I’ve posted about a couple of DSO catalogues, the Messier and the Caldwell, that beginners should start out with. Heck, even supposedly “more advanced” astronomers like myself are still working on these lists, as they represent the vast majority of the most gorgeous items in the sky. But if you’re looking for your next challenge, there’s the Herschel 400, a compilation of even fainter, more difficult objects that require some serious aperture to see, or more to the point, to see well.
The Herschels were a family of astronomers who observed beginning at the end of the 18th through the middle of the 19th centuries. In 1781, William Herschel discovered the seventh planet, Uranus (must . . . resist . . . making . . . obvious . . . joke). His partner in astronomy was his sister Caroline, who made a number of discoveries herself.
In 1757, at the age of 19, William emigrated to England from Germany (technically, from Hanover, as Germany would not become one united country until 1871). At the time, Hanover and England were united under the rule of King George II – father of King George III, that guy who we Americans had our little rebellion against. At the same time, Hanover was at war with France. So William’s father sent him and his brother to England to avoid military service. In 1772, Caroline came over to England to join William after their father’s death. The Herschels were a family of musicians; William and Caroline earned a living giving concerts and performing at churches.
William’s church performances put him in contact with Reverend John Mitchell, who was a former math professor. Mitchell was developing some hypotheses regarding astronomy, and these sparked the interest of the naturally curious William. He started grinding and polishing mirrors made of speculum metal for use in telescopes.
Speculum metal and modern mirrors
Speculum is an alloy of copper and tin that was fairly reflective, at least by 18th century standards. The technology to make glass telescope mirrors (or even glass bathroom mirrors) as we now know them did not yet exist at that time. Speculum was the only material known that would hold the parabolic shape needed for a telescope mirror. However, speculum had three important drawbacks: first, it was only about 68% reflective at very most; and 60-64% is closer to the true amount of reflectivity that Herschel was able to achieve. Since a reflector has both a primary and a secondary mirror, only 0.64 x 0.64 = 36% of the light was reaching the eyepiece.
Second, speculum tarnishes very quickly, reducing the reflectivity even further. This led to many astronomers of the day having two mirrors for each of their scopes, one that was being used in the scope, and the other, back in the shop, being repolished to remove the tarnish. Finally, speculum does not hold its shape well under varying temperatures.
To overcome these problems, in 1856-57, Steinhall and Foucault (yes, the pendulum dude) invented a method of silvering a mirror – depositing a thin film of silver onto the front of glass that had been ground into the correct parabolic shape. This process – along with the fact that a reflector only needs one ground surface versus four or more for a refractor – eventually led to the rise of reflectors over refractors in the late nineteenth and early twentieth centuries.
Today’s modern telescope mirrors have this same thin film of metal on glass, although it’s aluminum instead of silver, which yields a reflectivity of 91%. With the two mirrors found in most reflector telescopes, that gets 83% of the light to the eyepiece. This means that for any given aperture, a modern telescope will be sending well over twice as much light to the eyepiece as compared to the speculum ones that Herschel was using. While silver itself has an even higher reflectivity at 95%, silver tarnishes; aluminum does not.
Nowadays, there are various advanced coatings that can be deposited onto mirrors to increase their reflectivity to 96, 98, or even 99%. These 99% reflectivity mirrors are generally called “dielectric”. The term refers to these coatings, usually made of alternating layers of silicon dioxide and titanium dioxide – up to one hundred separate layers! Obviously, coating a mirror this many times can become prohibitively expensive, which is why they are not generally available on primary mirrors. Instead the dielectric process is usually reserved for diagonals, which have far smaller surface area than a primary mirror. (Far smaller area = far less expensive.)
Herschel’s discovery of Uranus (ahem)
On March 13, 1781, while they were in the middle of one of their methodical surveys of the sky using a 7″ reflector, William came across a previously unknown object that was clearly not a star. At first, because it was not a pinpoint the way stars are, he thought it must be a comet. However, because it was so bright at 6th magnitude, if it were a comet, it should have been relatively close to the sun, and therefore should have been moving quickly against the background stars. That close in to the sun, it should also have developed a tail the way that comets do. It did not. Based on how slowly it was moving, it had to be orbiting the sun further out than Saturn; and based on its brightness, it had to be a planet.
