draper improved on this image in 1882 with a 137-minute exposure revealing more structure within the nebula (Abrams 2000). In 1883 Andrew Ainslie Common, using his 36-inch (91 cm) telescope, captured stars in Orion that had never been seen before by eye.

A few years later, in 1885, brothers Paul and Prosper Henry discovered the nebulosity around the Pleiades star cluster through a long-exposure photo using their 13-inch (33 cm) refractor (Tenn 1989). To their amazement, when they produced images of the Milky Way, they discovered more stars than they could count (Tenn 1989; Brasch 2017)! It seemed that the more astrophotographers branched out from the familiar, the more they discovered, once again demonstrating how astrophotography extends far beyond human vision. Professional astrophotography also began in earnest at this time with observatories around the world beginning to catalog the sky and acquiring large collections of photographic glass plates in the process. The largest of those collections now resides at the Harvard Center for Astrophysics–a collection of over 500,000 plates covering both hemispheres spanning from 1885 to 1995. The collection is currently in the process of being digitized!

Through the end of the nineteenth and beginning of the twentieth centuries, there were incremental advancements in dry plate technology, increasing plate sensitivity to light along with producing finer emulsion grains, allowing for higher resolution photos with shorter exposure times (Brasch 2017). Concurrently, larger and more advanced observatories were popping up around the world. The reflecting telescope came to dominate these new observatories given the difficulties in building ever larger refracting telescopes. Mount Wilson observatory near Pasadena, California was established in 1904. This observatory would eventually be the site of the famed 100-inch (2.5 meter) Hooker telescope.

Edwin Hubble Blows Our Minds

Built in 1917, the Hooker was the world’s largest telescope until 1949. Astronomer Edwin Hubble used this telescope to finally settle the “Great Debate” regarding the scale of the universe. At the time, astronomers were split over the question of the size of the universe–with some holding that the Milky Way galaxy contained the extent of the universe–and others arguing that the “spiral nebulae” that observers were finding and cataloging were actually entire galaxies in their own right separate and distinct from the Milky Way. In 1924 Hubble pointed the Hooker telescope at the great spiral nebula in Andromeda and meticulously measured the changes in brightness of a unique type of star known as a Cepheid variable (Trimble 1995).

Cepheid variables have a very well-understood pulsation period that is directly linked to their intrinsic brightness…something known as a period-luminosity relationship. Harvard astronomer Henrietta Leavitt discovered this relationship through her meticulous measurements (by eye on glass plates) of stellar brightness from observations of the Small and Large Magellanic Clouds. One may wonder how we know the distances to the clouds, how could Leavitt be certain of the distance measurements? At first, the distances were assumed within a scale factor relative to the Milky Way, but eventually independent measurements of stellar parallax could confirm Leavitt’s work. By measuring a Cepheid variable’s apparent brightness (how bright it appears to the observer) and determining its intrinsic brightness through the period-luminosity relationship, it is possible to obtain a rough estimate of their distance from the observer (Shapley 1927). Edwin Hubble used this information to find that the Cepheid variables in Andromeda were over 900,000 light-years away, a distance far greater than any estimate of the size of the Milky Way galaxy at that time. This dramatically changed our understanding of the size of the universe (Trimble 1995).

Of course, Hubble didn’t rest on his laurels; just a few years later in 1929 he measured the distances to several nearby galaxies and determined that they are all moving away from us, implying that the universe is expanding. What’s more, the speed at which they are expanding actually increases with distance…this relationship came to be known as the Hubble Law (Trimble 1995; Gendler and Gabany 2015). The determination that the universe is constantly expanding naturally leads to the conclusion that if you run everything backwards, there must have been a beginning time to this cosmic expansion…and thus the concept of the Big-Bang was born! Edwin Hubble, armed with the most powerful instruments of his day, used the barely perceptible light of the cosmos that he captured on glass plates to completely transform our understanding of the universe. It was only fitting that the world’s most advanced space-based telescope be named after him! 

The Universe–Now in Color!

This “brief” post is getting pretty long, so we’ll jump ahead about thirty years to 1958, the year that the very first color astrophotography was produced. In the intervening years, the Palomar observatory was built (1948) along with its 200-inch (5.1 m) Hale telescope and the 48-inch (1.2 m) Samuel Oschin Schmidt telescope. Great advances in photographic film had been made allowing for even more sensitivity to light, even higher resolution, and most importantly color emulsion. In 1958, William Miller, using the Schmidt telescope, produced the first color images of the Andromeda Galaxy and several nebulae (Gendler and Gabany 2015). Miller’s image of the Ring Nebula (M57) taken with the 200-inch telescope shed a whole new light on planetary nebulae. Colors that could only be inferred through black and white photography and a solid understanding of the underlying physics of emission nebulae were now vividly displayed in a single color photo. The combination of a very powerful telescope and long exposure times with color emulsion film produced a real image of something that could not be seen with human vision.

Jumping ahead once again to the late 1970s, astronomer David Malin of the Anglo-Australian Observatory used the color techniques first described by James Clerk Maxwell to produce true color images by combining black and white images taken through red, green, and blue filters. Malin also used traditional photography techniques such as unsharp masking to emphasize more subtle information contained within his photographs. (Gendler and Gabany 2015). Variations of these same techniques are still used today (of course in the digital darkroom) in the production of images from Hubble data!

CCDs Change Everything

By far, the biggest advancement in photography in modern times has been the development of Charge-Coupled Devices (CCDs) which have essentially replaced film in cameras. These sensors are designed to capture photons of light and convert them into electricity, which is then read out and interpreted as an image. The first CCDs were developed by Willard Boyle and George E. Smith of AT&T Bell Labs in 1969, and within ten years professional observatories had already started replacing photographic plates with CCDs (Brasch, 2017; Gendler and Gabany 2015). It is not an exaggeration to say that CCDs have revolutionized modern photography and astrophotgraphy. It is easy to take this technology for granted today because it is so ubiquitous. Anyone with a camera phone is carrying a sophisticated camera in their pocket at all times, made possible by CCDs.

