Sixty-inch Hale Telescope at Mount Wilson Review
This review is a bit of a cheek. After all, the Mount Wilson 60-inch is one of the most famous and historic telescopes in the world. At the time it was completed (first light was December 8th 1908) the Sixty-inch was the largest telescope in the world and remained in the vanguard of research for many years afterwards. It effectively ended the era of giant refractors that preceded it and was involved in many ground-breaking discoveries. There is a cabinet of astronomers’ lockers in the basement of the Sixty-inch dome; one of them is simply labelled ‘Hubble’.
But whilst putting it through my usual review format may seem cheeky, it’s also appropriate. I spent a night with Sixty-inch recently, critically viewing a range of objects in near-perfect conditions and with time to linger at the eyepiece. So I’m in a good position to write this review. What’s more, it’s not just of theoretical interest for you. Why? Because you could do the same thing I did and book an evening on the Sixty-inch for yourself. In this review you’ll find out whether it’s worth it (spoiler alert: it is).
Please note: This review is just about the Sixty-inch reflector. My visit to Mt Wilson also included a personal tour of the Hundred-inch telescope and interferometer array that merits a separate article in due course.
That’s me, standing on the gantry of the Hundred-inch Hooker telescope dome with Session Director Heven Renteria, taken by the webcam on the Solar Tower.
At A Glance
Sixty-inch Hale Telescope
1.5m (Sixty inches!)
F16 in Cassegrain mode
Design and Build
The Sixty-inch was one of the first big reflectors, but it was also one of the last big professional telescopes designed to be looked through and this is a key point. Most professional telescopes are just giant camera lenses. Even if you could physically slot in an eyepiece, the optics just aren’t designed for it. In fact, the Sixty-inch is currently the second largest ‘scope in the world available for visual use (the largest is the Hundred-inch Hooker telescope just a few hundred metres away on Mount Wilson).
Because it was designed for visual as well as photographic use, the Sixty-inch is a bit different from most modern professional telescopes. It has a longer focal length and optics intended for high magnifications on the Moon and Planets, unlike modern professional instruments that are intended for astrophysical research using cameras and other instrumentation.
Optics - design
The Sixty-inch is basically a Cassegrain. I say basically, because it could originally be used in several different optical configurations simply by swapping out the secondary cage (the other cages can now be seen under the observing floor – you pass them on your way to the loo). The Newtonian cage replaced the curved Cassegrain secondary with a flat mirror, whilst the Coudé focused the light out through the RA axis to a static instrument package, such as an early spectrograph (the tiny Coudé-focus room past the end of the RA shaft can still be seen below the Sixty-inch pier).
Newtonian and Coudé secondary cages stored in the basement.
The Cassegrain design is a proper reflector, using only mirrors to focus the light and so avoiding the false colour (chromatic aberration) that plagues large refractors like the ones the Sixty-inch superceded (such as Lowell’s 24” Clark refractor at Mars Hill – see below). The Cassegrain uses a parabolic primary, like a Newtonian, with a hyperbolic secondary that usually focuses the light back through an aperture in the primary.
The Classical Cassegrain isn’t found much these days. For professional telescopes, it’s been superceded by the Ritchey-Chretien, which allows faster focal ratios more suited to imaging, whilst most Cassegrains in amateur hands are cheaper-to-make Dall-Kirkhams that sacrifice the Classical Cassegrain’s flat field.
Nonetheless, the Classical Cassegrain makes an ideal visual/photographic instrument. A moderately long focal ratio allows a relatively small secondary mirror, which is ideal for high-power planetary work, but the design still has a flat field for extended objects and imaging.
The Sixty-inch is currently fixed in a modified Cassegrain configuration, converted for visual use by directing the light out of the side of the mirror box using a flat placed in the light path.
The Sixty-inch currently operates at a fairly ‘standard’ Cassegrain F-ratio of F16. But with a 1.5 metre mirror, that means a colossal 24m focal length!
Optics – primary mirror
Green ‘Champagne bottle’ glass primary mirror with entrapped air bubbles of the Hundred-inch.
The 860 Kg blank for the 1.5m primary mirror was cast by Saint Gobain in France in the late Nineteenth Century. Legend has that it’s Champagne-bottle glass and it certainly has that exact same bottle-green colour. The blank is 19cm thick, so it was cast in one go, unlike the larger 100-inch mirror which suffered from entrapped air bubbles and had to be cast in three layers.
