A Visit to Chamberlin Observatory (and their 20” Alvan Clark Refractor)

Just east of the Denver University’s main campus, sandwiched between E. Evans Avenue and E. Iliff Avenue are two small parks of a block each, surrounded by an upmarket residential area of big, respectable old mansion houses.

The southern of the two parks is graced by an area of pine trees surrounding a century old observatory building of red stone overlooking grassy lawns. That observatory is the University of Denver’s Chamberlin Observatory, now mostly run for outreach by the Denver Astronomical Society.

Chamberlin Observatory would be worth a visit anyway, but its big draw are the public observing nights every Tuesday and Thursday, which take place on the observatory’s wonderful 20” Alvan Clark refractor. I booked on one, on a Thursday night in late February.

Normally, I write up my viewing evenings in telescope review format; when I’ve actually been able to do some viewing that is. Sadly (or maybe not), when I arrived for my viewing session, I slithered my rental car to a snow-crunching stop on East Warren Avenue and climbed onto a side walk drifted with new snow and a blizzard of flakes falling from the dark sky.

There may not have been any stars out or prospect of looking through a historic telescope, but I had to admit it was a beautiful scene, the old observatory glowing frostily among the pines and drifting snow.

The viewing was snowed out, but Denver A.S. still show visitors around the telescope and observatory whatever the weather; so I had an interesting visit anyway and I managed to get some photos too.


The construction of Chamberlin Observatory was begun in 1888 and finished by 1890. Its patron was one Humphrey Barker Chamberlin, a real estate speculator who had moved to Denver from NYC for health reasons in 1880.

Chamberlin made various donations following his financial success in Denver, including one to the university for a telescope and observatory. Perhaps foolishly, he instructed his brothers to ‘pay whatever you wish’, which ended up as a bill of $56,000, perhaps ten times what he had expected!

Still, the observatory ended up with the fifth largest telescope in the U.S. at the time and the highest.

Getting There

Chamberlin Observatory is very easy to access – just a mile west from junction 204 of I-25.

From I-25, drive south on Highway 2 for a couple of blocks and then take a right onto East Warren Avenue. Then it’s a short drive until the observatory is on the left, at the top of the park and set amongst the trees by some tennis courts. You can park on either side of Warren Avenue.

Warren Avenue parking.

What to see

The park and grounds are open and free during the day, so you can just pitch up and snap some photos, as I did, if you’re passing through. But if you want to see the telescope and the observatory building, including the 20” Clark and the exquisite transit telescope, you need to book on a viewing evening.

20” Alvan Clark Refractor

Chamberlin’s main instrument is a 20” refractor by Alvan Clark and Sons on a Saegmüller mount. Both the refractor and mount are in very original condition.

The Denver Clark is of the unusual Boyden design, which allows it to be converted into an early astrograph. This was achieved by putting the front element is in its own cell. That cell is designed to be reversed, changing the spacing and thus improving the colour correction towards the blue end and shortening the focal length by about a metre for faster exposures.

According to a photograph at the observatory, both telescope and mount look very much as they did when installed in 1894, something that’s all too rare among remaining Clark refractors (even the beautiful restoration at Lowell is of slightly dubious originality – it’s now a much shinier and brassier object than it would have been).

Despite the original condition, the Alvan Clark objective was carefully cleaned in 2010, for the first time since its installation, using time honoured methods like the application of collodion under a cheesecloth. At the time of cleaning, the elements were discovered to be remarkably thin and relatively light, apparently a characteristic of Clark objectives. The glass was also found to be in perfect condition underneath a century of grime.

After the cleaning, the telescope was found to perform much better and views are now supposedly excellent – something I will have to go back and try for myself one day, God and virus pandemics permitting.

After visiting the 24” Clark at Lowell and the 36” at Lick recently, my first thought on seeing the 20” at Denver was how small both the telescope and especially the mount seem, the dome too. A wealthy amateur could conceivably build an observatory to house a refractor of this size and the telescope operator easily and safely pushed the telescope around with one hand.

The 20” Clark is effectively a twin of the telescope at Van Vleck Observatory operated by the Wesleyan University in Middletown Connecticut. However, that telescope has recently been restored with goto drives, so it’s perhaps more functional but certainly a little less original than Denver’s. Van Vleck do host outreach viewing evenings too and I hope to book on one eventually and report back.

We were given plenty of time to look around the 20”. There was an eyepiece ready and waiting in the diagonal, but the snow was still falling thick outside so sadly no chance.

Transit Telescope

Another interesting and beautiful little instrument at the observatory is the transit telescope in the library (itself a space any astronomer would covet for his own study).

Transit telescopes are designed to move in altitude only, being very carefully aligned with the Meridian (i.e. N-S) to measure exact transit times, often for practical purposes like navigation and timekeeping in the pre-modern era.

Things to do

Viewing Nights

As I said at the start, the main events are the public viewing evenings every Tuesday and Thursday and run by the Denver A.S.

These viewing evenings can be booked online and cost just a few dollars, but note that there are no refunds for bad weather because they show you around and give a talk anyway.


I’d come a long way and should have felt cheated by the weather, but I didn’t. The snow and the silence and that special light that comes with thick snow made for an unforgettable experience in the twilit dome with the historic 20” Clark.

With snow driving onto the windshield, the car slipping and sliding over thickly snowy and almost deserted roads, I just made it back to my nearby motel before Denver shut down for the snowstorm.

I’d had a great evening seeing a historic and interesting old refractor, so I was far from disappointed. I just hope I’ll be able to go back someday for a look through that recently cleaned objective.

The Chamberlin observatory is well worth a visit, even if the actual viewing gets stopped by Denver’s fickle Rockies weather.