From an astrophotography perspective, the development of space-based observatories such as Hubble would have been very difficult without an electronic means of capturing and transmitting images. Hubble was first conceived in the 1940s (then known as the “large space telescope”), well before the advent of CCDs, and scientists actually considered using film on the observatory (Spitzer 1990). Images would be captured on film, and then jettisoned from the observatory in capsules that would later be intercepted by high-altitude flights. A similar design was successfully used in spy satellites in the 1970s. (Guillemette 2011) Alternatively, images could be captured, developed, and then scanned and transmitted all from the observatory, a technology that was actually used in the 1960s when NASA launched several Lunar Orbiters to survey the moon for suitable landing sites for the Apollo missions (Machemer 2019). The Voyager satellite, launched in 1977, digitized its images using an imaging system based on vacuum tubes, the Vidicon Camera, a precursor to CCDs (Tiscareno NASA Planetary Data System 2021). All of this is to say that moving from the analog to digital realm is what really allowed space-based observatories to take off (pun very much intended).

Rise Above It

There has been a clear progression of ever more sensitive instruments designed to capture the faintest light of the cosmos in order to extend our own vision and provide insight into our deepest questions about the fundamental nature of the universe. Although the twentieth century saw tremendous advancements in both telescope and image capture technology, the fundamental limitation to ground-based astronomy is the nature of Earth’s atmosphere. Observatories are generally built high on mountaintops, not to get closer to the stars, but to rise above as much of Earth’s atmosphere as possible. We essentially live at the bottom of a large ocean of air, complete with currents and temperature gradients that greatly distort the faint light of the universe. Hubble was designed to take advantage of being above Earth’s atmosphere to get a clearer view of the heavens. Contrary to popular belief, Hubble doesn’t see better because it is closer to the objects it observes. Hubble’s orbit is really only a few hundred miles above Earth, far enough to rise above atmospheric turbulence, but a negligible distance compared to the many trillions of miles to distant galaxies. This same principle is true for the next generation space telescope, the James Webb Space Telescope, which is scheduled to launch on October 31^st of this year. Webb will peer deeper than ever before into the early universe to see the first galaxies and uncover the mysteries of the origin of the universe.

Hubble was born out of a desire to push the boundaries of knowledge, to advance the frontiers of science, to observe distant galaxies, to answer the tough questions about the nature of the universe. Hubble’s optical system is a direct descendant of generations of ground-based observatories before it and its imaging detectors were built on the many decades of advancement in capturing light as electronic information as outlined here. The Webb telescope will build on Hubble’s accomplishments with its next generation optics and instrument capabilities.

We had a very detailed look at Hubble’s inner workings in part I of this series, and we now have a more complete picture of the advances in astrophotography which put Hubble into a larger historical context. In part III of this series, we’ll take a closer look at why images from Hubble are real, what they reveal about both space and the nature of human vision. If you’ve made it this far, thank you for sticking it out and embracing your own curiosity…I know I learned a lot in the process of writing this post, and I hope you learned something new by reading it. If you’re inspired to learn more, please use these references as a springboard towards that goal!

References:

1. Tenn, J. S. (1989) The Rise and Fall of Astrophotography  http://www.phys-astro.sonoma.edu/people/faculty/tenn/pubs/riseandfallhires.pdf
2. Ré, P. (2010) History of Astrophotography Timeline http://www.astrosurf.com/re/history_astrophotography_timeline.pdf
3. Hughes, S. (2015) Catchers of the Light http://www.catchersofthelight.com/
4. Gendler, R., Gabany, R.J., Breakthrough! 100 Astronomical Images That Changed the World, DOI 10.1007/978-3-319-20973-9_1
5. Abrams, P. (2000) The Early History of Astrophotography http://www.europa.com/~telscope/astrphot.txt
6. Brasch, K. (2017) A Short History of Astrophotography: Part 1, Journal of the Royal Astronomical Society of Canada, Vol. 111, Issue 2, p.52, http://articles.adsabs.harvard.edu/pdf/2017JRASC.111…52B
7. Brasch, K. (2017) A Short History of Astrophotography: Part 2, Journal of the Royal Astronomical Society of Canada, Vol. 111, Issue 6, p.252, http://articles.adsabs.harvard.edu/pdf/2017JRASC.111..252B
8. Trimble, V. (1995) The 1920 Shapley-Curtis Discussion: Background, Issues, and Aftermath PASP 107: 113,DOI 10.1086/133671
9. Shapley, H. (1927) Investigations of Cepheid Variables. II. The Period-Luminosity Relation for Galactic Cepheids Harvard College Observatory Circular, vol. 314, pp.1-6 http://articles.adsabs.harvard.edu/pdf/1927HarCi.314….1S
10. Spitzer, L. (1990) Astronomical advantages of an extra-terrestrial observatory Astronomy Quarterly, Vol 7, Issue 3, 131 https://doi.org/10.1016/0364-9229(90)90018-V
11. Guillemette, R. (2011) Declassified US Spy Satellites Reveal Rare Look at Secret Cold War Space Program https://www.space.com/12996-secret-spy-satellites-declassified-nro.html
12. Machemer, T. (2019) Moon photos from the 1960s were developed in space—here’s how https://www.nationalgeographic.com/science/article/moon-photographs-from-1960s-were-developed-in-space-heres-how
13. Tiscareno, M. NASA Planetary Data System (2021) Voyager Imaging Science Subsystem https://pds-rings.seti.org/voyager/iss/instrument.html