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82” Struve Telescope at McDonald Observatory Review

It still just says ‘McDonald Observatory’ above the door to the 82” dome. That’s because in its early years, McDonald observatory and the 82” Otto Struve reflector were essentially the same thing. Not only was the then second largest telescope in the world (after the 100” at Mount Wilson) McDonald’s main instrument, but the dome unusually housed the accommodation and offices for the observatory as well.

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

The 82” made some important observations and discoveries. In 1944 Gerard P. Kuiper, arguably the first modern planetary scientist, used it to discover methane in Titan’s atmosphere. Continuing his solar system observations, Kuiper went on to discover moons Miranda (of Uranus) and Nereid (of Neptune) with the 82”. Of less import to science but of even more interest to me personally, in 1956 Kuiper observed Mars at opposition with the 82”. He published the following statement that was effectively the last nail-in-the-coffin for Percival Lowell’s Martian canals:

“There is no evidence anywhere on the planet of a distinctly geometrical pattern or a systematic streakiness.”

Unlike most large old telescopes available for outreach, Kuiper’s instrument is still heavily used for research, particularly into white dwarfs. But of course, these days research isn’t done with an eyepiece like much of Kuiper’s, it’s done with a spectrograph. Still, a few times a year, McDonald fits a focuser to its classic instrument and invites a small group of the public to pay a modest sum and come for an evening’s viewing. Browsing the web one New Year’s Eve, I found one of those evenings. Given that the 82” may just be the finest visual instrument available anywhere, I had to be there, so I booked the last place and then a big trip to west Texas to do it.

This is my review of that experience and the instrument itself. Most unfortunately, my evening on the 82” ended up curtailed by the weather. But at least I got some photos (not normally possible as the 82” isn’t open to the public and viewing evenings are conducted in low red light from the start). Hopefully, I’ll go back and update this review with more observations on another viewing night at some point.

The observatory building and dome for the McDonald 82”.

At A Glance

Telescope

82” Struve Classical Cassegrain

Aperture

2.08m (82”)

Focal Length

Cassegrain: 23.91m (78.4 ft)

Focal Ratio

F13.7 (also F17.7)

Central Obstruction (incl. cage/baffle)

~35% (my measurements from photos)

Length

8.2m (27 ft)

Weight

41 tonnes

 Data from McDonald Obs./Univ. of Texas.

Design and Build

The 82” was designed by Russian astronomer and first McDonald director Otto Struve, whose name it has borne since the 1960s. The telescope and mount were built by Warner and Swasey of Cleveland Ohio, a fanous builder of large observatory mounts, including the one for the Lick 36” refractor.

McDonald calls the 82” a work of art. If so, it’s of a highly functional kind – this telescope doesn’t (to me) have the beauty of the 24” Clark at Lowell, for example. It’s appearance, finished in dark grey, is certainly unique and has a definite inter-war aesthetic (some of the castings remind me of parts from a Spitfire’s Merlin engine), but without the nods to the ‘Streamlined Moderne’ movement seen in the smooth simple lines of the 200” at Mt Palomar. In fact, the fussy braced truss-tube of the 82” looks more like the previous generation of big reflectors at Mount Wilson.

Optics

Unlike most modern professional reflectors (including the 107” onsite at McDonald) which employ the Ritchey-Chrétien optical design, the 82” is a Classical Cassegrain.

In the Ritchey-Chrétien, both primary and secondary mirrors are hyperbolic, giving a wide field, round stars off-axis and typically a fast focal ratio for a Cassegrain. The downside for visual use (for which such instruments are not designed) is often a large central obstruction.

The Classical Cassegrain has a hyperbolic secondary, but its primary is parabolic like a Newtonian. The Classical usually has a longer focal ratio which makes it slower photographically, but can also mean a smaller central obstruction and super-sharp on-axis visual performance.

The 82” has a basic Cassegrain-focus F-ratio of F13.7 (which is a little faster than some) and a central obstruction, including the prime-focus cage, of between 30% and 36% - typical of good visual reflectors (the secondary mirror itself is 15”). And of course, the 82” was actually designed for visual use, perhaps one of the last big professional telescopes to be so.

The primary mirror is made of Pyrex and is monolithic, unlike modern big reflectors that have honeycomb mirrors (or segmented ones like the Hobby Eberly at McDonald). The mirror is around 30cm thick and weighs 1900 kg.

That mirror was the first major optical commission by Warner and Swasey. It was figured by a young optician, Carl Axel Robert Lundin Jr, whom Warner and Swasey had appointed head of their optical shop especially for the Texas contract. Lundin had learned the trade making big refractor lenses at Alvan Clark and Sons, just like his father (of the same name), who had figured some of Clark’s most famous optics, including the giant 40” lens for Yerkes.

Corning’s first attempt at casting the Pyrex blank was a failure – it had big cracks in it. They promised the cracks could be ground out, but Struve was having none of it and insist it was re-cast. When the mirror came out of the mould it was actually two inches larger than the 80” originally ordered.

