Alvan
Clark 36” Refractor (Lick Observatory) Review
This
is the third of my (slightly cheeky) reviews of world-famous telescopes and the
experience of visiting and using them (I have also reviewed the 60” Hale
Reflector at Mount Wilson and the 24” Clark refractor at Mars Hill).
The Lick 36” is one of a number of giant
achromatic refracting telescopes built for (essentially visual) professional
use in the late nineteenth century, many of them by the firm of Alvan Clark in
Massachusetts. Most of the big refractors were a size down from this one,
though, at 20-25” aperture. In fact, the 36” at Lick is the second
largest operational refractor ever made for general astronomy and is the
largest still in use. The largest of them all – another Clark product,
the 40” at Yerkes in Wisconsin – sadly ceased operations in 2018
and may never see starlight again.
One thing is certain, no astronomical lenses of this
size will be made in future: not enough aperture for the pro’s; too big
and expensive for amateurs. That makes the 36” ‘Great
Refractor’, at Lick Observatory on Mount Hamilton above San Jose in
California, a singular experience for an observer today (as well as a unique
piece of history).
James Lick stipulated that the Great Refractor should
be made available for public viewing sessions and it always has been. These
days, the sessions are generally scheduled as an encore to other events, such
as ‘Summer Series’ concerts. Such truly public open evenings sell
out quickly and I wanted a more astronomy-centred experience, so I booked on an
observatory tour and viewing evening for ‘Friends of Lick
Observatory’ members. The tour was very in-depth and will get its own
article. Here I want to concentrate on the viewing session with the 36”
refractor which started in early evening and went on until past Midnight.
Private viewing? Oh yes, that’s possible. But
the costs (at Lick effectively via a hefty donation) are even higher than at
other similar observatories like Mount Wilson and Lowell (and so beyond my
budget, sadly).
The 36” refractor at Lick is second only to
this, the 40” at Yerkes (sadly no longer operational, here pictured when
I visited Yerkes in 2009).
History
Unlike
other founders and funders of great 19th C. observatories like
George Ellery Hale, James Lick had no particular interest in astronomy. Rather
he was looking for a grand way to memorialise himself. Lick had bought up big
chunks of San Francisco after the Gold Rush, so his first thought was to build
himself a giant pyramid there – you know like the ones in Egypt, only
bigger obviously – in downtown at Fourth and Market.
Fortunately
for the residents of San Francisco, Lick had a cultured friend who owned a
6” Clark refractor and regularly went into the local hills to observe.
That friend suggested Lick build an observatory instead of a pyramid, which
Lick did.
In
the event, James Lick died a decade before the observatory was finished, but
his involvement didn’t end there. For James Lick was later exhumed and buried
at the observatory (he’d built the place instead of a pyramid mausoleum,
remember) and not just somewhere in the grounds as you might expect. No, James
Lick was re-interred in the black-brick pier base for the Great Refractor and
there he remains. His tomb is marked by a plaque just out of sight
beneath the observing floor and accessed by a narrow stairwell at the side of
the dome.
The
era of great refractors lasted less than a century. In 1824, the newly arrived
9” refractor at Dorpat was the largest in the world and just a couple of
inches larger than my own refractor today. Just seventy years later came the
40” at Yerkes, the largest refractor ever mounted for astronomy (a bigger
refractor was constructed for the Paris Exhibition, but never used). By the
early 20th C., reflectors like the 60” at Mount Wilson had
made the big lens telescopes obsolete.
The
history of the Lick 36” refractor starts in 1880, when the glass blanks
arrived from Paris, France by ship and were ground into the lens elements by
the Alvan Clark works in Massachusetts. The finished lenses were transported to
the west coast by train (sorry, railroad), then up the mountain by horse and
cart, to arrive onsite in 1886. The long delay was because one of the lenses
broke on the journey and it took many attempts to grind another.
Lick
observatory was the first to be built on a mountain-top (at 4200 ft on Mount
Hamilton) and an interesting piece of trivia concerns that twenty mile road up
to it. I had never driven anything like it. Gruelling hairpin after hairpin
that lurch dizzyingly this way and that but hardly seem to climb at all. The
reason is that the road was built with a maximum 6% grade for pack animals and
carts to lug components for the 36” up the mountain. For me, getting up
it in the late afternoon sunshine was tough; getting back down in the pitch
dark after midnight was truly horrible.
