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 unusually the dome housed the accommodation and offices for the observatory as well.

The observatory was founded on 23^rd November 1932 and within a year work had begun 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. 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, the 82” is still used for research, particularly into white dwarfs. But of course, these days research isn’t done with an eyepiece, it’s done with a spectrograph (and apparently a new prime-focus CCD is in the offing too – the old one got broken years ago when it collided with part of the dome during slewing).

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. The 82” Struve is then the largest telescope on Earth available for public sessions (as far as I know).

The first time I attended one of these ‘Special Viewing Nights’ it was clouded out. Then two years later I booked on two more and got lucky with two nights of clear skies, hence this heavily updated ‘V2.0’ review!

McDonald Observatory from Highway 118 to Fort Davis; the 82” dome is on the left.

The observatory building and dome for the McDonald 82”.

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

Otto Struve in a group photo from the lobby of the 82” dome.

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)	~30% (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 (see photo above), whose name it has borne since the 1960s. The telescope and mount were built by Warner and Swasey of Cleveland Ohio, a famous 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. Its appearance, in dark grey enamel, 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. The 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, again like the earlier big reflectors at Mount Wilson.

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 really intended) 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 Cassegrain-focus F-ratio of F13.7, but for Special Viewing nights a custom-made F8 reducer is installed behind the focuser. Why? Because otherwise the field of view is just too small for many DSOs.

The central obstruction, including the prime-focus cage, is perhaps ~30% - typical of good visual reflectors (the secondary mirror itself is only 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 (one of the first) 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 insisted 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 a 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.

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.

All the upgrades make it remarkably swift and easy to slew – just select an object and press a button. The dome takes longer to re-position than the scope itself.

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 for outreach. It’s modern but has a period, Art Deco look.

There is a separate professional control room below the observing floor.

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.

As I noted above, an F8 reducer is fitted at the visual back to give a wider field of view.

In Use – Astrophotography

The Struve reflector was been used for astrophotography in the past and a new CCD is coming, 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”).

The dome has a moveable viewing floor – an original feature – which moves swiftly and efficiently. Getting up to it means climbing a few rungs up a stepladder, but thereafter the eyepiece was always easy to reach.

The only eyepiece used, a Tele Vue Panoptic 41mm, 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 406x, even at F8 with the reducer – a high power on any telescope, given Earth’s turbulent atmosphere – and a true field of view of only 0.16°.

The 41mm Panoptic usually has plenty of eye relief, but here it seemed limited for use with glasses, perhaps due to the F8 reducer. This wasn’t ideal in an outreach situation where re-focusing isn’t possible.

A full list of the objects viewed follows:

Uranus

Though Jupiter was visible outside, at just 15° altitude it was too low to view. That left just one planet on the nights I was there – Uranus.

By amateur-scope standards, Uranus was large and bright, despite being just 3.6” across, but still the familiar pale blue-grey, not the green you see in images. In dusk seeing that hadn’t yet fully settled, no cloud detail was visible, but moons Titania, Oberon, Aerial, Umbriel and Miranda were. This was a thrilling first for me and especially poignant because Miranda was discovered with the 82”.

Deep Sky

I viewed the following deep sky objects across my two nights on the 82”:

NGC 604

Like famous and familiar M42, this is a star-forming region but in this case located in another galaxy in our local group (M33 in Triangulum). Big and bright through the 82, arcs of structure in the nebulosity were clearly visible.

NGC 891

Not quite all of this large spiral galaxy in Andromeda, seen side-on, fitted into the field of view. Nonetheless, its central dark band of dust was dramatically apparent, including variations in its width and density.

NGC 1535 (‘Cleopatra’s Eye Nebula’)

This magnitude 10.5 planetary nebula revealed the two distinct shells you see in images, with some structure in the inner shell and the central star clearly visible. Possible hints of a blue-green colour.

NGC 2158

This small and faint open cluster, very close to M35, looks insignificant in smaller scopes, but here showed as a mass of brilliant stars – much like M35 usually does, in fact!

M42

Only a small central section around the (at this aperture, star-rich) Trapezium fitted in the field, but nonetheless masses of structure was obvious with direct vision including part of the prominent dark region: a real wow moment through the 82”. Some colour too.

M79

A dense and small globular cluster to the right of Sirius, the 82” resolved individual stars right to M79’s core, with brighter and dimmer stars picked out in a way you just don’t see at typical amateur apertures. A really spectacular sight.

