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Mount Lemmon Sky Center’s ‘Sky Nights’ Observing Program

The cultured and likeable city of Tucson in southern Arizona is a real epicentre for astronomy, professional and amateur. Home to ‘Optics Valley’, including AU’s ‘Mirror Lab’ which makes many of the world’s largest professional mirrors, Tucson is surrounded by mountain-top observatories like Kitt Peak and Mount Graham with world-class research programs.

One of the lesser known, but closest to Tucson, is the University of Arizona’s Observatory on Mount Lemmon, one of the sites that comprise their Steward Observatory.

The Mt Lemmon observatory used to be a military early warning radar facility and still hosts a couple of (empty) geodesic radar domes, but was taken over by the university in the 1980s to host a range of professional telescopes, including the 60” and 40” telescopes of the Catalina Sky Survey that looks for Near Earth Asteroids.

The university also has an active outreach program at the observatory through its Sky Center, which offers tailored individual programs on its RC Optical 32”, but also regular public viewing ‘Sky Nights’ that include a session on the telescope with some unusual additional activities. This is my experience of one of these sessions.

I originally included more details on the scope and observing session, but this article ended up too long, so I’ve split the telescope review out here.

Catalina Highway from Tucson to Mount Lemmon.

View from the Iron Door Restaurant parking lot used by SkyCenter visitors, looking north towards Oracle and Biosphere 2.

Sky Center sessions start at these locked gates: you can’t drive all the way.

Getting There

Tucson has an international airport and Mount Lemmon is easy to access from it without going through central Tucson. Similarly, hotel prices in Tucson can be high, but places east on I-10 are cheaper and easy to get to from Mt Lemmon. Whilst you’re there, the Tucson area has lots of other space-themed visits, including Mirror Lab, Pima Air and Space Museum and Kitt Peak.

The Sky Center is about 45 miles and 1.5 hours’ drive from central Tucson or I-10, but that’s no hardship because the Catalina Highway up to Mt Lemmon is one of the most scenic drives in the southwest.

The drive starts among giant Saguaro cacti and ends up among snowy (in winter) pine forests, via numerous curves, huge views and some spectacular rock ‘hoodoos’ by the wayside.

Towards the top of the Catalina Highway, you also pass – almost unseen but quite close to the road – a couple of domes belonging to another Steward Observatory site, mount Bigelow, including the one containing the 61” NASA telescope (through which one of my guides reported amazing solar system views).

The drive up through the Catalina Mountains ends, and the viewing night begins, at a locked gate just past the Mt Lemmon Ski Valley and its big parking lot. But the ski area parking closes at dusk and the Sky Center advise you to park across the road at a smaller (free) parking lot that serves the Iron Door Restaurant.

My ‘Sky Night’ session in late January started at 15:45 when two minibuses with chains arrived at the locked gate and we were ushered onboard for the drive up the steep and snow-covered road to the observatory.

The Sky Center warn you to dress warmly and they weren’t kidding. It had been in the 60s down in Tucson, but by the end of the night at the 9,157 ft altitude of the observatory it was in the low 20s and felt much colder on deep snow and with wind chill. You’ll need more than jeans and a thin puffer!

Snow chain equipped minibuses take you up to the dome for the Sky Center 32” Schulman Telescope.


Orientation and First Observing Session

The minibuses dropped us at the observatory and the guides - Will (a graduate astronomer and keen amateur too) and Gustavo (an electrical engineer at the Large Binocular Telescope) - gave an orientation talk about the history of Mount Lemmon.

They also pointed out several interesting sights around, including Biosphere 2 clearly visible in the valley to the north and the Large Binocular Telescope dome on snow-capped Mount Graham in the distance opposite. The latter is very distinctive and I was really interested to see it through my bino’s (it’s not an easy facility to get to see otherwise).

After that, we were ushered into the Sky Center dome whose slit was already open and where they introduced us to the telescopes – a 32” truss-tube Ritchey Chrétien built by RC Optical, on a remotely-controllable fork mount, and a piggy-backed SkyWatcher Esprit 150ED triplet apochromat refractor for wide-field views.

They (bravely, I thought) passed around the expensive eyepieces we would using later: a 31mm Tele Vue Nagler giving about 206x magnification in the 32” and a 21mm Ethos giving 50x in the refractor.

Then – unusually given it was still broad daylight – we viewed the bright star Deneb (Alpha Cygni) and also a pale but surprisingly sharp and detailed Jupiter!

Daylight viewing with the Sky Center’s Schuman Telescope – a 32” F7 by RC Optical.

Introductory Lecture and Dinner

A snack meal at the main building, just down from the observatories, was included in the price. It followed the initial viewing session and was preceded by an introductory astronomy talk and slide show.

Thereafter, we were each offered a pair of 10x50 binoculars to use for the rest of the evening (though I opted to stick with my trusty Canon 12x36s) and a complimentary red keylight for getting around whilst maintaining that all-important night vision.