At
first light in 1888, the Lick 36” refractor became the most powerful
telescope in the world and would remain so for some years. Much of its early
success was in the field of planetary science. James Keeler discovered the
Encke Gap (named by Keeler in honour of Johann Encke) in Saturn’s rings
with the 36” in 1888. Later, Edward Emerson Barnard discovered Amalthea,
Jupiter’s fifth Moon and the first discovered since Galileo, with it in September
1892. Early spectroscopic work (by William Huggins) with the 36”
established the paucity of the Martian atmosphere and the absence of
significant water vapour. At the Mars opposition of 1892, the same E. E.
Barnard observed Mars and failed to see the canals that four of his Lick
colleagues believed they had seen with the 36” refractor. Barnard’s
Mars observations – he was renowned as a peerless observer - were an early
source of doubt about Lowell’s Martian canals.
Other
seminal work by the 36” refractor included the discovery of some 25
comets, along with numerous double stars and spiral nebulae.
Lick’s tomb – in the pier base for the
36”!
Lots
of visitors to Lick come for the sunsets and the stars.
At A Glance
|
Telescope |
36” Clark Refractor |
|
Type |
Air-spaced Fraunhofer doublet achromat |
|
Aperture |
36” (914.4 mm) |
|
Focal Length |
57 ft (17373.6 mm) |
|
Focal Ratio |
F19 |
|
Length |
57 ft |
|
Weight |
c. 11,000 kg (OTA) |
Design and Build
Optics
The optics for the Lick 36” refractor were (like
most large US refractors of the time) ground by the famous optician Alvan
Graham Clark, using blanks cast in France. The design is a conventional
Fraunhofer achromatic doublet, in which the bi-convex crown element is in
front, the concave flint behind. Like other Clark designs, the objective incorporates
a large air gap between the elements. This design allows access for cleaning,
but can also can give more freedom in correcting aberrations.
The exterior of the Lick dome looks outsized.
Inside, the reason is clear – this is one very long telescope, even by
the standards of big refractors. The reason is that the Lick 36” has a
focal length of 57 ft to give a focal ratio of F19, longer than many big
refractors of that era and most large achromats today. In contrast, the
24” Clark at Lowell Observatory on Mars Hill at Flagstaff has a focal
ratio of F16.
The reason for this long focal length is to reduce
the problematic false colour and spherochromatism (change of focus point with
wavelength) that affects all achromatic doublets, but especially big ones. A 19th
C. paper on the chromatic aberration of the Lick 36” suggests that this
approach was largely successful. The author of the paper measured the deviation
of focal point for different colours to be no more than another large refractor
that had undergone similar testing, the F15 27” Imperial Refractor in Vienna.
Even so, the spherochromatism was on a gigantic scale compared to amateur
telescopes – several inches, apparently. This was so pronounced,
the paper claims, that Wolf-Rayet stars, which emit mostly in two distinct
wavelengths, can be identified purely from having two discrete and widely
separated best focal points.
Storage box for the 36” Clark objective.
This historic image (hanging in the Lick Observatory
lobby) shows the 36” as it once was, with a giant brass visual back.
Tube
The tube of the 36” Clark is in three
sections. The short middle section that attaches to the mount is a massive
casting. The front and rear tubes are made up of riveted steel bands and give it
that ‘ship’s-hull’ Steampunk look so typical of big Victorian
refractors If slightly less so than the earlier Lowell refractor which is made
of individual plates riveted together).
The steel and brass of the Lowell 24” I
visited a couple of years ago have been beautifully ‘restored’ with
a glossy enamel finish. The Lick refractor retains a more workmanlike and
original flat grey painted finish.
Focuser
As
you can see from the historical photo above, the 36” OTA once terminated
in a giant visual back made of brass with a threaded port for mounting various
instruments. Nowadays, this has been blanked off and fitted with a very
ordinary 2” focuser, just like the ones fitted to many fast-food backyard
scopes.
With
the 55mm eyepiece and a diagonal fitted, the focuser was almost fully racked
in, suggesting that it may not accommodate a wide range of eyepieces and
magnifications (55mm being about the longest focal length and so lowest
magnification you can get in a 2” eyepiece).
Mounting
The 36” Clark has a huge German equatorial
mount, like other big Clark refractors. The mount sits on a tall pier
constructed of four cast iron segments and accessed by a spiral staircase at
the back. A plaque on the pier states it was designed and built by Warner &
Swasey of Cleveland, Ohio.
