JOHT 86: What the Telescope Sees

[stickmaker]

The Joy of High Tech

by

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. And if you find a mistake, please let me know about it.

What the Telescope Sees

These columns have repeatedly pointed out how technological requirements have spurred developments (sorry about the photography pun) in our understanding of the universe. Likewise, the quest for a better understanding of the universe has repeatedly driven developments in technology and science. As noted below, these are also connected with the pursuit of artistic expression. The challenge of accurately recording new scientific discoveries is a fascinating topic. One small branch of how that challenge was addressed is discussed briefly in this column.

Before the telescope what was known of the heavens was, of course, limited to what could be seen by someone with good, unaided vision. Sometimes from a place with natural or artificial positional references. This was recorded in drawings, but also in the constructs made to aid observation. Cultures around the world at the very least built some of their structures aligned to astronomical events. Some people created actual observatories, with references built in to help locate what was seen in the sky when. Much important work was done with these aids. However, telescopic examination of the skies provided multiple revelations. As well as revolutions. 

The telescope was probably invented around 1608, most likely in the Netherlands, actual creator unknown. Just a year later it was greatly improved by science and engineering tutor Galileo di Vincenzo Bonaulti de Galilei, who designed a better lens arrangement using mathematical modeling of light rays. Apparently without actually seeing a telescope first, but just from reading about the concept. Galileo was the first person on record to turn a telescope towards the heavens. (As often happens in science, it seems that many people were doing similar things at the same time. Thomas Harriot, a British ethnographer and mathematician, also used a spyglass to observe the Moon. His August 1609 drawings of the Moon predate Galileo's, but were not published at the time, so Galileo gets the credit.) Regardless of who first directed a telescope upwards, ever since then astronomers have been trying to preserve what they see, in part to show others. (Galileo was insistent about having others look through his telescopes, which is probably how he got multiple eye infections. Most likely because of these, he was effectively blind for his last few years.)

As a method to record what the telescope revealed, drawings of course came first. That was all the early observers had available, after all. Astronomers had to make do with that method of recording what they saw for centuries. Even when photography finally became available in the early Nineteenth Century, the images through a telescope were generally too faint for practical photography using the early methods. The first known successful photographic recording of a telescopic image was a Daguerreotype of a particularly bright object: the full Moon. This process used activated silver on a prepared copper surface (tintypes came later; they used a different process on treated iron (not tin) and were somewhat more light sensitive; ambrotypes came between, using a wet emulsion process on glass). Daguerre himself is believed to be the first person to take a photograph of the Moon, on January 2, 1839. Unfortunately, in March of that same year his entire laboratory burnt to the ground, destroying all his written records and much of his early experimental work. Including that historical image of the Moon. A year later, John William Draper, an American doctor and chemist, took his own Daguerreotype of the Moon. This resulted in a beautiful - though small - image of the Moon in silver. In 1850 Draper collaborated with astronomer William Cranch Bond to produce a Daguerreotype of the star Vega. 

The event of photographing the Moon on a Daguerreotype was recreated by modern astronomers using a period telescope in the late Twentieth Century. The result was again an exquisitely beautiful image of the Moon in delicate silver. (Anyone who thinks there are no links between art and science doesn't know enough about both. As mentioned above, art has repeatedly driven the development of science and technology, and developments in those have provided new media for artistic expression. A sculptor invented the method of making large acrylic castings without bubbles, which made deep submersibles with acrylic pressure spheres possible.)

As photography improved, its use for astronomy increased. However, even towards the end of the widespread use of photography for astronomy (in some cases this use actually continues as this is written) more sensitivity and resolution were desperately sought. The problem is that there is a tradeoff between sensitivity and resolution. All other factors being equal, greater sensitivity means larger photographic grain size, which reduces resolution. As well, chemical photographic media are subject to what is known as reciprocity failure. That is, below a certain level of light, what is recorded diminishes far faster than the light level. Which means that, regardless of exposure time at very low light levels your negative will be blank. Still, chemical photography improved markedly through the decades and became vastly capable. There are also multiple methods of increasing the sensitivity of existing chemical negatives with little or no effect on resolution. These sensitizing techniques are generally lumped together under "hypoing." 

