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A Visit to McDonald Observatory

Most of the US observatories I visit are historic sites that have seen their heyday in terms of professional discoveries or, like Lowell, have moved their real science offsite somewhere darker and more remote. McDonald in remote west Texas is different. It is still a major research hub, with one of the world’s very largest optical telescopes and new observatories still springing up on site. Somewhere like Lick in California is all but deserted much of the time, but by contrast McDonald is a thriving small community with all its own amenities (and even its own water, sewage and power provision).

Also unlike most observatories in the continental US, McDonald does still have really dark skies. Apparently, light pollution has increased a bit in recent years, not from towns but rather from drilling rigs out in the Permian Basin oilfields (another good reason to ditch fossil fuels then). To me the skies still looked stunningly dark.

The main reason to go to McDonald as a tourist though is that they have a really comprehensive and varied outreach program. In this article I’ll tell you what the major outreach events are and what they’re like. The only caveat being that you will need to plan ahead and book onto programs well in advance: even as remote as it is, McDonald’s viewing nights in particular get quickly sold out.


McDonald observatory started as a collaboration between the University of Chicago, who already had a prestigious astronomy department but wanted to expand beyond their light-polluted observatory at Yerkes, and the University of Texas who didn’t have an astronomy program. A wealthy Texas businessman and amateur astronomer named William J. McDonald thought Texas ought to have a premier observatory and left the thick end of a million dollars in his will to achieve it.

The observatory was founded on 23rd November 1932 and within a year work had begin on the 82” reflector that would be the second largest in the world when it was completed in 1938 and would (thirty years later) be named after the observatory first Russian-born director, Otto Struve.

A number of major discoveries were made with the 82” within a decade of its inauguration (see below for details), most by the famous astronomer and early planetary scientist Gerard Kuiper.

Much later, in the 1960s, after the University of Chicago had pulled out, director Harlan J. Smith instigated a renaissance at the aging facility by partnering with NASA to be build a huge new instrument, the 107” which now bears his name.

Road from Fort Davis up to McDonald Observatory.

Getting There

Texas is a big state. You knew that. But driving to McDonald is a good way to really get it: wherever you come from the drive will be a long one. Drive? Probably, because remote as McDonald is, you’ll struggle with public transport.

McDonald is easily accessible from Highway 118 in west Texas, either from I-10 in the north or from the small town of Fort Davis in the south. But unlike many of the older observatories I’ve visited, it is a long way from anywhere else, like much in west Texas. So you’ll be needing somewhere to stay – this won’t be a day trip.

Fort Davis is 15 miles and 25 minutes’ drive from the observatory, but it really is a tiny sleepy place, with no chain motels and not even a McDonalds (!) If you want a standard small US town, then Marfa is 37 miles.

In the other direction, there is Van Horn – it’s much further off at 74 miles distant, but half of that is on I-10 and it’s certainly much easier to get to if you’re flying in (likely to El Paso, the nearest city).

My pick, though, would be Alpine, just a couple of miles further away than Marfa. Admittedly, Alpine isn’t very alpine, but it seems a much more quirky and interesting place. It has the usual facilities – motels, shops and food - but a walkable downtown with historic buildings and a lively arts scene too.

If you want to combine a day (or two – might as well do it properly if you’ve travelled a long way) at McDonald with other attractions, the obvious ones are Guadalupe Mountains National Park to the north and Big Bend to the south (though much of what’s worth seeing at Big Bend is a long way off). More offbeat things of interest include the Marfa Lights viewing station between Marfa and Alpine and Blue Origin’s secretive rocket testing facility north of Van Horn.

I you need a walk after your long drive, there is a state park on the Fort Davis side and a wildlife hike between the observatory and I-10 on the other.

Be careful driving Highway 118. It’s scenic and fast (esp. from the north), but packed with wildlife. I saw deer, raptors, hogs and a Great Grey Owl; all actually on the tarmac.

What to see

The Frank Bash Visitor Centre

The VC is new and large, with restrooms, a café and terrace, the usual gift shop selling T-shirts and small scopes and a big movie theatre where they show interpretive films but also conduct remote solar viewing sessions. The VC is the most accessible part of the observatory from Highway 118 (from either side) and has plenty of parking.

