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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


24” Clark Refractor


Air-spaced Fraunhofer doublet achromat


24” (61cm)

Focal Length

384.6” (977.2 cm)

Focal Ratio



About 32 feet


About 2 tons (tube only)


Design and Build


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.


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”.


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.


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.


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.


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.


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.


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


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.


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.


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