Pointing the telescope in the modern era was
assisted by eight illuminated Selsyn dials above the eyepiece (four each for
Dec and R.A.), but these apparently no longer work accurately. During my
session, the telescope was pointed manually by the two operators: one at the finder,
the other operating the mount by hand from high atop the pier and equipped with
a hard-hat and safety harness (essential in the darkness of an observing
session).
The colossal length of the 36” Lick refractor
means that the eyepiece height changes a lot between target objects at
different altitudes. This was originally accommodated by a movable observing
floor, with a range of some 5 metres, supported on a massive radial fan of cast
girders, operated by hydraulic presses and a system of ratchets to keep the
huge circular parquet floor level. Sadly, the system has fallen into disrepair
and many of the ratchets have teeth missing. This means an extremely tall step
ladder is used for viewing, one of several available. Even so, objects at low
altitude (which in 2019 means all the planets) end up with the eyepiece high
above the steps and unreachable.
Accessories
On the evening I was there, they only used a 55mm Tele
Vue Plössl in the 36” Clark, an eyepiece that seems a favourite at
observatories with classic, long-focal-length instruments. In this case, that
55mm Plössl gives a magnification of 316x and the widest possible field
given the 2” focuser (no huge bespoke eyepieces like the ones I used at
Mount Wilson here). That maximum-possible field is just 0.15 degrees!
The 36” Clark has a single 6” finder/guide
scope, also by Alvan Clark. The finder retains its original brass focuser.
The Clark was at various times fitted with
professional instrumentation, including an early spectrograph; but it is now (almost)
solely used for visual outreach.
Dome
The dome is a brick base, lined with wood and
supporting a lattice of girders clad in plates. It’s a more sophisticated
thing than Lowell Observatories ‘upturned wooden pale’, but without
the neoclassical grandeur of Yerkes. The dome has a narrow balcony running
around it (with a rather flimsy and low railing). Steps at one side lead into
the basement which houses the mechanism for the movable floor, various bits of
equipment (including a storage/transport box for the 36” lens); and, of
course, James Lick himself!
Underneath the observing floor; Lick’s tomb
is just out of sight to the right.
The 36” at Lick is a giant among refractors.
Operating the telescope these days requires a hard
hat and harness.
In Use – Astrophotography
The
36” Clark has been used extensively for astrophotography in the past. I
had no opportunity to try photography through the Clark.
In Use – The Night Sky
General Observing Notes
My visit to Lick was straight after returning from
Hawaii and the seeing at Lick, whilst good, was not as good Mauna Kea’s.
For that matter, it didn’t seem to me as good as the outstanding seeing
on Mount Wilson a few years back, but obviously individual nights vary.
Whilst some violet blur was evident on the double
star we viewed, we didn’t view anything really bright like a planet or
the Moon (see below). The professional astronomer helping on the tour said that
on a night some years before, when he had viewed the planets, false colour had
been a problem and the Great Refractor had been stopped down in the way the
24” is at Lowell Observatory.
The observing session was long and you might wonder
at so few targets. Part of the explanation is that this was a group session,
with short queues for the eyepiece, but mainly it was because the scope took a
long time to point by hand, with one operator at the eyepiece of the 6”
Clark finder and the other high up on the pier in his safety harness in the
dark, working the mount clamps and controls with the giant setting circles.
Even so, all the targets we viewed were in the same small region of sky.
All of our viewing was with a TeleVue
55mm Plossl giving 316x.
What’s it like?
The sheer experience of using it isn’t
something I generally discuss with telescopes, but this is different.
First there is the dome. The interior is cavernous,
echoey and dimly lit, even with the white lights on. The circular walls are
lined with dark planks and the dome rafters – a network of steel beams
supporting the external plates – arch high overhead. The floor is of
concentric rings of polished parquet, picked out in light and dark woods. The
57-foot-long telescope virtually fills the space, the objective extending
almost to the dome slit, which is open and showing stars.
Getting to the eyepiece involves climbing fifteen rungs
up a step ladder. It feels secure enough and is edged with lights to help see
the steps, but ends up at a surprising height. Some folks need a bit of help
and encouragement to get up there. Looking into the eyepiece then involves stooping
below a big wooden capstan wheel and the ring of switches that surrounds the
visual back.
For the first target, a double star, they kept the
white lights on. Thereafter, it was just dim red lights and the dome became
very dark indeed, so that climbing those tall steps seemed a bit more daunting
for some.
The Lick’s modest altitude and the balmy
California summer meant it never got really cold in the 36” dome, but I
did have to don a jacket towards midnight.
Deep Sky
Gamma Virginis (Porrima)
was our first target.