Facilities close at about 5:30 pm, but you’ll find yourself waiting around outside for the bus up to the domes if you’ve booked on a Special Viewing Night (see below). In the evening you might see some wildlife, including a herd of small wild hogs, whilst you wait.

Hobby Eberly Telescope

Looking up from the VC, the odd-looking silver dome on the peak to the left is the Hobby Eberly Telescope. You can visit on a tour from the VC or drive yourself up during the day and park outside. The dome has a large foyer area with its own interpretive displays and video feeds explaining the telescope and its work. The displays are really worth perusing, but you’ll immediately be drawn to the telescope itself – visible through a big window at the back. To understand what you’re seeing, though, you’ll probably have to head back to the displays after all!

The HET is McDonald’s premier research tool. As I write it’s still tied for second place in the list of largest optical telescopes worldwide at 10m (shared with the Keck twins on Hawaii). It is a very different beast from Keck though and interesting for it.

The thing about the HET is that it was designed to be cheap, radically so at around 20 million (though it’s current upgrade will cost twice that). In order to achieve that super-low (for a big research telescope) figure, the HET has a unique design and looks … well, not much like a telescope at all.

Unique? How? For a start, even the dome looks unusual, because it is really a radar dome with a slit chopped out. The telescope itself cannot move in altitude, it’s fixed at an elevation of 55 degrees, only in azimuth. This radical design makes it much smaller and spindlier and less telescope-like than other big instruments. This sounds like a huge limitation, but the HET can cover about 80% of the sky because as the Earth spins and moves around its orbit, various target objects drift into view at different times.

The HET has other distinctive design features too. Its mirror is made of thin segments aligned by computer-controlled actuators, like the Kecks’. But the HET has many more segments: 91 in place of the Kecks’ 36 each.

Even the instruments are unusual. The HET dispenses with a secondary mirror and the usual Cassegrain and Coudé foci by having that main mirror focus its light onto a giant array of fibre optic cables in a special assembly at the prime focus. Those cables then feed not one, but numerous spectrographs that give the telescope a low-resolution but ultra-wide field of view and some special capabilities.

The HET has an odd way of tracking objects as the Earth turns, too. Instead of having the mount moving like usual, those fibre optics feeds and a corrector lens are contained withing an 11 ton moving ‘tracker’ that moves to counter the Earth’s spin.

As I hinted before, the HET is undergoing a major upgrade for a new study into Dark Energy, for which its ability to look at the spectra of numerous objects simultaneously is a big advantage.

HET Dome and foyer with window onto the telescope.

Hobby-Eberly Telescope: bottom to top - mirror support (turquoise), primary mirror, detector arrays, tracker secondary assembly.

The 82” Otto Struve Telescope

The Struve telescope was built between 1933 and 1938 and made some famous observations and discoveries. In 1944 Gerard P. Kuiper discovered that Saturn’s moon Titan has methane in its atmosphere. Continuing his solar system observations, Kuiper went on to discover moons Miranda (of Uranus) and Nereid (of Neptune), also with the 82”.

The 82” has a distinctive appearance, with a semi-closed tube with wire-braced trusses and a huge yoke mount, all in dark grey. The McDonald website calls it a work of art and in a way it really is.

The 82” dome is unusual too. Unlike most observatories it is a proper building with lots of windows in the lower stories. That’s because, as the McDonald themselves puts it, the dome ‘housed all of McDonald’ – those windows open onto rooms that now serve as offices and a library, but once housed living quarters for resident astronomers Is it me or is that really romantic – living below a giant telescope; when can I move in?

Unlike the 107”, the 82” is a Classical Cassegrain (not a Ritchey-Chrétien), which means it has a long focal length and is ideal for visual use (for which it was originally intended – astronomers still did much work visually in the 1930s). Gerard Kuiper (one of the first ‘modern’ planetary scientists) observed Mars with the 82” in 1956 and recorded seeing no sign of canals, perhaps the final nail in the coffin for Lowell’s Martian fantasies.