The mirror was ground between 1904 and 1906 and from my observations was clearly very well figured. The observatory currently re-silver the mirror every few years.
Open lattice-work tube is very long seen side-on (standing back as far I could, it didn’t quite fit in the frame of my 23mm lens!)
The Sixty-inch looks relatively compact in the flesh, much smaller than the enormous Hundred-inch Hooker telescope next door, though it is long due to the long focal length primary.
The tube is an open lattice-work of truss tubes and bracing girders, currently painted pale blue. The telescope appears in fine condition, but it is a much more utilitarian device than the brass-and-lacquer showpiece that is the 24” Clark refractor at Lowell observatory (the class of instrument it effectively replaced).
The telescope is very robustly constructed, more like a piece of civil engineering than an instrument. It is ruggedly and heavily built: viewing at moderate to low elevations involves leaning on the mirror box, hands either side of the eyepiece; doing so doesn’t induce the slightest vibration.
One interesting piece of trivia is that the tube has a number of US plug sockets next to the eyepiece. These were installed after WWII to power war-surplus heated flying suits favoured by the astronomers to help endure long cold nights at the eyepiece.
Focuser for the Sixty-inch with a four-inch-barrel eyepiece (100mm focal length), alongside modern paddle for slewing.
The current focuser is not the original. It is a massive Crayford design, machined from a solid brass block and capable of taking enormous four-inch barrel eyepieces (most amateur telescopes take 1.25” or 2” barrel eyepieces). This allows the Sixty-inch to have quite a good field of view, despite its long focal length.
The focuser was built by an expert machinist from a local Los Angeles astronomy society and reminded me of a huge ‘Moonlite’ focuser. It is accurate, but a bit squeaky in use.
Note that the Hundred-inch Hooker telescope has a standard Astro-Physics (3”?) focuser that probably only allows for conventional eyepieces with 2” barrels; given that and its greater focal length, it will have a much smaller field of view than the Sixty-inch.
Sixty-inch’s compact fork mount and mercury-bath RA bearing.
Massive pier assembly below the observing floor.
The other end of the pier, RA Shaft and old Coudé focus instrument room behind it. Note the azimuth adjusters.
Huge RA worm wheel and modern (surprisingly small) motor assembly.
The Sixty-inch has a massive fork mount. Nonetheless it is much more compact than the Hundred-inch’s yoke mount. The RA axis employs a mercury-bath instead of a conventional bearing. This is now safely sealed, but used to regularly leak silver beads over the observing floor which had to be scooped up and poured back in!
The RA axis is isolated on a massive pier that extends through the floor and basement. Below the observing floor, the RA axis is supported by a cast iron frame that can be aligned to take into account minor movement due to earthquakes. The RA is driven by an enormous worm and wheel. Slewing was originally effected by separate gears mounted on the RA shaft; these still exist but all slewing is now done by the main drive gear and worm (running in an oil bath), turned by a surprisingly small modern electric motor.
The mount is controlled from a modern desk behind the RA bearing, that includes a computer and an infrared camera overlooking the observing floor (no bum-scratching in the dark).
There is no GOTO, but the mount can accurately and quickly slew to a given DEC/RA, read from planetarium software.
The mount/scope has an elevation limit of about thirty degrees (we tried to look at Antares at 26 degrees, but couldn’t quite reach it).
Note: If you are deciding whether to go for the Sixty-inch or splash-out for a session on the larger Hundred-inch Hooker telescope, it is worth taking into account that the bigger telescope is slower to move and has a more limited range of elevations, so you probably won’t get to see such a wide variety of objects.
Control desk, log book and infrared camera feed.
This is a long telescope for its aperture compared to most professional telescopes and so it needs a large dome.
The dome slit is wide and so the dome doesn’t often have to be moved, but it does so with quite a lot of vintage whining and clanking that only adds to the historic atmosphere. The dome switch gear includes three original Edison light bulbs more than a century old and some Frankenstein-style switches that give a satisfying green flash when they’re thrown.
Dome switch-gear includes three century-old Edison bulbs that light up when the shutter is closing.
That 24m focal length means that familiar eyepiece sizes take on new uses. A 55mm TeleVue Plossl delivers higher powers than most observers ever use! Consequently, Mt Wilson have various enormous eyepieces to give lower powers and wider fields. Meanwhile their highest power is delivered by a 27mm Panoptic!