William wanted to name his discovery Georgium Sidus, Latin for “George’s Star”, after, who else, King George III. But the non-English rest of the world wasn’t going to stand for this, as the other planets were all named after Roman gods, not after British kings. In “keeping” with this nomenclature, it was named Uranus, after the father of Saturn. However, it didn’t exactly keep with the previous scheme, as Uranus was a Greek god, father of Cronus. The Greek Cronus became the Roman Saturn, with his back story altered a bit.
The discovery permanently affixed Herschel’s star in the astronomical firmament. More importantly, King George III named him “Astronomer Royal” and gave him a lifetime pension of 200 pounds per year. Caroline became “Assistant Astronomer Royal” and received a pension of her own. This allowed the Herschels to retire from having to be musicians to support themselves, and to devote themselves to astronomy full-time.
William was a prolific builder of telescopes, constructing hundreds over his lifetime. His first reflector had a 6-inch mirror that was 7 feet long and magnified 40x. (As I discussed in my Messier article, the concept of telescopes with interchangeable eyepieces had not yet come into being, and they were more frequently referenced by their focal length rather than by their aperture.) He then moved on to a 9-inch scope that was 10 feet long. After that, Herschel came down with an incurable case of aperture fever.
He was notable for building some true whoppers. With money from the king in 1783, he built a 20-foot telescope, which had a 19-inch mirror.
Talk about aperture fever, 19 inches still wasn’t enough for Herschel. So between 1785 and 1789, with an outlay of 4000 pounds from King George III, he built the 40-foot, 48-inch telescope. The tube was made of iron. The original mirror weighed over 1000 pounds, but he ground it too thin. It was only 1.1 inches at the center, so it could not keep its shape. He cast a second mirror that was twice as thick, and this became the main mirror for use in the scope:
I don’t know how much 4000 pounds translates to nowadays, but let’s do some figuring. William’s 200 pound per year pension was more than enough for him to live as an English gentleman very comfortably for the rest of his life, so let’s call that the equivalent of $100,000 per year. That puts the cost of the 48-inch telescope at twenty times that, or $2,000,000. Gulp. Thanks, King!
Herschel built his larger telescopes with tilted primary mirrors to eliminate the need for secondary mirrors. We’re generally familiar with how a newtonian telescope works – it has a primary mirror aimed to reflect light directly back up the tube to a point perfectly in the center, equidistant from the walls of the tube. A secondary mirror with a 45-degree angle at that location then directs light out through a hole in the side of the tube, where a person can observe through an eyepiece placed in a focusing mechanism at that hole.
Herschel’s design tilted the primary mirror slightly so that the focal point would naturally arrive just outside of the tube towards the top end. One merely had to place an eyepiece at that point, looking essentially down the tube at an angle, to use the telescope. This design produced much brighter views – 60-64% of the light transmitted to the eyepiece versus 36% in a two-mirror system. He made his first observation with the 40-foot telescope of the Orion Nebula while it was still under construction, by crawling inside the tube and using a handheld eyepiece.
Herschel rarely used the 40-foot scope – it was just too much trouble, taking too long to prepare it for use. He instead used the 20-foot scope, abandoning the 40-foot scope in 1815. Herschel therefore conclusively proved the old adage, “The best telescope is the one you use.”
The Leviathan of Parsonstown
Herschel’s 48-inch telescope was the largest telescope in the world for over fifty years. That is, until 1845, when William Parsons, the third Earl of Rosse, built his 72-inch monster, the Birr Telescope, otherwise known as “the Leviathan of Parsonstown”. He built it smack in the center of Ireland, about 80 miles west of Dublin. Wouldja take a look at this thing!
I know this is far afield from the topic of Herschel, as he was dead for over 20 years when this was built, but jeez, I can’t just mention Herschel’s giant scopes and move on without talking about the Leviathan. Here are some fun facts about the Leviathan and the man at the eyepiece:
- The speculum mirror was 5 inches thick and weighed 3 tons; there were two of them, so that one could be used while the other was repolished;
- Together with the mirror box and the 54-foot tube, the telescope weighed 12 tons;
- The scope could be moved up and down in azimuth, but could only move about one hour in right ascension (one-twelfth of the sky) between the two walls of the supporting building;
- With the telescope, Rosse was able to discover the spiral structure of galaxies, particularly, the Whirlpool Galaxy, M51;
- Rosse also named M1 the Crab Nebula based on its shape through a 36-inch scope he started out with before building the 72-incher;
- Rosse discovered 226 objects in over 30 years of observing, between 1848 and 1878;
- The Leviathan was the largest telescope in the world for over 70 years, until the 100-inch Hooker telescope was built in 1917 at Mt. Wilson. Hubble (the dude, not the scope) used the Hooker scope to discover that Andromeda was not a nebula in our galaxy, but in fact, is a galaxy of its own, far outside of the Milky Way.
