Alvan Clark 24” Refractor (Lowell
Observatory) Review
Percival Lowell died on November 12th 1916 –
exactly a century before I wrote the original version of this review (I’ve
since updated it following another visit in 2020). Lowell is famous for having
imagined a dying race of Martians who built the giant canals that Lowell
believed he saw through his 24” Clark refractor at Mars Hill observatory near
Flagstaff in Arizona. His observations and his vivid (if mostly quite wrong)
descriptions of Mars and its supposed inhabitants arguably launched our obsession
with aliens and space travel. Lowell’s influence extended to writers like H.G.
Wells and Edgar Rice Burroughs and planetary scientists, such as Carl Sagan
(who was seen at the Clark refractor during an episode of his famous TV series
‘Cosmos’).
I have spent several nights at Lowell observatory, once a few
weeks after the 2016 Mars opposition, viewing through the Clark refractor at
public sessions. My intention was to see Mars through the same instrument as
Lowell himself. That intention was largely thwarted, but I spent quite a lot of
time with the recently (2015) restored giant refractor, hence this review.
This is one of a series of reviews of world-famous telescopes
and the experience of visiting and using them.
At
A Glance
|
Telescope |
24” Clark Refractor |
|
Type |
Air-spaced Fraunhofer doublet achromat |
|
Aperture |
24” (61cm) |
|
Focal Length |
384.6” (977.2 cm) |
|
Focal Ratio |
16.3 |
|
Length |
About 32 feet |
|
Weight |
About 2 tons (tube only) |
Design
and Build
Optics
Lens cell
(with cleaning ports) and iris mechanism.
The optics
for the Clark refractor were (unsurprisingly) ground by the famous optician Alvan Graham Clark in 1895-96 and may have been his last.
The design is a conventional Fraunhofer achromatic doublet, in which the
bi-convex crown element is in front, the concave flint behind. It seems that
the objective employs a substantial air gap between the elements, a design that
can give more freedom in correcting aberrations (as opposed to having the
elements cemented or separated by thin foil spacers).
The lens
elements, crown and flint, are quite thin – about 1.5 inches in the centre –
but the objective lens still weighs a hefty 150 lbs (68 Kg). The lens sits in a
cast iron cell with ports that allow access (only by slim hands, apparently) to
the back of the elements via that large air gap.
Like most big
Victorian-era refractors, the focal ratio of F16 is short for an achromat. It
had to be: according to one commonly used rule of thumb - that the F-ratio
should be 1.22 times the objective diameter in centimetres - it would need to
be about F70 (giving a focal length of about 40m) to achieve good correction
for chromatic aberration, an impossible length for a practical, moveable
telescope.
One
consequence of the short F-ratio is that the Clark refractor is fitted with an
iris diaphragm in front of the objective, which allows it to be stopped down,
to as little as 150mm aperture, to reduce chromatic aberration. The diaphragm
is like the multi-blade type used in more expensive camera lenses. From that
same rule of thumb 1.22D formula, you might expect to have to stop it down to
about 12” (300 cm) to kill most of the chromatic aberration, but I found that
stopping it down to about 20” improved things dramatically. Even so, some have
suggested that this diaphragm mechanism is to blame for Lowell’s canals: he
stopped it down to the point where the exit pupil was so small
he was actually viewing the capillaries in his own retina.
The lens was
carefully cleaned and re-assembled during the recent refurbishment of the Clark
and it is carefully cleaned by observatory workers every two years or so, by
swabbing it with alcohol, front and rear, in-situ.
Tube
The tube of
the Clark is in three sections. The middle section that attaches to the mount
is a massive iron casting. The tapered front and rear tubes are riveted steel
plate and give the telescope its ‘ship’s-hull’ Steampunk look, familiar from
that classic photo of Lowell seated on a ladder, wearing a jaunty cap and
observing Venus during the day.
From what I
can gather, the centre section has at least one baffle and the interior is
painted flat black to stop reflections from the tube walls. The tubes were
restored and powder coated as part of the restoration process.