The following eyepieces were available for my use on the night (but you could bring your own):
· A 100mm with a four-inch barrel giving 240x.
· An 80mm with a four-inch barrel giving 300x.
· A 55mm TeleVue Plossl (my overall favourite and most used eyepiece in the Hale Sixty-inch) giving 436x.
· An Explore Scientific 30mm eyepiece with a three-inch barrel (the largest eyepiece I have ever seen – like a huge 31mm Nagler) giving 800x.
· A 27mm TeleVue Panoptic giving 888x.
In Use – The Night Sky
At the 100mm eyepiece of the Sixty-inch Hale.
General Observing Notes
The seeing at Mount Wilson is widely regarded as the best in the US and on the night I visited it was good to really outstanding (according to my Telescope Operator). At best, it was almost perfectly steady, well below an arc-second. Even at its worst, movement was gentle and slow, nothing like the boiling I get back home.
The sky was flooded with Moonlight from a twelve-day-old gibbous Moon, but smaller DSOs were still visible to great effect (though more diffuse ones were a bit washed out as you would expect). Sky-glow from nearby LA wasn’t very noticeable and inversion fogs regularly shut it out to leave a dark sky.
My Telescope Operator (John) was able to quickly slew from object to object on my viewing list and made helpful suggestions as well (Campbell’s Hydrogen Star was his idea). Both the Operator and Session Director are highly trained so you get the most out of your session, but they are also keen and enthusiastic amateur astronomers in their own right.
I noted that the 100mm (240x) eyepiece gave a distracting shadow from the secondary during twilight, but not at night when it gave a fabulous ‘low power’ view for extended objects. My favourite high power eyepiece was the 55mm Plossl at 436x. The higher power (800x and 888x) eyepieces were still sharp, but harder to focus and delivered no more detail except in rare moments of total stillness.
Viewing comfort depended on the altitude of what we were observing. Objects near the Zenith mean you can sit comfortably on a stool. Objects lower down mean leaning off the ladder high above the observatory floor, one foot on the mirror box and a kind of frozen push-up, hands around the eyepiece!
Observing Log (in order of viewing)
I arrived back from my tour of the observatory with the Session Director in Twilight. I could already hear slewing from within the Sixty-inch dome. Through the door and left up the short flight of industrial-grey stairs to the observing floor, I found that John, my friendly Telescope Operator, had already set the ‘scope on Jupiter. He urged me not to waste time and get observing. I followed his advice, nipped up the yellow viewing stepladder and leaned out to the eyepiece …
(I didn’t have time to make observing notes, so I recorded my observations on my phone. I’ve reproduced them, object by object, here in case you are interested in the details of what you can see through a 1.5m ‘scope)
The view of Jupiter, at ~436x with an old-style Televue 55mm Plossl was stunning. The Great Red Spot was bright red (rather than the usual pale salmon pink) and showed concentric whorls within it, with thickening of the South Equatorial Belt and white vortices upwind, also a leading dark streak being drawn up and around the GRS.
The North Equatorial Belt contained masses of small dark storms and white spots. To the north of the NEB were two smaller dark bands and a number of white storms in the greenish-grey polar hood. Belts below the SEB were visible too, but not as clear as the pair north of the NEB. I managed to get my phone up to the eyepiece for a quick snap of Jupiter – see below.
Jupiter’s Galilean moons looked surprisingly different from the small-telescope view I am used to. They were all resolved into disks of slightly different size and showed markedly different hue and brightness: Io and Europa were both bright, with Europa a whiter hue; Callisto was most surprising, having a notably grey-buff colour and lower brightness.
I watched Europa occulted at ~20:25. I Could clearly see the disk of the moon disappearing gradually ‘into’ the atmosphere, rather than suddenly winking out.
Later, John slewed Ganymede into centre field. I could detect a dark mark just off-centre and a check-up by zooming in on Sky Safari confirmed that this was Galileo Regio. I recorded:
“An incredible moment for me – seeing surface detail on Ganymede with my own eyes”.
Again at 436x with the 55mm Plossl, I was able to see the central rille in the Alpine valley, another exciting first for me. As that part of the Moon was already well away from the terminator, one side of the rille was brightly illuminated and it appeared as a fine silvery thread running down the middle of the valley.