Sunset and the Green Flash

The next part of the program was a complete surprise and (for me) a unique experience. We were led out to a snowy slope in front of one of the domes facing west. The deep orange of the setting Sun was sparkling prettily off the snow and we looked to the left of it to find Kitt Peak National Observatory silhouetted in the far distance, its solar tower and the dome of the 4m Mayall telescope clearly visible despite being 55 miles away.

We then waited and shivered as the sun sunk onto and then past the horizon. At the very last moment, just as the final remaining sliver of the Sun’s limb was about to set, the guide instructed us to look at it through the binoculars.

This really surprised me. It’s not something I would have dared to do otherwise, but it was worth it because I clearly saw the famous ‘green flash’ for the first time and even a hint of even-rarer blue too. This is caused by refraction and is something I’d looked for many times but never seen. The exceptionally clear skies, unobstructed horizon - and yes the binoculars - all helped, but apparently it’s far from guaranteed even so.

Please don’t repeat this experiment unsupervised! The guide knew precisely when it was safe to look: a few seconds too soon and it could blind you!

Waiting for the green flash.

Dark sky binocular viewing outside the Schulman dome.

Catalina Sky Survey up and running across the way from the Schulman Dome.

Two guests view through the Schulman 32” RC and the 150ED simultaneously!

Dark-sky Telescope and Binocular Viewing

As darkness fell under very transparent skies, we made our way back up to the dome and started with some guided binocular viewing of easy deep sky objects like the Andromeda galaxy and Orion’s Great Nebula, whilst the dark skies sparkled overhead and various other observatories opened for the night and began whirring around.

Then it was back into the dome to view a number of objects in the 32” Schulman telescope and its piggy-backed 150mm Esprit apochromatic refractor. There was plenty of time at the eyepiece on both scopes and plenty of chairs to sit on in between. Adding to the comfort factor, the dome does have a small warm room.

The guides were both informative and relaxed. We were allowed to tweak the Feather Touch focuser for ourselves, and even try a spot of impromptu imaging through the eyepiece with our phones, something I’d never done at a public session before.

We started with views of bright stars and Jupiter again, the latter showing much more detail this time. After that it was a good selection of deep sky objects. Highlights included seeing Jupiter’s cloud belts in quite strong colours, internal structure in the Crab Nebula and lots of fine structure in M42 through the 32”.

The Esprit 150ED with its ultra-wide Ethos eyepiece did provide a beautiful low-power overview of the objects we were viewing in detail with the 32”. Orion’s sword region encompassing the entire Great Nebula and virtually the whole of M31 showing hints of a spiral structure, were especially memorable through the refractor under Mt Lemmon’s dark sky.

For a full description of the 32” viewing experience and the object list, see my separate review of just the telescope and viewing session here.

My Rocket Fuel Dump Imaging Accident

So there I was, with my Canon EOS 6D MkII and 20mm F1.8 Samyang lens, taking long exposure photos of the sky, the watchers and the domes for this article. Meanwhile, everyone else was enjoying binocular views.

I was pleased with some of the photos and tweeted them at the Sky Center later that evening, who promptly asked what the nebula was to the right of Rigel, that looked much the size and brightness of nearby M42.

I confess that I hadn’t even noticed it, but on inspection I was puzzled too, speculating that it might be a fuel dump or rocket stage ignition; and so it proved. I asked my readers for help and Victor Boesen discovered that this was in fact a fuel dump from the USSF8 Atlas V launch! My tweet about it went semi-viral and ended up in a re-tweet from Tori Bruno, the CEO of ULA who make the Atlas V (that’s my 15 seconds of fame over).

Inspection of all the images I took that night show it in five taken over two minutes between 02:25 and 02:27 GMT. You can spot it in one of the other photos above. Tellingly, those images show it fixed relative to the stars as it moves with the turning Earth.

USSF8 Atlas V fuel dump.

Leaving the Schulman dome for the drive back down to our cars at the Iron Door parking area in the minibuses.

Exit Via the Gift Shop!

At the end of the evening, we went back into the main building to collect our things, warm up and purchase a few souvenirs (I added to my collection of observatory beanies). Then they drove us back down the icy road to our cars for 9 pm.

The drive back down the curves of the Catalina Highway allowed some evocative views of the Tucson lights (see below).


The Mt Lemmon Sky Center experience was a little different from viewing nights I’ve done on otherwise similar telescopes at McDonald and Kitt Peak.

The list of objects viewed with the 32” RC telescope at Mt Lemmon was a bit shorter, but that’s because those other viewing nights were limited to just the telescope viewing, where the Sky Center experience adds in binocular viewing – of the deep sky, of the observatories around and that bucket-list sunset green flash.

I also really liked being allowed to get best focus for myself and the opportunity to try some phone imaging. The guides were particularly knowledgeable, enthusiastic and friendly too.

Finally, Mount Lemmon is easier to access (much more so than remote McDonald). Overall, one of my best astronomy outreach experiences to date!

A ‘Sky Night’ at the Mount Lemmon Sky Center is a unique, varied and hugely enjoyable public viewing experience that’s easy to attend and ranks amongst my all-time favourites.

Lights of Tucson on the way back down from Mount Lemmon.