Aberration. It’s not a defect. It’s a characteristic of the optical system, but it gets more noticeable and more intrusive with larger telescopes and at higher magnifications. Modern Apochromat refractor designs can effectively eliminate it by using more exotic, low-dispersion glasses, and more than two lens elements. These can provide superior images but they are also much more expensive to manufacture. At the time the 40” objective of the Yerkes refractor was built, Achromats were still the state-of-the-art, regardless of the inherent Chromatic Aberration. Through the great Yerkes refractor, Jupiter would have certainly been surrounded by an obvious violet halo if it weren’t for that light-green filter.
For most of us observing the night sky through a large observatory telescope is a rare, maybe even once-in-a-lifetime experience. Most of the large telescopes used in modern observatories are designed to focus their light onto a camera sensor and are optimized for data acquisition. There is no practical way to even look through these telescopes. For most recreational astronomers, it’s an unobtainable fantasy. However, the largest telescopes of a century ago or more were designed and made before photography was essential for astronomical study. At that time astronomers were still observers. Luckily, there are a few places where such instruments still exist. With a little planning and luck, it is possible to look through some of them. I recently had an opportunity to observe through the historic 40” refractor at the Yerkes Observatory in Williams Bay, Wisconsin. Below is an account that experience.
As a young visual observer growing up in Wisconsin I had toured the Yerkes Observatory in the mid-1990s, about 100 years after it was first founded by the University of Chicago. I can remember visiting the grounds on a cold winter day and stepping into the equally cold large dome housing the principal instrument for which the facility was known. The refractor was huge, over 63 feet long and 6 tons in weight. It was supported by an equally huge equatorial mount situated on top of a very tall pedestal. The domes wooden floor is approximately 75 feet across and moves up or down to position the astronomers comfortably at the focus of the telescope. The 40 inch objective is a classic Achromat doublet and was made from both crown and flint glass by Alvan Clark & Sons to a focal length of over 19,300mm and a focal ratio of f/19.
At the time I never thought there would be an opportunity to view through it, but two decades later such an opportunity finally presented itself while on a visit back to Wisconsin. We made our reservations for July 2nd, 2018, about a month and a half in advance. The session was planned to begin shortly after sunset and would conclude around midnight. The Moon would be over 80% illuminated, but being a Waning Gibbous phase it would not rise until nearly the end of the program. By luck, the weather was perfect. The sky was cloudless and calm with temperatures predicted to drop no lower than the mid-60s. The total group of observers was 12-14 with only a few experienced observers present. The rest were novices and, I hope, appreciative of the rather special opportunity they had for their first view through a telescope.
By Chad Quandt
Practical considerations for making a smooth running program limited the prospective targets to within a couple hours or Right Ascension of the Meridian and there would be only enough time to see six different objects. Luckily this included some favorites; Jupiter, M13, M57, M11, M17, and ending on Saturn. I had hoped we could view Mars as well which was less than one month from opposition, but it would be too low in the sky during our session.
Having observed all of these objects before with a variety of telescopes, I was curious how they would appear in the world’s largest refractor. Just what does 40” of aperture get you? Well, it turns out that the comparison isn’t as straightforward as you might think. For example, in this session we were not permitted to change eyepieces. For convenience, a 40 mm eyepiece with a 2” barrel was used throughout the night providing a little over 480X. That’s a lot of magnification, far more than I typically use on my scopes except under especially steady seeing. The use of such high magnification also results in a very narrow field of view. This is of little consequence when viewing the planets, but many deep sky objects benefit from being viewed within the context of their region of the sky.