The craterlets in Plato (at least 5) were clearly resolved into ‘proper’ simple craters, rather than just the usual dots.
Astounding detail in the Gassendi region with numerous rilles and craters within Gassendi itself. Also, an interesting group of rilles nearby centred on a ghost crater – de Gasparis.
A great view of Aristarchus and its striped walls. Schroter’s Valley looked very wide and lots of domes and craterlets, small rilles, were visible on and around the Aristarchus Plateau.
Moving to a dome field near Sinus Iridium, the 900m craterlet on Mons Gruithuisen Gamma dome, (at under an arcsecond, a supposedly stiff test of any telescope) looked big and clearly resolved as a simple crater with a bowl-shaped floor.
Overall, I could see much more detail than the Rukl atlas: a real smorgasbord for a lunar observer.
My main reason for being at Mount Wilson was to view Mars, less than two weeks after closest approach at opposition (at 35° altitude and eighteen arc-seconds in diameter), as research for a book I am writing. So, I spent a significant part of my observing session on the Red Planet.
The relatively low altitude of Mars meant that the mirror box was high above the observing floor and it required a bit of gymnastics to get to the eyepiece –one foot on the steps (fortunately locked), another on the edge of the mirror box and hands around the focuser for support. For this reason, I was unable to get a camera or my phone to the eyepiece to record a snap, as I had with Jupiter.
Mars was a delicate ochreish red-orange, not the ‘whiteout’ I had been led to believe you get with a really big aperture. The level of detail was astonishing.
Margaritifer Sinus and Meridiani Sinus were particularly dark, with bright Aram in between giving that ‘eye’ effect. Sabaeus Sinus and Deuteronilus were clearly bounding the large bright area of Arabia, that nonetheless had a huge range of subtle variations and gradations in tone and colour, as did Tharsis. On the other side of Mare Acidalium was Solis Lacus, containing lots of fine albedo detail. I thought I could see linearity in the Coprates region: a hint of Vallis Marineris?
The Mare Australe region was very bright with cloud that extended northwards later in the evening, more so in the west (the Solis Lacus side which was right in the eyepiece), perhaps over Argyre. Later, John plugged a blue filter into the 55mm EP, giving an even clearer view of the bright cloud.
I noted that Margaritifer Sinus comes much closer to Niliacus Lacus at the foot of Mare Acidalium, than the atlas showed. I thought Aurorae Sinus was darker than in the atlas too.
Of Mars’ two moons, Deimos was very easily visible, but Phobos was close in and much more difficult – it blinked in and out of view as the seeing steadied.
Mars looked like a perfect little world through the 60”, ripe for exploration rather than just occasional glimpses of detail - interesting because this telescope superceded the era of big refractors like Lowell’s and no-one could have seen this Mars and believed it had canals.
Perhaps the most impressive view of all was Saturn. It’s hard to describe what an extraordinary sight it made, surrounded by moons – just like one of the more distant shots from the Cassini space-probe.
The Cassini division was gaping wide all the way around, with the bottom limb of the planet visible through it. The rings were composed of numerous individual bands. The Encke division was clearly visible in moments of extra-quiet seeing. The Crepe ring was astoundingly wide and obvious, much wider than I thought.
Bands were visible in Saturn’s atmosphere, including a broad cream equatorial band, with a central dark belt and two thin belts at around the two o’clock position and the dark polar hood, subtly shaded, above it.
There was a whole cluster of Moons to the bottom left in the telescope field: Enceladus, Tethys, Rhea and Dione, with Mimas very occasionally blinking into view close and above the planet. Titan was way off to the left and I was astounded by the fact that it was a clearly resolved disk and a startling orange colour – very surprising!
I booked a night on the Sixty-inch to indulge my personal interests in the planets (especially Mars) and the Moon. I didn’t really expect to do much deep sky observing, but was thrilled by what I saw. Under a Moon-less sky with an inversion to block the lights of LA, the Sixty-inch would make for an incredible deep-sky fest’.
At 436x the M3 globular cluster was resolved to the core with direct vision – masses of tiny stars, looking more like the Pinwheel open cluster than a typical view of a globular. An even better view with the monster 4” 100mm eyepiece giving ‘just’ 240x: still able to resolve whole core at 240x, but with the whole cluster, with all its outer stars, hanging in dark space.