It is
interesting to compare the 24” at Lowell with a slightly smaller Clark
refractor. As you can see in the image below, the next size down, the 20” at
the Chamberlin Observatory in Denver, is a much more manageable tube assembly
on a substantially smaller mount and pier.
20” Clark
refractor at Chamberlin Observatory in Denver is much smaller than the Lowell
24”.
Focuser
The Clark
refractor retains the original brass focuser, nowadays equipped with a 2”
diagonal (historical photos show Lowell and others using the Clark ‘straight-through’,
i.e. with the eyepiece in the focuser itself).
One
consequence of the decision to retain this beautiful piece of late-Victorian
era engineering is that the telescope can’t take the huge 4” eyepieces that
give nearby Mount Wilson’s 60” reflector such a relatively wide field of view.
Visual back
with brass focuser and mount control handwheels, the eyepieces for the 12”, 4”
and 3” finders, surrounding it.
Mounting
The Clark has
a huge German equatorial mount, in principle just like a super-sized EQ5! The
mount sits on a four-metre high pier constructed of cast iron segments. Together,
the mount and pier weigh a total of about eleven tons. One of the main reasons
for the recent renovation was a problem with the mount – the century-old RA
bearing wheel (you can see it in the photo below after renewal) had worn to the
point where it was no longer smooth.
The mount was
originally turned by a clock drive that was revealed in the restoration, but it
is now operated by stepper motors. The old clock drive is on display behind the
pier.
The mount has
large hand-painted setting circles. The declination circle is read at a
distance by (yet another) small brass telescope mounted at the eyepiece end.
Later, a synchro mechanism (sometimes called a Selsyn), employing WWII surplus
equipment, was installed to point the telescope in right ascension.
The mount’s
slow-motion controls to adjust pointing are operated by the wood-and-brass
handwheels that surround the focuser.
Accessories
Whilst I was
there, they only used a 55mm TV Plössl in the 24” Clark, but one of the guys kindly
explained that they had a whole range of eyepieces at their disposal, but only
used them when ‘playing around by ourselves’. Well that’s alright then.
The 24” Clark
has two larger finder/guide scopes – a 4” and a very fine 12”, both also by Alvan Clark, along with a smaller (and more recent?) 3”
mounted atop the Selsyn. The 4” finder retains its original brass focuser, but
the 12” has been fitted with a modern unit.
The 12” is a
very nice refractor in its own right and I had several good views of the Moon
and Jupiter through it, again with a 55mm Plössl, giving about 90x, that
confirmed its quality.
The Clark was
at various times fitted with professional instrumentation, including an early
spectrograph; but it is now solely used for outreach.
Dome
It has been
said that the 40ft dome for the 24” Clark resembles an upturned wood pail. It
was built that way to be as light as possible, something Lowell wanted to
enable quick cool-down. The whole structure is made of the local Ponderosa pine
and was designed and built in 1896 by the Sykes brothers – bicycle repairmen based
in Flagstaff, who claimed (rightly, it turned out) to be able to build
anything. It was originally erected in Tucubaya,
Mexico and only later moved back to Flagstaff, to a location on Mars Hill at an
elevation of 7246ft, overlooking the town of Flagstaff (just 15 years old when
it arrived) and the volcanic San Francisco peaks.
The wooden
latticework of the dome was originally covered with canvas, replaced later with
wooden slats and tin. For much of its early life, the dome turned on a number
of iron wheels. For a time, a flotation system was tried, with pontoons buoyed
up by salt water in a trough, but this was plagued with leaks and was scrapped.
In 1960 the old iron wheels were replaced by 24 Ford car wheels and tyres which
are still used today (though flats and blowouts are not uncommon).
The dome is
accessed up a flight of steps and through a wooden door under an expansive
porch. The inside of the dome is all polished wood and perfectly matches all
the gleaming brass and grey-painted cast iron of the telescope and mount to
create an air of Edwardian technology that would thrill a Steampunk. The floor
is likewise all polished wooden decking that descends in steps to the pit for
the mount’s foundations. The dome is a really evocative space and they often
play 1920s music to enhance the Lowell-era ambience.