Figuring the big Pyrex blank took a long time, partly due to problems in the Warner and Swasey optical shop and perhaps also because Lundin had never polished a large mirror; and in fact, another optician, Dr John Stanley Plaskett, was called in to finish the job after Lundin botched it. Even so, the 82” mirror may be little better than one wavelength error across its surface. That sounds bad, but is probably sufficient for such a large optic.

The mirror is cleaned by blasting it with carbon dioxide every few weeks (the CO2 falls on the mirror as snow which brushes off the dirt then sublimates). Every two years it gets re-aluminised in a tank right on the observing floor.

Tube

The tube is half-truss design, with an enclosed mirror box at the base and a section of cross-braced truss tubes at the top supporting the secondary mirror assembly and prime focus cage. The entire back of the mirror box can be removed, by unbolting it and lifting it with a small crane, when the mirror needs re-aluminising.

Focuser

Since the 82” is regularly used for spectroscopy, the focuser has to be fitted especially for Special Viewing Nights. The focuser chosen is a large Astro Physics rack-and-pinion with a 4” drawtube and even a micro-focus knob (an unusual luxury on a big scope like this).

Astro Physics focuser and Tele Vue Panoptic 41mm.

Mounting

The giant cross-axis yoke mount with its huge offset counterweight was actually built before the optics were finished. A big mount was much more Warner and Swasey’s regular line of work and it seems they may have had far fewer problems than with the mirrors.

The massive castings that comprise the mount are, I would guess, in their original glossy dark grey enamel finish. As I said, the whole has a definite pre-War appearance. Despite being made so long ago, much of the mount is original. Like a vintage car, they regularly have to re-oil the bearings … with the same special oil specified in 1939 and still made by the same company.

The telescope is driven in RA by a couple of small motors and a huge 720-tooth gear wheel hidden within a cast shroud. The gear is surmounted by an old-fashioned setting circle fully 24 ft in diameter, but cast and machined by Swasey to an accuracy of a few thousandths of an inch. The HA circle at the back has been fitted with an encoder and a PEC system in recent years that has hugely improved tracking accuracy.

The declination drive has also been replaced recently after a number of spectacular fires (!) and the whole mount has been upgraded with a modern goto system, including (I think) permanent encoders. The console is also modern, but has been given a period Art Deco appearance in the lettering, switches and wooden casing.

RA drive and setting circle for the huge yoke mount. Recent PEC encoder and tape are housed in the corrugated pipe.

Concrete art: towering Brutalist pier for the RA axis.

Detail of the control console. It’s modern but has a period, Art Deco look.

Accessories

Despite the 4” focuser, the Struve telescope is equipped with a standard 2” diagonal and eyepieces. This is unlike some large professional telescopes outfitted for visual use, which employ over-sized, 3” or even 4”, diagonals and eyepieces (often made by Siebert) for a wider field of view.

For our viewing session, only a Tele Vue Panoptic 41mm eyepiece was used. This eyepiece gives the maximum possible field in a 2” eyepiece and in most amateur telescopes a low power to match. But crunch the numbers for the 82” and you get a power of 583x – a high power on any telescope, given Earth’s turbulent atmosphere – and a true field of view of only 0.11°.

In Use – Astrophotography

The Struve reflector was been used for astrophotography in the past, but in recent years has been dedicated to spectroscopy or visual outreach. The standard focuser and visual back mean I was itching to just slot in my DSLR, but no idea if anyone’s done so.

In Use – The Night Sky

General Observing Notes

Despite being little more than a quarter century newer than the 60” at Mount Wilson, both the telescope and dome seem much more modern. The dome moves relatively fast and smoothly with just a few creaks and without the arcing Frankenstein throw switches and clanking at Mt Wilson. Similarly, the telescope mount has been upgraded to computer control with goto pointing, making it faster to slew from object to object than the big refractors at Lick or Lowell at least (though actually not much faster than the mount for the Mt Wilson 60”).

For the object I viewed, chosen to be high in the sky, just a short step ladder was needed to access the eyepiece. I’m not certain how the 82” is able to offer views of targets at lower altitude, but I believe the floor can be raised and lowered.

Deep Sky

The sky had been dodgy all day and as I rode the curves in the night bus up onto Mount Locke I fretted and searched for stars through the window – there were a few in dark holes, but mostly it was cloudy. So when I stepped up to the Panoptic eyepiece I was both excited and worried. As it happened, I was in luck.

The Crab Nebula (M1) was our first target, high in the south east and to the left of a bright red star peeking through the clouds and the dome slit – Aldebaran in the Hyades. I view and image M1 quite often, but this was the Crab Nebula as I had never seen it, filling the 0.1° field of view.