If you do the daytime tour, you’ll park just below the dome for the 82”, so you will be able to go and have a look at it on your way to the 107”. Unfortunately, though, the 82” telescope isn’t open to the public except on Special Viewing Nights (see below).

The 82” dome: astronomers once lived, as well as worked, behind those windows. I always dreamed of living in an observatory among the pines!

The Otto Struve 82” Classical Cassegrain, equipped with a focuser for a Special Viewing Night.

The 107” Harlan J. Smith Telescope

The 107” was built between 1964 and 1968, following a request by NASA to use the 82”. NASA wanted to survey the Moon and planets before sending space probes to them. But the Director of McDonald at that time, one Harlan J. Smith, told NASA this wouldn’t work because the 82” was already heavily booked up by astronomers. Smith proposed that NASA fund half the budget to build a new telescope instead. It was a ballsy move, but it paid off and Smith got his big new research instrument, still doing science long after NASA lost interest, probably just as he planned.

The 107” is a Ritchey-Chrétien with an F8.8 focal ratio, but it can also employ Coudé and Cassegrain foci by swapping out the secondary mirror cage. The mounting is a giant equatorial cross-axis yoke mount, much like the 82”, but with a really huge counterweight. The tube, as you can see, is unusual (for such a large instrument) in being enclosed.

Another unusual feature of the 107” is the system of weighs and pendula around the visual back. These are an ingenious and purely mechanical system for cancelling the distortion suffered by the mirror at different orientations (where a modern telescope might have computer-controlled pistons).

A visit to the dome on the tour includes one lucky member of the group getting to press a button and slew the 190 ton telescope and move the 220 ton dome, also of raising the observing floor. Even if that person isn’t you, the 107” is an impressive sight and well worth the trouble.

You might get to ride up to the observing floor in the lift, which is a proper big one like in an office block. Oddly, the lift misses a floor, because that floor is where the spectrograph and other sensitive instrumentation is housed and is off limits.

The Smith telescope continues to do research work and I was lucky enough to see it fitted out with McDonald’s VIRUS-W spectrograph, that attaches to the side and gimbles so it always stays vertical. The spectrograph is able to take up to 267 spectra simultaneously via fibre optical cables which makes it ideal for studying the stellar velocity variation within a galaxy (which is exactly what the astronomer working that night would have been doing if the clouds hadn’t closed in!)

A reminder that the 107” also has a foot in the past is the presence of an optical guide scope and (really) a Telrad red-dot finder! Our guide said the Telrad is a joke, but a someone has gone to a lot of effort to attach it properly if so.

Another interesting piece of trivia about the 107” (and the 82”) is the unusual way they clean the mirror. Every few weeks, they lay the scope flat and someone walks down it. They then spray liquid carbon dioxide from a bottle at the mirror. The CO2 falls on the mirror is snow, sweeping away dirt and dust without leaving traces (the snow sublimated soon after).

Outreach guide explains the 107” Harlan J. Smith Telescope on the day tour.

Other telescopes

Right by the visitor centre is a large white radio telescope dish that was brand new when I went. This is a 12m antenna owned by NASA and is part of their Space Geodesy Project (SGP), specifically part of the Very Long Baseline Interferometer (VLBI).

SGP has also recently completed a satellite laser rangefinder in a small dome on Mt Fowlkes near the HET. What’s SGP? It’s all about measuring the Earth’s precise shape, gravity and rotation and establishing a Terrestrial Reference Frame (TRF). This is essential to all sorts of science and technology, including oceanography, climate change, geohazards research and tracking interplanetary spacecraft.

McDonald hosts lots of other smaller instruments, including a 0.8m made from the glass trepanned from the 107” mirror and a 0.9m (36”) that is regularly available for viewing nights (again, see below).

NASA SGP VLBI Dish at McDonald, with HET dome above and left.

Things to do

First off, take binoculars! For the dark skies, but for perusing the many domes of the observatory and for wildlife viewing too.

Just turning up to the observatory and enjoying its scenic location, having a coffee on the terrace, nosing about the gift shop and checking out all the domes and dishes, is a pleasurable way to spend a few hours. But McDonald has one of the best outreach programs of any major observatory and as I noted at the start, you’ll need to book well in advance to make the most of it.