When I was there they were holding a gala dinner at the observatory
ranch house next door, where Lowell once lived. As dusk was falling, the air
still warm and scented with pine and Mars glowing first in the darkening sky,
Lowell’s old car roared around to the house, bringing the first gala guests and
completing the time-travel fantasy that Lowell might still be at the eyepiece
of the 24” Clark, observing Mars and dreaming of canal-building civilisations.
In
Use – Astrophotography
The 24” Clark has been used extensively
for astrophotography in the past, including some of the best early planetary
images taken by Lowell and his workers a century ago. I had no opportunity to
try photography through the Clark.
In
Use – The Night Sky
General
Observing Notes
On the
evenings I was there, the staff opened the dome early and set the Clark on the Moon
or Jupiter in daylight. Early on, before the queues, was a great time to
observe and even later there was nothing to stop me going to the back of the
queue and waiting for another look (often at a different object by the time I
got back to the eyepiece). During a later, winter visit, there were no queues
and it was easier to linger at the eyepiece.
I had read
(what amounts to an urban myth) that very large refractors give dim and
yellow-cast views due to the thickness of their lenses. The Clark proved this
just isn’t true.
Adjusting the
setting of the telescope can be accomplished manually with the handwheels
around the visual back, which operate with a satisfying click-click-click.
Jupiter
The first thing I looked at through the
Clark was Jupiter. With the sun throwing long golden shadows through the trees
around and lighting up the sky-blue tiles over Percy’s Saturn-shaped mausoleum
next door, the shutters of the Clark dome were already open and 1920’s music
was playing through an old radio. They’d fitted a modern 2” diagonal and a 55mm
Tele Vue Plössl eyepiece giving 178x
magnification. I stepped up to the gleaming brass focuser, amidst all those
wood-and-brass handwheels and finder-scope eyepieces, just as Lowell must have
done countless times during long observing sessions on Mars.
Jupiter looked really good, as planets
often do at dusk with a bright sky to cut the contrast down a bit. There was a
lot of detail in the north and south equatorial belts, light storms and dark
ones; other cloud belts were easy to pick out too. I hung about at the eyepiece
too long though and a nice member of staff called Kevin had a go at me (even
though there was no one else waiting); apparently,
it’s a pay-as-you-go Universe at Lowell.
Later, against a dark sky, Jupiter was
very bright and showed a lot of violet blur around the limb (the way it might
look in say a 5” F5 achromat). The telescope operator closed the iris in front
of the objective, stopping it down to F18 (about 20” aperture) and the violet
blur dulled a lot and the view sharpened noticeably. Now the Great Red Spot
stood out to good effect, with all the vortices upwind clearly visible and some
nearby white storms too.
Mars
Given that it was a couple of weeks
past a good opposition and that Mars was hanging brilliant overhead at a good size
and altitude for viewing (18 arcsecs and about 32 degrees), you might expect
that they would have been keen to get the 24” Clark set on the object for which
Lowell originally commissioned it. They never did; and I stayed until late for
several nights, hoping. When I asked one of the outreach guys why not, he
replied, ‘Mars is always disappointing’ – a bizarre (and indeed disappointing) statement
made all the stranger by the fact that I had stupendous views of Mars through
the nearby 60” at Mount Wilson a few days later.
Finally, very late on my last night, I
did get a brief view of Mars with their 16” Cassegrain before the observatory
closed – but it was out-of-focus as usual and showed no surface detail at all,
just a bright orange ball.
I got chatting to a gentleman from
Tucson, a fellow refractor fan called Craig, at the Grand Canyon Star Party, a
few days after my first visit to Mars Hill. Craig told me he had seen Mars
through the 24” Clark using a pair of 13mm T6 Naglers
in a binoviewer (giving 750x on my slide rule) at a club event for the
super-close 2003 opposition. He said the detail resolved then had been
‘amazing’.
The
Moon
The Clark may
be most famous for Lowell’s observations of Mars, but it also made a major
contribution to Lunar research in the 1960s, when it was used to create new
Moon maps that were eventually used to select Apollo landing sites. At that
time – between 1961 and 1969 – both visual and photographic techniques were
used to create the maps, as part of a program for the Aeronautical Chart and
Information Centre (itself a branch of the U.S. Air Force).