The ‘Double Double’
Tried to get onto Antares to see if we could spot its companion, but too low, so settled on Double Double. Huge black space between components with at 426x: they looked like Mizar and Alcor through binoculars.
Campbell’s Hydrogen Star
This is a tiny and little known planetary just 5” across in Cygnus, but remarkable through the Sixty-inch – a blue 11.3 mag central star with a halo of really deep red gases surrounding it that have a slightly squareish appearance. Extraordinary that this doesn’t make an NGC number and isn’t in most atlases!
Cats Eye Nebula
With a 27mm Panoptic giving 888x and a high enough altitude to view comfortably sitting on a stool for once, it was easy to see the central white dwarf and detail in the double rings of nebulosity, with the striations in the central oval ‘eye’. With averted vision, the intersecting lobes of shock had an almost square shape, despite bright moonlight.
M92 in Hercules
Resolved to the core like M3, with arcs of stars coming out of the centre and lots of stars in the core with a sense of individual stellar brightness and colour. M92 was high enough to view standing on the first rung of the step ladder on tiptoe.
Really bright and obvious with its typical dumbbell shape, but a bit washed out at this magnification due to the Moonlight.
The Ring Nebula
The central star was easily picked up with averted vision, but again the nebulosity was quite washed out due to the Moonlight.
Viewing through the Sixty-inch was an astonishing experience for me that exceeded my expectations: memories to last a lifetime. Lots of personal firsts, including detail on Ganymede, the Alpine Valley, The Enke Division, Deimos and Phobos, Mons Gruithuisen Gamma dome and seeing Titan as a tiny world instead of a star, not to mention all the fine detail on Mars.
In Use – Astrophotography
If you bring a DSLR or a manageable CCD, there’s no problem slotting it into the massive focuser, so you could do some serious astrophotography (though you’d have to discuss dark frames and flats beforehand).
I was intent on just getting the most from my observing session and I had none of these things, just my iPhone plus, which didn’t even fit the iPhone holder they have. Nonetheless I took some pretty decent snaps of Jupiter just holding it to the eyepiece:
Left to right: Callisto, Io, Europa and Jupiter snapped through the Sixty-inch with an iPhone. Note how dim and grey Callisto is.
Booking a night on the Sixty-inch
You can book either a full or a half night on the Sixty-inch. The costs are currently (mid 2016) $950 for half a night and $1700 for the full night.
I did the ‘half night’, but in fact I ended up observing for over five hours - from 8pm until after one in the morning.
Viewing with the Sixty-inch often involves leaning out and supporting yourself in some challenging positions and I was quite tired physically at the end (and I am not unfit). So I would recommend the half night if you do this on your own. Even my Session Director admitted people get pretty bombed in the small hours and often take a nap. If you can afford the full night, why not do two half-night sessions at different times of year to broaden the experience?
In my opinion, a full night would make more sense for a group (of up to fourteen participants), to allow more individual eyepiece time.
The mechanics of booking are as follows:
1) Choose your night. The Mt Wilson Institute currently publishes a list of available dates early in the year (the 18th Feb in 2016).
2) Fill in the Time Request and Agreement form and email it to the observatory Telescope Coordinator (a very helpful lady named Shelley Bonus at email@example.com when I booked, but best check the website).
3) Mail your payment and a hard copy of the form to the address below. If you are a foreign national, you will need to organise a draft in US Dollars. Your bank should be able to do this for a small fee; I had to turn up in person at my branch for this.
4) You will then receive confirmation when your draft clears.
5) The Session Director for your night on the Sixty-inch will then contact you about a week before you are due to view, attaching some advisory information and directions.
6) You will get a final go/no-go text or email from your Session Director on the day of your planned visit, depending mainly on the weather.
7) Your Session Director will meet you at the observatory gates at around 1930. It’s a winding road up from Pasadena, so leave plenty of time to get there!
Mt. Wilson Institute
P.O. Box 94146
Pasadena, CA 91109
Mt Wilson Sixty-inch vs Lowell Observatory Twenty-four-inch Clark Refractor
The eyepiece end of the beautiful 24” Clark refractor used by Lowell at Mars Hill in Flagstaff
The Sixty-inch Cassegrain at Mt Wilson superceded the generation of large refractors epitomised by Lowell’s own 24” at Mars Hill near Flagstaff, so I thought I’d do a comparison (more cheek, I know).