Filter aside, the view of Jupiter was pretty nice. Atmospheric seeing is often best shortly after sunset and just before sunrise and viewing the gas giant early in the evening benefited from this pattern. Initially, I had difficulty finding best focus which I attributed to the relatively long f/19 focal ratio and variations in the seeing. Compared to the optically faster telescopes I am accustomed to using, with their comparatively narrow depth of focus, it was somewhat difficult to be confident that the image was the sharpest possible. The problem was enhanced by the high magnification and atmospheric seeing. Furthermore, once best focus is achieved, the observer has to exercise a little patience. The image was a little bloated, somewhat blurry. Then, for just a moment, it would steady and a great amount of detail was visible including many cloud bands. The Great Red Spot, which seems like it spends more time than not on the far side of the planet, was clearly visible approaching the limb. That was a nice surprise. All four Galilean satellites were also visible. In most telescopes they appear like four little stars next to the planets disc, but here they themselves appeared as very small but distinct discs. Jupiter is a stunning sight, even in small telescopes and I can’t say the 40” refractor provided the best view I’ve ever seen of it, but so much depends on atmospheric seeing that I have to say it was a satisfying view considering this was a singular opportunity to view it.
After Jupiter, the dome interior lighting was further dimmed until only a few dim red lights remained. The floor was lowered and the telescope pointed near the zenith and centered on M13, the Great Globular Cluster in Hercules. Stepping up to the eyepiece with the telescope hanging vertically directly overhead, the program guide reminded us all that the telescope weighs 6 tons. Viewing this time without any filters, I eagerly awaited my turn at the eyepiece. Like Jupiter, I’ve viewed M13 on countless occasions. It is said by many to be the best globular cluster visible from mid-Northern latitudes. I think M5 gives it a good run, but okay, M13 is probably it. Refocusing for my eyes the field of view was full of stars. Seemingly resolved to the clusters core, the density of stars in the center of the field of view was somewhat greater, but not substantially so. While I’ve pushed up the magnification on M13 to 200-300X before, here I think 480X was too much. It’s beautiful for sure, but I think a little space around the cluster would help define it and set it apart from the surrounding sky. In this scope, all we were seeing was the center of the cluster. To the casual observer, most globulars tend to look alike, but with some patience and experience they often have signature characteristics. In M13, there are several dark features that appear as short segments or lanes, two of which cross to make a tell-tale “X”. These were visible, but again, I think backing off on the magnification would have made them stand out a little more against a visibly tighter starfield.
After the group had examined M13 the telescope was moved again, this time to the famous Ring Nebula (M57) in Lyra. As before, the field of view was very small, but in this instance the high magnification was a great asset. The Ring Nebula is one of the few nebulae that obviously resembles its appearance in photographs. It looks like a smoke ring. At 480X, it’s a rather large ring set against a very dark background. There were a few stars visible in the field that proved useful for checking proper focus, and the expected shape of the nebula was very apparent. The space inside the ring, the hole, was not as dark as the background sky and indicated the presence of fainter nebulosity. A closer look revealed what I consider to be the highlight of the evening, the star at the center of the Ring. M57, like all planetary nebulae, demonstrates the inevitable fate of moderate mass stars like our Sun. Late in their life-cycle, these stars begin to shed their outer layers. Then, when fusion ceases, the core collapses to form a white dwarf. The central star in the Ring Nebula is a white dwarf, the exhausted core of a dead star. It is also especially difficult to see. Only once before was I confident that I had seen it using averted vision with a 25 inch telescope at 360X. Through the Yerkes refractor, it was easily visible with direct vision. Visually, stars appear as points of light. Through a telescope, increasing magnification tends to dim the view. Extended objects like a nebula or even the glow of the background sky get dimmer as the amount of light per unit of area is reduced. Point-like stars don’t follow the same rule. Higher magnification makes fainter stars more visible, even though the view as a whole gets dimmer. I had to go back to the eyepiece two more times.
The next object was lower in the sky, M11. This classic open star cluster is also popularly known as the Wild Duck Cluster. Its resemblance to a duck or flock of ducks is debatable, but it’s a beautiful cluster in small scopes. While it is fairly compact for an open cluster, the narrow field of view made it more difficult to identify the boundary of the cluster against the background stars. It was a beautiful view, but not as pleasing as I’ve seen it through many smaller scopes.