Enjoying McDonald’s dark skies with binoculars, outside the 36” observatory.

Daytime (obvs) Solar Viewing

The visitor centre has several small observatories clustered around it and one of these houses a pair of solar telescopes, one with a Herschel wedge for white light viewing of sunspots and granulation, the other with a special filter that reveals the chromosphere for filaments and prominences. But don’t expect to look through an eyepiece, because all the solar viewing is done remotely via video cameras.

You’ll watch the show in the movie theatre, with the sun projected live onto a big screen via a computer. The live feed from the telescopes is accompanied by an interpretive talk that covers a lot of interesting stuff about the Sun and its cycles, as well as explaining the views you’re getting through the telescopes.

It was an interesting show and projecting the solar images enables a much larger audience to view in comfort. Personally, I missed looking through an eyepiece though.


You can book onto an afternoon tour which lasts several hours and takes you up onto Mount Locke for a visit to the 107” Harlan J. Smith telescope and then Mount Fowlkes for the Hobby Eberly Telescope. People with walking difficulties or disabilities can take a small bus; the rest drive themselves up.

The tour isn’t strictly necessary for the HET, which has an interpretive area you can access independently. However, it is the only way you’ll get into the 107” observatory apart from on a viewing night, to see the telescope from the observing floor and watch it and the dome being moved around.

The tour I took was excellent, the guide friendly helpful and knowledgeable. I’m not a big fan of tours in general, but this one is well worth it. However, note that if you were hoping to see the historically-important 82” Struve telescope you will be disappointed. You do get to see its unusual dome close up because it’s right by the 107” dome, but the telescope isn’t accessible, perhaps because of the long and narrow stairs to get up to the observing floor.

Star Parties

The visitor centre hosts regular star parties, which look fun and should be good given the dark skies, but I didn’t attend one during my visit as my evenings at McDonald were taken up doing Special Viewing Nights (worth noting that you can’t do both on the same night).

Special Viewing Nights

These events – where you actually get to observe through a big professional telescope - are currently available on three of McDonald’s instruments: the 36”, the 82” Otto Struve telescope and the 107” Harlan J. Smith. These are some of the best such experiences available and at ridiculously low prices (the most expensive, the 82”, was $125 at the time of writing). However, be prepared for weather problems. Two of my nights were clouded out and other visitors spoke of blizzards and thick fog on previous attempts!

By far the most desirable Special Viewing Night is the 82”. Not only is it the most famous telescope of the three, but it was designed and configured specifically for visual use (unlike the 107”) and is likely of much higher optical quality than the 36”. I’m told it gives the best viewing experience of any telescope, anywhere. Unfortunately, the 82” is still used for research and only rarely available for viewing nights. My night on the 82” was frustratingly curtailed by the weather, but was enough to convince me to make the trip again some day if I can. Even so, I got to tour the dome, see and photograph the telescope and hear about its history.

The 107” experience is very different, with more of the evening spent on interpretation with a professional astronomer and just one or two objects viewed at (I’m told) extreme high power, via an eyepiece in the control room (you don’t get the experience of climbing a step ladder up to the telescope itself to view). I ended up doing the interpretive part of the 107” Special Viewing Night – and very interesting it was, seeing the scope and control room in operation and talking to the astronomer - but not the actual viewing, again due to the weather.

The 36” Special Viewing Night is the cheapest, the most frequently available (because it’s no longer much used for research, unlike the 82” and 107”) and probably covers the most objects due its swift goto slewing and pointing. I did it on a night of clear dark skies and I can highly recommend it.

But again, please do note that demand exceeds supply on these Special Viewing Nights, so you will have to book early: don’t expect to just turn up and view the way you can at Lowell, for example.


McDonald observatory is a must-do if you have an interest in astronomy. I liked all the activities I took part in, but a Special Viewing Night (especially, but not only, if it’s actually clear) is the highlight. If you are a keen observer then a night on the 82” in particular should be on your bucket list, but the 36” was a really excellent few hours of deep sky viewing and the 107” very interesting in its own way too.

Dome of the 107” looms into a starry sky at McDonald Observatory (spot the Pleiades, Andromeda Galaxy and Double Cluster).