The Herschels’ Observations and Discoveries
William and Caroline observed the sky by simply leaving the telescope alone, pointed in one position relative to the ground, and letting earth’s rotation carry different portions of the sky to the scope in long, skinny strips to his eyepiece. And boy, were they skinny. Doing some quick calculations, the 20-foot scope had a focal length of 6.1 meters = 6100mm. Widefield eyepieces did not exist in Herschel’s time, so he would be using an eyepiece with an apparent field of view of 40 degrees or less. A 40mm Plossl (which, again, did not yet exist) would fit that AFOV – and yield a true field of view of just over a quarter degree across at 151x. That’s barely wide enough to show you half the moon at a time.
Fortunately, because of the design he was using, he didn’t have to worry about seeing the shadow of the secondary by going down to a very low magnification. If he had had an 80mm eyepiece – and I have no idea if an eyepiece of that focal length ever even existed, or if Herschel had one – he could have doubled the width of his field of view at half the magnification. Because the area of a circle is Pi multiplied by the square of the radius, doubling the width means you are quadrupling the amount of area you’re seeing at any one time.
William would stay at the eyepiece and call out what he was seeing to his sister. Caroline would write these observations down, noting the exact time so as to later be able to determine the precise location of the object in the sky. Each night, objects rise 4 minutes earlier than the night before. Using this method, over a number of years, they were eventually able to observe the entire sky as viewable from England.
With the enormous increase in aperture he was able to build, William was the first to discern that the nebulae he was observing were actually made up of individual stars. Remember, back then, it was called the Andromeda Nebula, because the concept of a separate galaxy had not yet come about.
Additionally, he discovered two moons of Uranus (heh, heh) – Titania and Oberon. Uranus’ moons are mostly named after characters from Shakespeare. He also discovered two moons of Saturn – Mimas, also known as the Death Star moon, due to its gigantic central crater, and Enceladus.
Enceladus is especially intriguing, as the Cassini probe discovered that it has liquid water geysers shooting out from near its south pole. Scientists have further determined that this water emanates from a gigantic subsurface ocean. This makes Enceladus one of the top places to look for life in our solar system.
Additionally, Herschel was the first to coin the term asteroids for the first four of a group of bodies that were discovered in the early 1800s orbiting the sun between the orbits of Mars and Jupiter. It wasn’t until additional asteroids were discovered starting with the fifth one, Astraea, in 1845, and many others in the 1850s, that asteroids came to mean something less than a planet.
Herschel was also the first to discover infrared light. He used a prism to split light into its color component spectrum, much as Newton had done over a century earlier. He noted that just beyond the red end of the spectrum, there was something warm. He deduced that there was a light source there, but that our eyes could not detect it – the infrared part of the electromagnetic spectrum.
William and Caroline put together three lists of DSOs: the first 1000 starting in 1786, another 1000 in 1789, and a final 500 in 1802, for a total of 2500 DSOs. These three lists together were called the Catalogue of Nebulae and Clusters of Stars (CN). Herschel studied the nature of nebulae. He discovered they were formed of stars, disproving the long-held belief that nebulae were composed of some sort of “luminous fluid”.
If you’re thinking that that’s an impressive leap over Messier’s “paltry” 109-110 DSOs*, you’re right. While Messier had his Mechain to assist him in observations, William had his Caroline, who was a full-fledged observer in her own right. More important than that, William was using much larger and therefore better scopes than Messier was, with significantly more aperture. This again proves that aperture is king in this hobby.
The critical differentiating factor between the two is that Herschel also wasn’t limiting himself to looking in particular areas of the sky for comets, as Messier was doing. Comets can more or less can be seen along the Ecliptic, the plane of the solar system projected onto the sky. These include the 12 constellations familiar to us from astrology, Pisces, Libra, Scorpio and such, as well as at least as many neighboring constellations. Messier confined himself to this general area. On the other hand, with Caroline, William was doing full-sky surveys, cataloguing everything he could see, everywhere in the sky.
