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A Visit to the Lowell Discovery Telescope (LDT)

Lowell Observatory above Flagstaff is truly historic, home to Percival Lowell’s famous and much-photographed 24” Clark refractor. Lowell still hosts active research as well, but not at Flagstaff – it’s too light polluted. They’ve long had a site south of town at Anderson Mesa, but their main big science tool is now the Lowell Discovery Telescope, located forty miles south of Flagstaff at a remote dark-sky site called Happy Jack.

I’d previously driven down to Happy Jack just to catch a glimpse of the dome for the LDT, but then Lowell started offering public visits to the LDT and I grabbed the first one available in early January.


The Lowell Discovery Telescope is a 4.3m Ritchey Chrétien, with a focal length of 26.04m (F6.06, though they confusingly state F6.2), on a computer-controlled alt-azimuth mount. The primary mirror is a thin, light-weight honeycomb fabricated by Mirror Lab down in Tucson. Adaptive optics deform the primary via pressure actuators, rather than by a separate deformable element in the down-stream light path.

It’s a thoroughly modern installation, equipped with various cutting-edge instruments doing research in astrophysics and exoplanets on every clear night.

The LDT became fully operational in 2015 (first light was in 2011) and cost $53 million. The F6.2 Ritchey Chrétien prime focus is currently the only one used, but it has other foci too. The LDT hosts various instruments, including spectrographs and large imagers in the visual and near-IR. It also hosts Yale’s EXPRES exoplanet radial-velocity instrument. A night on the LDT currently costs ~$25,000.

Getting There

Accommodation can be expensive in Flagstaff and I stayed in Williams, a twenty minute drive west. Alternatives include Sedona, Oak Creek and Camp Verde to the south, all of which are at much lower altitude if things get seriously snowy (which they did for me).

You’ll have to make your own way to Lowell Observatory in Flagstaff where the tour starts/ends, but it is very easy to find – just a few minutes’ drive off I-40. You exit at junction 340B and take Highway 89A north towards the centre that becomes Route 66. After bearing right under a railway bridge, the observatory is then accessed from a left turn up West Santa Fe Avenue, where Route 66 angles sharp right into town.

The tour includes travel to and from Happy Jack, starting at the Flagstaff site visitor centre. I met my two guides – senior and knowledgeable members of Lowell’s outreach team - at the VC reception at midday and they drove me out to the LDT in the observatory’s own car (a 4wd to account for the winter conditions when I was there). The trip takes around an hour one way, but passes some very pretty forested scenery on the way along Highway 209. You can meet the guides at the LDT gate if you prefer and the (only) other visitor on my tour did so.

The LDT is west of the highway, up an unmade (and very snowy on my visit) track that winds into the forest and around the back of the little mesa the LDT dome stands on.

LDT dome from the highway and entrance track.

What to see

The LDT dome

The hexagonal dome is small and unassuming for such a large telescope, constructed from aluminium sheeting and with more of a pre-fab vibe than others I’ve seen (including the modern examples at Apache Point). Surrounding it is just red earth (and lots of snow when I was there). Those big shutters all round it open for cooling.

Behind the dome is another sheet-sided building, the Telescope Support Building, connected to the dome by a trackway.

Nearby is a diesel power unit to provide continuous and stable electricity in this remote area.

The Lowell Discovery Telescope

The main event is the 4.3m LDT itself. If you’ve just visited the Steampunk 19th C. observatory for Lowell’s  own 24” Clark refractor back at the Flagstaff site, you’re in for a shock. The LDT is a 21st Century big science instrument in an industrial space, accessed up several flights of stairs that could be in factory, or maybe a ship. Forget brass focusers, wooden chairs, ropes and tweedy old-guy astronomers here. More than anything, the whole place reminded me of the drill floor on an oil rig, but without the mud.

Another surprise is that, especially compared with the older observatory instruments I’ve seen, the LDT is really quite small! It’s much more compact and the dome interior a smaller space, than the 100” at Mount Wilson or the 82” at Mc Donald. That’s because the LDT is a truss-tube Ritchie Chrétien on an alt-azimuth mount, not a Cassegrain on an equatorial English yoke.

Another big difference is that the work-floor around the telescope is clear and uncluttered – no dusty old instruments or tarped-over secondary cages here. But they did warn us to stay inside the yellow line because behind it is a high-voltage rail.