Unlike many planetary nebulae that show a somewhat regular shape and internal structure from stellar eruptions, the Crab looks like what it is – the expanding remains of a single massive explosion, supernova SN 1054. Usually you need to look at a photo to see that expanding blast of gas, with its chaotic arcs and tendrils, but through the 82” those twists and skeins of gas were visible to my eye, obvious and bright. It was a stunning view, but all too soon someone else wanted a look.

Then, a few viewers later, someone called out, ‘I can’t see a thing!’ The guide confirmed it, the clouds had thickened to cover the whole sky. We waited for a bit, then the lights came on …

Frustration doesn’t cover it.

Simulated view of the Crab Nebula within the narrow 0.11° field of the 82”.

The Broader 82” Special Viewing Night Experience

As usual with my reviews of big outreach instruments, this is really two in one: of the telescope and of the broader experience too. If you are interested in what a Special Viewing Night at McDonald is like, read on; or skip it if you’re only interested in the telescope itself.

Special Viewing Nights are currently available on three of McDonald’s telescopes: this the 82” Struve telescope, the 107” Harlan J. Smith telescope and a 36” Dall-Kirkham.

By far the most desirable is this 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 much larger than the 36”.

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. The 36” Special Viewing Night is the cheapest, the most frequently available and probably covers the most objects due its swift goto slewing and pointing. You can read my review of it here.

The night starts with parking in the free lot near the observatory entrance and checking in at the foyer of the large and modern visitor centre at around 7 pm. The VC is at the bottom of the observatory, well away from most of the professional instruments and easily accessible from Highway 118, either from I-10 in the north or from the small town of Fort Davis in the south. But note that you’ll likely need to drive out to Marfa or Alpine if you need to fill up with more than a chocolate bar or gas and the same goes for lodging. The observatory might let you stay in its dorms, but it will be expensive.

The other facilities at the VC - the gift shop, interpretative displays, film theatre and café - close by 5:30, so after signing in it’s a short wait for the bus at around 7:15 pm. Being driven up the mountain at dusk on the red-lit night bus is fun (‘cos you get to look at the other domes opening for the night) and convenient, but it does mean you can’t leave early.

Buses up to the domes for Special Viewing Nights depart from outside the visitor centre.

Model of the 82” observatory building at the McDonald Visitor Centre.

All the Special Viewing Nights start at dusk and by the time you arrive at the dome, low red lights will be all the illumination at the observing floor. That’s just enough to get some grainy photos inside the small 36” dome, but the 82” just disappears up into impenetrable blackness. Paradoxically, getting the photos for this review was the result of my viewing night being cut short by clouds, after which they turned the lights on.

For the 82” Special Viewing Night, we were bused up to the main entrance to the dome and ushered inside the lobby for a short presentation. After that, it was up a series of narrow stairs (which may explain why it’s off limits during the day tour), past landings and passageways leading to the old accommodation rooms arranged radially around the outside of the building and now converted into offices, a library and crucially for us … the restrooms.

Climbing the last flight of stairs, a grey girder appeared in the wall on the right and I knew we had arrived at the viewing floor. Stepping inside, the dimly red-lit dome looked bigger than it does in bright light. Several rows of chairs were available at the side for the group to sit in between turns at the eyepiece.

The environment in the dome is cold because the slit is open and there’s no heating to avoid turbulence (as for all observatories), but it’s much more comfortable if less magnificent than being under the open sky in a roll-off like the one at Kitt Peak’s outreach observatory.

Same as for the 36” evening, we had a guide who was a local amateur with an interesting back story. The guide had made the trip to McDonald many times, he said, to view there and then had finally decided to retire to Fort Davis, after which he became the regular guide for the 82”. Again, as for the 36” evening, he had prepared a set of objects to view in advance, with representative images of each one on the computer at the control desk to prepare us for what we would see at the eyepiece. Too bad we only got the one to actually view.

At the end of the session, we exited by an external fire-escape that needed the light of our phones to navigate, down to the bus back to the VC. Unlike Kitt Peak, there’s no restriction on car lights when you drive away down Highway 118, but take it slow because that road is full of wildlife after dark.

On my 82” Special Viewing Night, we entered the observing floor through this door, but left again much too soon!

Summary

The 82” Struve reflector is one of the largest three telescopes worldwide regularly available for viewing (some others may occasionally be, for a select few already working on them professionally, e.g. the 2.4m at Kitt Peak).

The telescope operator at McDonald said that, especially given the dark skies, the 82” offers the best deep sky observing experience available to the public anywhere. Due to the weather I only briefly observed one object out of a target list of perhaps 15, so I can’t say for sure if that’s true, but it’s certainly possible from my brief view.

Despite such a brief and curtailed viewing session, my evening with the 82” was a wonderful astronomy experience and I’d hope to return on a clear night for more observing.

Special Viewing Nights on the 82” at McDonald must rank as among the best astronomy experiences anywhere (and inexpensive too), but they are infrequent and in high demand, so book early and be prepared for (considerable) disappointment if the weather is bad.

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