The Clark
wasn’t only used for mapping the Moon. Hidden in the Lowell Observatory Guest
Book is an entry for January 16th 1963, a rather modest signature of
one ‘Neil Armstrong – NASA MSC, Houston, Texas’. A group of astronauts,
including Armstrong, Lovell and Borman, visited the Clark for a night of
training – viewing the Lunar surface at high power until the small hours and
comparing what they saw with nearby Meteor Crater, which they had visited
earlier that day.
My first few
views of the Moon through the Clark – still at their favourite 178x
magnification – were spoiled by significant defocus. When one of the
professional astronomers finally took over, he got it perfectly in focus and
the Moon revealed stunning detail. All of the Plato craterlets appeared as
proper small craters with simple, bowl-shaped floors, not just the usual
specks. Nearby Mare Imbrium was a mass of craterlets
and ridges. The peaks of the Tenerife range looked like proper mountains and
Hadley Rille looked wide and deep. However, there was significant chromatic
aberration on the bright limb, even stopped down to F18.
On a later
look, the crater Tycho displayed unprecedented detail in its slumped walls and
central peak that had me wanting to linger and explore.
Deep
Sky
M5
This globular
cluster was very bright and easily resolved to its core through the 24” Clark:
a mass of stars, with arms of stars arcing into space around, more like a tight
open cluster (think M38) than the usual fuzzy-blob you get with small
instruments: a really beautiful sight. Some of the stars clearly showed their
redder colour too, something I had never seen in smaller scopes. It is worth
remembering that the Clark is an unobstructed aperture as large as all but the
largest modern Dobsonians.
M42
On a later
winter visit to Lowell, the 24” stayed set on the Orion Nebula all evening and
I was able to take plenty of time to observe it.
The Orion
Nebula only showed its bright core region around the Trapezium, but this showed
a huge amount of structure in the nebulosity that you only usually see in
images. The Trapezium was easily and fully resolved. The guide said that the
24” was open to its full aperture on this occasion, but even so none of the
stars showed excessive false colour.
In 2020,
Lowell had switched over to red lights all across the observatory to preserve
visitors’ night vision.
Summary
If you’re a ‘refractor guy’ like me, or
a fan of Victorian and Edwardian telescopes, your heart will beat a little
faster when you first see the Lowell’s 24” Clark Refractor. In its newly
restored state it feels like a time machine – back to
the era of Lowell and HG Wells’ Martians, of Canals and the Barsoom
of Edgar Rice Burroughs.
The Clark refractor isn’t just a truly
beautiful and evocative telescope to look at; it is also an extraordinary
experience to look through it,
something that is completely free if you pay the modest Lowell entry fee and
wait until twilight. You may get the chance to view a planet – Jupiter or
Saturn - or a fascinating deep sky object like a globular cluster or the Great
Nebula in Orion. Sadly, though, you may not get to see Mars, even if it’s
hanging like a brilliant orange star high above the Clark’s Ponderosa-wood
dome.
On a recent visit my former troubles
with rude and incompetent outreach assistants had been fixed – everyone was
polite and helpful and most of the time the telescope was properly focused too,
unlike the previous times.
Viewing through a beautifully restored
classic refractor the size and quality of the 24” Clark is a bucket-list experience
available at only a few other observatories. The only real disappointment was
that the Clark was never used at the high powers I am reliably informed it is
capable of and never set on Mars, which seemed a shame. Something else to be
aware of is that they are typically now leaving it set on one object all
evening to reduce wear and tear (which makes total sense, given it’s open
almost every clear night).
The word ‘iconic’ gets over-used, but
if there is such a thing as an iconic telescope, this is it. I highly recommend
a visit to Lowell observatory to see it (and, if it’s a clear night, through it).
But if you’re a Mars fan like me, be prepared for a disappointment, even if you
turn up near an opposition.
Viewing the Moon through the 24” Clark