We were able to crawl around under the prime focus to check out the instrument package, which contains five separate instruments swappable in minutes using fold mirrors. However, one of the most interesting instruments – Yale’s EXPRES (Extreme Precision Spectrometer) – is off-limits in a clean room, with light fed from the prime focus instrument cube through fibre optics.

Prime focus instruments and adaptive optics wiring.

Inside of the shutters.

Giant screen for dome flats.

Telescope Support Building

This green building was out first stop on the tour. It contains just one object – a huge white vacuum tank for re-aluminising the mirror. When the time comes, the mirror is transferred on a dolly via the trackway that connects this building with the dome.

Re-aluminising is done the same way it has been for almost a century at older observatories. A small, thin coil of aluminium wire (imagine wrapping some fuse wire around your finger) is vapourised in the chamber and settles onto the mirror surface in a layer just a few atoms thick that’s semi-transparent. Unlike your Dob’, there’s no overcoat of silicon on the mirror to protect it: when it tarnishes it’s back to the support building for a re-coat.

Attached to the roof of the support building at the back is an all-sky camera they’re using for comet and asteroid research by measuring meteor trajectories.

Atrium and Pier Base

The dome entrance door opens onto a high atrium with a garage door to the outside, where the mirror is lowered onto its dolly and moved out to the support building for cleaning and re-coating. The base of the mount is visible behind a mezzanine above and a door leads to the telescope’s pier.

The pier is a cast concrete column just a few feet in diameter covered in wiring. It looks too insubstantial to support the telescope and mount. The guide said it reaches down a long way into the rock to damp out vibrations.

Computing racks.

Control Room

The Control room seemed more basic than some I’d seen, with just a bunch of screens on one desk. The guide said they’d moved and covered some stuff to protect sensitive research; whether sensitive competitively or militarily she wouldn’t say.

The control room has a fun and geeky clock with equations instead of numbers.

The World’s Largest Visual Telescope?

Years before, an astronomer doing outreach work on the 24” Clark told me that they attached an eyepiece to the LDT (before it was fully commissioned) and looked at Mars. He said it showed amazing detail.

I wanted to believe this but honestly thought it was idle bragging (sorry!) I owe the guy an apology because I asked my guide and she said it was true. But in fact, this wasn’t a once-off as he’d implied. They infrequently run viewing nights to this day, but they’re for staff only and are very over-subscribed.

To use the LDT visually, there is a focuser (a small lime green single speed Moonlite Crayford with a 2” visual back, to be exact) permanently installed in the instrument package. The LDT’s Swiss-Army-Knife design, which allows multiple instruments to remain attached and soft-swapped via movable mirrors, is presumably the reason they can do this.

When I posted about this on social media no one seemed impressed, but it blew me away because professional instruments are very seldom usable visually and when they are it’s just for special occasions. I suspect that this means the LDT is the largest telescope in the world (cue the Jeremy Clarkson voice) permanently configured for visual use.

Given that the focuser has a 2” visual back, the lowest power they could achieve would likely be ~475x with a 55mm Plössl giving a ~0.1° true field (see below).

The guide reported having seen a bunch of objects on one of these viewing evenings, including the Dumbbell and Ring Nebulae, globular clusters and some doubles stars. She said it had been a ‘spectacular’ experience, memorably including seeing colour in M57.

LDT prime focus instrument package and focuser (!)

(Other) Things to do

The tour of the dome, LDT and support infrastructure was the main activity and took a couple of hours, but beforehand they did offer some other brief entertainment too…

Solar Viewing

When we first arrived, the guides set up a little Lunt LS50T solar telescope on a Vixen Porta mount (you didn’t need the specifics, I know, but hey I review gear!) for us to view the Sun. It was a hazy day and bitterly cold, so we didn’t spend long viewing, but on a fine summer day I’m sure you’d get a longer solar session. As it was, we had a good view of some sunspots in Hα.


As the tour started at midday and wasn’t inexpensive, I’d kinda hoped for lunch, but we did get a choice of crisps (chips!) and a bottle of water. The lunch offering might improve, as this was one of Lowell’s very first LDT tours.


If you want to see a modern big observatory scope and its instruments close up, there aren’t many options, and so the LDT tour was an amazing opportunity and experience for me.

That said, it’s an expensive tour and the time at the LDT quite short. It would have been great to see the telescope and/or dome powered up and maybe have a brief chat to an astronomer.

Lowell Observatory’s LDT tour is one of the only visits I know of to a truly modern research instrument. If that’s of interest it’s highly recommended.


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