Caroline contracted typhus at the age of 10, and her growth was stunted as a result. She was only 4-foot-3, and she lost sight in her left eye. As a result, there wasn’t any realistic chance that she would marry. Her father made sure she was educated and was taught to play the violin. After her father died, she left Germany and joined her brothers in England.
She initially earned a living with William giving concerts, at which she would sing. Once William made the transition from music into astronomy, so did she, as she found it as interesting as he did. She became his recorder as he shouted out his observations, writing down his observations and later matching them up to star charts.
Caroline was no slouch herself. She was more than just a recorder – she was an observer as well. She was the first woman to discover a comet, and discovered or co-discovered 8 of them in her observing career, including Comet Encke, the comet with the shortest period/orbit at just 3.3 years. She also discovered 14 nebulae, including NGC 205, better known as M110, one of Andromeda’s companion galaxies.
Because of her own astronomical abilities, both as William’s assistant, but also in her own right, in 1787 King George III appointed her assistant Astronomer Royal, and gave her an annual pension of £50. She was the first woman to hold an official governmental position in England, as well as the first female scientist paid for her work.
She was the one who did the actual work of compiling the records to form the three catalogues of DSOs that they issued in the last years of the 18th century. This involved her doing many tedious calculations to determine the precise positions of the observations. As a result of her lifetime of work in astronomy, in 1828 she received the Royal Astronomical Society’s Gold Medal for her work in relation to the catalogues. She was the only woman to win this medal for over 160 years.
William was knighted in 1819, becoming Sir William. He died in 1822. At that point, Caroline moved back to Hanover. She died there in 1848 at the ripe old age of 98.
William’s son John was also an astronomer. Well, in point of fact, he was more of a polymath – a genius at anything he tried, but someone who got bored with a field quickly, and moved on. One example of this is in the field of photography, where John had laid out many of the important principles of it in a paper he published in 1819. In 1839, Frenchman Louis Daguerre built upon these principles to invent modern photography. That same year, John merely heard of Daguerre’s work in photography, and within a few days, without having heard of any of the details, was able to take his own photographs. He could easily have been hailed as the discoverer of photography two decades earlier if he had only put his mind to it.
In the 1820s, he published papers on calculating parallax – the subtle shift in a nearby star’s position against the background of stars further away. This occurs as the earth moves in its orbit around the sun, providing a large enough baseline (like the distance between your two eyes) to allow the star’s parallax to be detected. By knowing the amount of this shift, the distance to the star could be calculated. Again, he did not pursue this work, and left it to another astronomer, a German named Friedrich Bessel, to follow up on his work and to become the first person to measure parallax and calculate the distance to a star, known as 61 Cygni.
In 1834, John travelled to Cape Town, South Africa, and observed there with a 21-foot telescope he had brought with him for four years. This telescope was roughly the same size as the one his father used and preferred. While there, he observed the return of Halley’s Comet. Based on discrepancies in its orbit, he correctly calculated that forces other than gravity were acting upon it, including outward pressure from the sun. He thus was the first person to discover the solar wind.
John was in South Africa to extend his father’s observations to the Southern Hemisphere’s skies and catalogue the DSOs there. However, he was not able to do so to the same extent as his father had done for the Northern Hemisphere. In 1864, he compiled “The General Catalogue of Nebulae” (GC) with 4,603 objects first observed by either himself or his father.
The New General Catalogue – NGC
Enter Danish astronomer Johan Ludvig Emil Dreyer (referred to as J.LE. Dreyer). Dreyer emigrated to Ireland to work with Lord Rosse at his 72-inch Leviathan. Observing lists were getting unwieldy; everyone and their brother had their own little lists at this point. Confusion reigned, and something needed to be done. Dreyer published a 1000-object supplement to John Herschel’s GC in 1878, and was about to publish yet another 1500 new objects in 1886, when the Royal Astronomical Society suggested to him that he compile a “catalogue of catalogues”. (At that time, all of Ireland was still firmly a part of Great Britain.)