The two stars of Porrima
– one blue, the other orange – look a bit like Albireo through your
scope (or mine). Except that Albireo has a separation of 35”, whereas Porrima just 2.9” – challenging if you
don’t happen to have a 36” of aperture and the seeing from a
California mountaintop.
The split was excellent and effortless, a yawning
black chasm between the two stars. The blue star did show some violet blur due
to chromatic aberration through the giant achromatic lens and some atmospheric
disturbance (contrast the almost unnaturally still seeing at Mauna Kea a few
nights before).
M53
This spectacular, tight globular cluster, is also
in Coma Berenices. A condensed mass of stars, surprisingly bright and resolved
to the very core and with the outer ones approaching the field stop (in a 55mm
eyepiece!), plus a stunning burst of brighter stars scattered across it like a
firework– wow!
M51
This famous double galaxy has pin-wheel spiral arms
and a smaller companion galaxy. The spiral arms and possibly some individual
stars were clearly visible through the 36”, plus the companion galaxy.
Note: one of the operators recounted that following
his training they had left him alone with the 36” for a whole night. What
did he do? Just moved from galaxy to stunning galaxy, he said, looking for
supernovae and revelling in the view. It had been an unforgettable observing
experience, but a spooky one. Apparently, the scope and mount creak and groan
and unknown creatures scurry about in the darkness outside. Then, of course,
there is the eternal presence of James Lick himself.
Simulation of M53 in the <0.2° field of the
36” refractor with a 55mm Plossl eyepiece.
Simulation of M51 in the <0.2° field of the
36” refractor with a 55mm Plossl eyepiece.
Planets
Jupiter was the only planet up, at an altitude of
about twenty degrees. Given the 36” refractor’s discovery of
Amalthea, I hoped some fine Jovian views might be in order. It wasn’t to
be. Because the moveable observing floor is out of service, none of the planets
will be high enough to view from the 36” for at least the next four years
(i.e. until at least 2023). Hopefully I will go back thereafter and report.
A professional astronomer who was there to help out
told me that he had viewed the planets some years before. They were an
impressive sight through the 36”, he said, with lots of detail and high
magnifications possible, but also lots of chromatic aberration.
We can get a more specific idea of how the
36” might perform on the planets from the following passage, taken from
the book ‘Is Mars Habitable?’ by the distinguished evolutionary
biologist (and the first astrobiologist) Alfred Russel Wallace when discussing
the nature of the Martian ‘seas’:
Professor Barnard, with the Lick thirty-six inch
telescope, discerned an astonishing wealth of detail on the surface of Mars, so
intricate, minute, and abundant, that it baffled all attempts to delineate it;
and these peculiarities were seen upon the supposed seas as well as on the
land-surfaces.
Viewing
with the Great Refractor involves quite a climb!
Summary
As
a physical presence and a historic instrument (the largest operational refractor
in the world and seriously huge), the 36” is a unique and wonderful thing
to visit and experience. No other operational refractor and dome come close to
this scale. It is a physically much more impressive instrument than even the
24” at Lowell (although it can’t match its restored
lacquer-and-brass elegance).
The
Lick operators were spot-on too: highly professional and knowledgeable, as well
as super-friendly and helpful (compare the rather rude interns at Lowell a few
years ago).
However,
if you want the best possible experience of viewing through a big Clark
refractor, the 24” at Lowell may ultimately be more satisfying. Why? Not
only is it newly-restored to a very high standard, but it is also much faster
to set onto new targets, so in a typical observing session you’ll see
more. Also, the Lowell refractor can easily be set onto the planets, which for
now the Lick Great Refractor cannot.
Then
again, if stunning views through the telescope are your only priority,
the Mount Wilson 60” Cassegrain reflector is incomparably superior to
either. It perhaps doesn’t have quite the wow factor and romance of a
great Victorian-era refractor, like the Lick 36”, though.
With
its slow-to-use manual mount and out of alignment Selsyns (and perhaps a lens
needing a good clean), the Lick 36” could benefit from some restoration
work, which I really hope it one day gets. I would love to view a bigger range
of objects with a wider range of magnifications, but that will need to wait for
those repairs and maybe a better focuser too.
Despite
some limitations, a night of viewing with the 36” Great Refractor at Lick
is a wonderful experience and a bit of a ‘must do’ for anyone with
an interest in visual astronomy and/or history of science. But for an expensive
private viewing session, I’d choose Mount Wilson, or even Lowell
Observatory, instead.