One improvement more essential to scientific photography - especially astrophotography - than to portraits or even landscapes is uniformity in the emulsion. When photographing stars and planets you want all of the emulsion - whether on glass or film - to have the same response. Otherwise you might find yourself declaring light or dark spots as significant, when they're actually due to flaws in the emulsion. Naturally, the techniques developed to improve scientific emulsions found their way into commercial and personal use, greatly improving the quality of all photographs. 

Note that, regardless of the specific mechanism used to record such faint light, the optics must be clean. Otherwise the astronomer will be spending a lot of time eliminating the recorded dust specks.

One of the many tradeoffs of chemical photography - for whatever purpose - is that, all other factors being equal, the larger the negative, the less light which strikes any particular portion of the emulsion. This is because the camera is literally spreading the light out over a larger surface. For this reason, medium and large format cameras tend to have much larger lenses than smaller format cameras of the same vintage. Telescopes, of course, have very large apertures in comparison with the lenses used on any conventional camera. This is to be expected, since they are observing images of very faint objects. Also, many astrophotos were made on fairly large glass plate negatives. The idea was to photograph as much of the sky as was practical per observing session. In part this was due to the expense of observing time; in part due to the optical limits of the telescopes themselves (above a certain magnification the images are just larger blurs); and in part due to the realization that anything which appeared interesting on these plates could be photographed in more detail later. Astrophoto negative plates (there is still some use of glass plate negatives, mainly for consistency in ongoing sky surveys) typically have very fine grain, and are frequently treated ("hypoed") to increase sensitivity. (This increase in sensitivity can also be performed on film negatives.)

One trick used in hypoing is to cool the emulsion to reduce noise. This is still necessary for modern, electronic detectors. Some of which need to be cooled with liquid helium for best performance. Heat - or thermal - noise is the bane of sensitivity.

As part of the drive (sorry about that) to record ever fainter objects, the mechanical tracking mechanisms used on telescopes improved concurrently. These mechanisms were originally driven by clockwork, and for the most part did very well for direct viewing. However, as photography improved and the need to record dimmer bodies for longer periods increased, astronomers often had to ride close herd on the telescopes. This included making frequent, fine adjustments to keep the telescope pointed at the precise part of the sky being recorded as the Earth turned. Later means of doing this involved electric motors and precise location sensors on the telescope mount. Yet that was not enough. For decades even the mighty Hale 200" - which was a technological revolution in may fields - often required someone to sit in the prime focus cage at the top of the telescope for hours. This in spite of all the work put into giving it the smoothest and most precise guidance system of the time. Today, of course, important astronomical instruments are digitally guided, with far better results than even the finest eyeball and most delicate fingers of old. Much to the relief of the astronomers and telescope technicians, I'd like to point out! Observatories are often built on mountaintops and have to be at ambient temperature during use to prevent air currents from causing distortions. It gets cold, sitting there for hours, staring through an eyepiece to make sure the telescope stays pointed at the right spot!

Speaking of the Hale, its robust design still makes it a favorite for trying out heavy, bulky prototype detectors of all sorts. Whether historic glass plate cameras or modern detectors which count each photon, just bolt it on and go! The sturdily built mount and directional actuators scarcely notice the extra weight.

Astronomers are constantly seeking something better when it comes to detectors, just as they have for over a century. Some tried using electronic methods of recording what a telescope showed as early as 1910. However, those primitive devices did little more than provide methods for standard measurements of how bright a particular body was. Still, early on there was great hope for the future of electronic astronomical observing. At the 1933 annual meeting of the Association for the Advancement of Science, one talk was on how the rapidly advancing technology behind television might soon replace chemical photography. This development turned out to be longer in coming than some expected. Many observatories and researchers indeed experimented with various types of video tubes during the following years. However, even when those had an output which was considered adequate, there remained the problem of recording what they revealed. In many cases, the only way to preserve that image was to photograph the image on the TV tube. The poor resolution and difficulty in recording the image made electronic images uncommon in astronomy for most of the Twentieth Century. However, because the contrast on such images could actually be better than direct astrophotography such work continued, though it was uncommon. Even in 1973, the National Academy of Sciences noted that astronomers did not yet have a suitable electronic device to replace chemical photography. Part of the delay was due to the development of electronic devices being taken over for military use during the Second World War.