I’m in a good position to compare the two telescopes, since I spent a couple of evenings at the Lowell observatory just a few nights before my session at Mount Wilson and I had numerous views through the 24” Clark.
I am generally a ‘refractor guy’ – I love small refractors for their sharp and sparkly views. On this occasion, though, I much preferred the bigger reflector. Yes, The Clark is a far more beautiful instrument. But in terms of performance (of the telescope, the mount and, frankly, the observatory staff), I overwhelmingly preferred the Sixty-inch Hale. If I was Jeff Bezos, it’s a big Classical Cassegrain I’d buy for my private observatory (dream on).
A summary of their relative merits follows:
· The Clark Refractor has been newly restored and it’s just the most beautiful thing – all brass and lacquer and redolent of a certain era of astronomy. The Sixty-inch looks utilitarian by comparison.
· The Clark gives fine views, but a lot of chromatic aberration on bright objects like planets. It really needs to be stopped down (easy to do with the iris in front of the objective) to about 18-20”, increasing the F-ratio to F18-20 (from F16).
· The Sixty-inch has better resolving power and (of course) reach.
· The Sixty-inch seemed to take high magnifications better.
· The larger focuser of the Sixty-inch means a similar (or larger) maximum field of view, despite its greater focal length.
· The seeing was much better at Mount Wilson (it’s famous for it), but obviously that could have been the individual nights in question.
· Slewing seemed easier and faster on the Sixty-inch. This meant a much wider range of objects to view over the course of an evening.
Some other points about the overall experience:
· The big refractor was often in a state of significant defocus that completely ruined the view; even novice visitors commented. This isn’t the scope’s fault (it was perfectly sharp when properly focused), but honestly Lowell Observatory, teach your outreach staff to focus a telescope or have the humility to accept assistance from someone who can.
· The staff mostly refused to use higher powers (they generally stuck with 175x) and didn’t want to set it on Mars which they said was ‘always disappointing’, though of course Mars was what the 24” Clark was installed by Percival Lowell (interred for posterity in his mausoleum a few metres away) to view.
· Both the Telescope Operator and Session Director at Mt Wilson were highly trained and enthusiastic. They really bent over backwards to give me the best possible experience. The outreach staff at Lowell … err, didn’t.
· Both telescopes can be theoretically booked for private viewing, but Lowell doesn’t offer a half-night on the 24” Clark and so it is much more expensive at the time of writing (though to be fair, when I asked the head outreach guy about the possibility of booking a night he cut me dead anyway).
I would still love to spend a night of private viewing through Lowell’s refractor, but I would hope that the outreach staff would be (frankly) more competent and accommodating than they were for the public sessions I attended.
Bucket lists are a thing these days. We’ve woken up to the idea that life isn’t endless and if you’ve got specific dreams and ambitions, better get on with it.
For some people, it’s a weekend in a Ferrari, or driving a racing car around a famous circuit; for others it might be flying a helicopter or trekking to Everest Base Camp. But if you’re an astronomer, amateur or professional, a night on the Sixty-inch at Mount Wilson should be at the top of your list. You’ll see things with your own eyes you never have before and never will again.
Of course, the Sixty-inch is a much bigger ‘scope than most, even big Dobs, even today. But it’s much more than that. It’s a superbly figured instrument in top optical condition and has a hugely stable mount capable of high-power tracking. It’s also at altitude under some of the best seeing anywhere. Put all that together and you get views that, if you’re used to amateur telescopes, will astound and enthral.
It’s not just the telescope itself, either. My Session Director and Telescope Operator worked really hard to give me the best possible experience (my Session Director, Heven Renteria, was also fantastic at interpreting the historical side, including an in-depth tour of the 100-Inch before my observing session).
I set out to tell you if paying for a night on the Sixty-inch was worth it. I can tell you that it is, and then some: it completely exceeded my already high expectations.
I’ll leave you with this sobering thought: if something were to happen to Mt Wilson, some of things I saw that night may never be seen by the human eye again, because no-one is likely to build telescopes of this size and quality for visual use in future.
Buying a night of observing on the Mount Wilson 60-inch gets my highest possible recommendation. Anyone with an interest in astronomy should do this at least once before they go.
Swapping eyepieces on the Sixty-inch Hale at Mount Wilson