Originally published in the Memoirs of the Royal Astronomical Society in 1888, the New General Catalogue of Nebulae and Clusters of Stars (NGC) contains details of 7,840 star clusters, nebulae and galaxies, arranged by Right Ascension – strips of the sky going from the north celestial pole (i.e., right near Polaris) all the way down to the south. Dreyer followed this up with two Index Catalogues (IC), published in 1895 and 1908 as further supplements to the NGC. The two ICs contained details of an additional 5,386 objects, raising the total to 13,226.
The NGC contains objects down well past 15th magnitude, a tiny fraction of which are even 16th and 17th magnitude. Most of these are galaxies, which demonstrates what you can see visually while observing with a 72-inch telescope. The NGC itself was compiled without the aid of astrophotography, but the second of the two ICs was compiled after astrophotography became much more widespread.
Astronomers today still use Dreyer’s NGC and IC numbers to refer to these brightest of the various celestial objects in the sky. Of course, with a catalogue of this size and magnitude, there were simply bound to be mistakes. In addition to the items Dreyer (or Lord Rosse) had discovered, he was also relying on reports from other observers, with greater or lesser degrees of reliability in their observations. It’s been estimated that close to 100 of the objects on the list simply don’t exist; astronomers have been trying to locate them for well over 100 years now, and they ain’t there. Other estimates state that up to 10% of the NGC objects are somehow erroneously listed. This has led to the development of the RNGC, the Revised NGC. However, this catalogue does not have the universal acceptance that the NGC itself has.
The First Astrophotos
If you’ll allow me to go off on yet another (hopefully) interesting tangent, here are the first astrophotos of DSOs EVER:
By 1888, astronomer Isaac Roberts, using a 20-inch f/5 silvered reflector he had had custom-built for the purpose (noooooice!), took this gorgeous pic of Andromeda:
The Herschel 400
The Herschel 400 is a subset of the best of the 2500 objects William and Caroline discovered. It was put together around 1980 by members of the Ancient City Astronomy Club in St. Augustine, Florida as a further list of objects to observe after the Messier Catalogue, but a bigger challenge to observe.
The Herschel 400 objects are all designed to be observable from mid-northern latitudes with a 6″ telescope. The lowest declination of any of them is about -32, putting them easily within reach of anyone at 40 degrees north, and, with some additional effort, those at almost 50 degrees North as well, as Herschel himself observed them from London.
The Astronomical League’s Herschel 400 observing program, while agreeing with that they’re observable with a 6-inch, says that a minimum of 10 inches is required. Looking at the list, I think the AL got it right, as a small handful of the dimmest of the galaxies are 13th magnitude, with a couple of planetary nebula at 14th. By the way, if you’re the member of an astroclub, then you’re automatically a member of the AL as well, so you can try out any of their observing lists to get a certificate saying you’ve completed it once you’ve submitted proof.
The spreadsheet I use for all my astronomical calculations says that a 6″ telescope will barely get you to 13th magnitude, but that’s a 13th magnitude point object – a star, not a diffuse object like a galaxy. Meanwhile, my C9.25 comes up just short of 14th magnitude; a 10″ just gets you there. If you’re going to tackle the 400, definitely get a 10″ scope to do it with, and find yourself some very dark skies to observe under.
17 of the 400 are Messiers; and 44 of them are Caldwells. The list is very galaxy heavy, with 231 of them, but there are also 107 open clusters, along with 33 globular clusters, 20 planetary nebulae, 2 halves of a single planetary nebula, and 7 bright nebulae.
If you’d like to tackle either the Messiers or the Caldwells or even the Herschels, there are a number of ways to do so. There are a number of books on the Messier objects, from the aptly titled The Messier Album, by Mallas and Kreimer, of my youth; to Turn Left at Orion, by Consolmagno and Davis, and, of course, The Year-Round Messier Marathon Field Guide by Pennington.
Fortunately, for the rest there is also Stephen James O’Meara‘s Deep Sky Companions series. This includes not only yet another guide for the Messiers, but one for the Caldwells, as well as his Herschel 400 Observing Guide. Of course, for all of these, there’s also the Astronomical League’s own free materials themselves at their website.
And if the Herschel 400 is too easy for ya, for the truly ambitious, there’s the Herschel II program – yet another 400 of Herschel’s objects to observe. This list was put together in 1997 by the Rose City Ancient Astronomers of Portland, OR. This list is even more skewed to galaxies – 323 of them – and the objects are mostly between magnitudes 11 and 13. A difficult challenge, indeed. Good luck!