One interesting hybrid technology combined electronic amplification with chemical photography. In 1934 André Lallemand began work on photomultipliers, soon producing a type of electronic camera which would soon be named after him. This, indeed, produced an electronically amplified image on a photographic medium. Unfortunately, preparation was complicated, and this device was limited to one exposure per setup. This was due to putting all the works of the device - including the electron-sensitive emulsion - in a vacuum, inside a single, glass container. After exposure, the device was broken open to retrieve and process the emulsion. Despite the shortcomings, some observatories used variations on this process into the Sixties. 

The Orthicon camera tube was one early type of electronic image detector, in use for television from 1946 to 1968. While it was also used in astronomy, purpose-made devices were soon more common.

In the Fifties the Carnegie Institute even formed the Carnegie Image Tube Committee, which was intended to produce an image detector specifically for observing. While they device they created - with considerable help from private industry - was used in many observatories, it still did not completely replace chemical photography. However, these instruments _were_ used. Among other ground-breaking applications, a Carnegie image tube was used by Vera Rubin to gather strong evidence that the outer parts of the Andromeda Galaxy were spinning faster than expected from the amount of visible mass. This was a major step in confirming Fritz Zwicky's proposed "dunkle materie" or dark matter. 

For the past several decades solid state image recording equipment - such as the Charge Coupled Device or CCD - has increasingly replaced chemical photography and vacuum tube-based electronic methods. Anyone who has switched from a film camera to a digital understands why. Modern solid state imaging devices have greater resolution than even professional grade photographic film, and greater sensitivity than even the best hypoing can provide. Another advantage of modern imaging is that, since the image is digitized, what a telescope reveals can be viewed anywhere in the world with an Internet connection. In fact, most professional observing these days is done remotely. An astronomer will get time on some large telescope and run it from his office or home, perhaps thousands of miles away. Even the directing of the instrument is done remotely, with technicians on site rarely having to help. However, chemical photography - specialized film and even glass plate negatives, as mentioned above - is still used for some purposes. The flat plane of a glass plate is much better suited to some types of precision photography than flexible film. Of course, sometimes large sheets of film are carefully fitted to curved backings for certain other types of photography.

The real current value of chemical photography, though, is for the record it provides. Do not underestimate the current importance of the previous use of photography in recording what was detected by telescopes. By the early Twentieth Century the emulsions on dry plate glass negatives had enormous resolution, and both that and sensitivity continued - and continue - to improve. By the middle of the century there was photographic proof of multiple theories about stars and groupings of stars. As well, those old photographic plates have far more than historical significance. There are currently multiple projects underway by observatories and college astronomy departments to perform high-resolution digital scanning of existing photographic plates. This serves multiple purposes, including creating a computerized library of standardized images of the same objects, going back many decades. This library of images can be examined by volunteers and computers to find changes which have previously gone unnoticed, or to provide history for changes which have only lately been noticed. Several announcements of discoveries made by such comparisons of old data with new have been made in recent years. Moreover, these scans are often bringing out details previously missed, perhaps by being too small or faint for the Mark I Eyeball which originally scanned the negatives.

Moving away from optical observations for a moment, modern astronomy of everything from Gamma rays to UV to IR to radio waves has also repeatedly revolutionized our understanding of the heavens. We're even detecting cosmic gravity waves now! All of this data is recorded and analyzed digitally, but often presented visually. 

Some of the gravity wave discoveries have been presented audibly. Sound also has been used to portray things such as recordings of magnetic and electrical and other data detected by probes around Jupiter and Saturn. There are a few other examples. The development of analogue sound recordings came after early photography, but that doesn't seem to be the reason astronomical data is far more often presented visually. Most astronomical data simply fits better with visual representations. Something to be expected from the type of data. 

We are well into the Big Data era of astronomy. Professional astronomers rarely look through telescopes with their own eyes these days, unless it's for their own entertainment. Of course, many still do just that. (If they weren't interested in what was overhead, they wouldn't have gotten into professional observing, would they?) Also, even the best current computer reviews of data sometimes miss things. This is where volunteers come in; they often find new discoveries missed even by the best supercomputers and search algorithms. (It's hard to tell a computer to make note of something you don't expect.) Regardless of how it is found, when something new _is_ discovered, somebody has to check and decide what it means. Modern digital imaging makes this far more easy and convenient, besides revealing information invisible to even the trained eye or best chemical photograph. Small wonder so many interesting discoveries are being made in the sky these days.