Peering lower in the south, the telescope was moved to M17, otherwise known as the Swan Nebula. M17, like the Ring, actually resembles its namesake to the visual observer. Even fairly small telescopes show it like a ghostly duck in profile, but the big refractor showed us just the gizzard. The nebulosity was obvious, appearing mottled and clumpy, but the field was just too narrow to take in the whole object at once. While studying M17, I reached over and knocked on the hand-rail at the back of the telescope that is used to manually move the telescope. The view appeared unaffected with no visible vibration. Despite the scopes impressive size, it was solidly mounted.
The last object of the evening was the planet Saturn, which had just risen high enough to the Southeast to get a decent view. Perhaps the most inspiring sight in the night sky, Saturn is almost everyone’s favorite. The iconic rings were at nearly maximum tilt, allowing a decent view of the Cassini Division and I could detect a slightly brighter band across the equatorial region of the planet. Six moons were visible set against a few stars, perhaps more but I couldn’t be certain. As expected, the atmospheric seeing had deteriorated somewhat by this point, now approaching midnight. Furthermore, without the use of colored filters, Chromatic Aberration due to the Achromat design was obvious though not overly distracting. Saturn is a beautiful planet, but again, I think dropping the magnification by half would have made it appear much sharper and more pleasing to the eye. Nevertheless, who would complain about viewing Saturn through such a large and historic scope! Not I.
As the program ended, dome lights were turned on and the group took turns taking pictures around the telescope. Some of the guests had probably not fully realized what they were getting into by signing up for this program, but I felt that the length of time spent at the eyepiece was plenty to satisfy my curiosity. Of all the objects we looked at, only the Ring Nebula appeared better than any previous view I’ve seen. The rest of the objects were all enjoyable and fit closely to my expectations. While seeing was not perfect, the program guide thought this was the best night they’d had in several months. Maybe he says that every time to make his guests feel like their hundred bucks were well spent, but for me I could believe it. Maybe I’d have told myself that even if we got to see just one object through the scope and got clouded out for the rest, but I also walked away with something else.
Observing the night sky can be inspirational, it can be humbling, and it can be therapeutic. It can also be an exercise of personal discovery, but for those who do it regularly they often end up doing it alone. On this night it was the best kind of discovery, that which includes a touch of fellowship. Accompanying me was my father, Richard, who has always encouraged and supported my interest in astronomy, the night sky, and other things. That alone makes this night a special one I will always remember. Also present were my Uncle Ray and cousin, David, who on more than one occasion have demonstrated their enthusiasm for observing the night sky and I am greatly appreciative to have been able to share this experience with them as well. I sincerely hope there are more such adventures to come!
These are the six objects viewed on the night of July 2nd, 2018, through the 40" refractor. These images were all created by Chad Quandt using a variety of amateur telescopes.
These images were created using a camera on a fixed tripod. The red light visible in the images was from the dim lighting inside the dome and was not as obvious in person as it appears here. The movable observatory floor is suspended on cables and as observers move about the dome, a subtle sway is detectable. Some experimentation was required to capture these scenes of the telescope in action.
At the start of the session, the floor was elevated to place the eyepiece at a comfortable viewing height with the telescope centered on Jupiter. The sky was mostly dark but some dim lights were left on in the dome to allow observers time to gain familiarity with the telescope. A light-green filter was placed on the eyepiece which accomplished two goals. Many observers find that a simple color filter can be used to increase the contrast of some cloud features. The effect is subtle but noticeable. The second goal is more fundamental to the design of the telescope.
Like a prism, a lens refracts light. White light is split into the full visible spectrum. What makes a telescope lens different than a simple prism is that rather than projecting a rainbow, the telescope lens focuses each color of light at a different location from the lens. The Yerkes refractor, like all Achromats, has a two-element objective made of two different glasses with different dispersion properties. Achromats are made such that the extremes of the visible spectrum, blue and red light, come to focus at the same place. With that design constraint, the other colors also happen to come to focus at nearly the same position, but not exactly. What do we see? Well, when we look at a focused image in an Achromat it is sometimes possible to see what looks like a violet halo around bright objects. This effect is called Chromatic
Last Updated 12 May 2020.
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