IC 418 (‘Spirograph Nebula’)

Another planetary nebula below Rigel. Two shells were obvious, again with hints of structure in the inner shell. Unlike Cleopatra’s Eye, colours were very obvious here – a rich blue in the centre fading to purple. It was interesting to note that the colour was only visible with direct vision, though – looking to the side made the nebula brighter and more structured, but in monochrome (only the central part of the retina has colour-sensitive cone cells).

NGC 2261 (‘Hubble’s Variable Nebula’)

A small nebula in Monoceros that varies in brightness and appearance, this looked very much as it does in images – a misshapen ‘comet’ with a bright head and long, roughly-triangular tail with curved boundaries and a distinct dark region within it. Hints of nebular structure too. Another truly spectacular sight through the 82”.

M1 (‘Crab Nebula’)

For me the Crab was among the highlights of my two nights viewing, mainly on the first night due to better seeing and because they inserted an OIII filter.

Without the filter, the Crab was large and bright and showed some structure. But with the OIII filter in place, the structure – the fibrous arcs and knots in the nebulosity you see in images – was just extraordinary to observe with my own eyes.

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 the Special Viewing Night experience, 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, (possibly) 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”.

Viewing nights start at 7 pm and end around 10 pm, so you’ll probably want to sleep locally. The observatory might let you stay in its dorms, but it will be expensive.

I stayed at the quaint and comfortable Hotel Limpia in Fort Davis, but have stayed in a motel at Alpine in the past. Fort Davis is much more convenient, but short on services. Cheaper options include motels at Van Horn to the north, but they’re more than an hour’s drive away.

The viewing night starts at 7 pm with parking in the free and spacious lot opposite the visitor centre. 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.

The other facilities at the VC - the gift shop, interpretative displays, film theatre and café - close by 5:30, so there’s nothing to do but wait for the bus at around 7:15 pm. Note that the bus leaves promptly and is the only access to the observatory, so you can’t join late or leave early.

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

Viewing groups enter through the dome lobby.

On my 82” Special Viewing Night, we entered the observing floor through this door.

The 82” dome open and waiting.

Let the views begin!

The fire escape offered exceptional dark sky viewing between turns at the eyepiece!

All the Special Viewing Nights start at dusk and by the time you arrive at the dome the observing floor will be illuminated by red lights only. They may turn the lights on for a few minutes at the end so you can take photos.

The whole session lasts about three hours and there are typically around ten participants, so everybody gets plenty of time at the eyepiece.

The 82” Special Viewing Night began inside the lobby of the historic dome, where there are some photos and displays, including books from the original library and a beautiful chronometer.

From the lobby, it was up a series of narrow stairs, 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… the restrooms!

A last flight of stairs led up to the observing 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 slit is open and they keep the fire-escape door open too, to keep the dome cold and air currents to a minimum, so you’ll need to dress warmly in winter. But the open fire escape and exceptionally dark skies meant I was able to enjoy fabulous binocular views outside whilst waiting for my turn at the eyepiece.

They had prepared a set of objects to view in advance, so slewing from object to object was surprisingly fast, but the dome is moved slowly to reduce wear. A session director was on hand to interpret and answer questions, with two assistants to operate the telescope.

At the end of the session, we exited through a separate control room below the observing floor and out down the narrow stairs, back to the bus.

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 - especially deer and Javelinas - after dark.

Summary

The 82” Struve reflector is the largest telescope worldwide regularly available for public viewing sessions, so you’d expect a singular and incredible viewing experience and it certainly delivered, with remarkable views of a range of nebulae and galaxies and all the (visible) Moons of Uranus too. It’s among the best public viewing experiences I’ve had.

With the telescope in tip-top condition and the mount brought up to modern standards with a moveable observing floor, the session encompassed a wider range of objects than usual for the biggest instruments.

But did the 82” deliver my best views ever, as I’d thought it might? No. The views of planets and DSOs with the Mount Wilson 60” (on a private viewing night) were significantly better. Why? A large factor is seeing, which was reasonable at Mc Donald, especially on the second night, but not up to the exceptional standard I was lucky to encounter at Mount Wilson.

Another factor is focus. Like most public viewing events, re-focusing wasn’t allowed and it was often quite out for me, even with my glasses on.

None of that detracts from a superb astronomy experience and an absolute bargain at current prices. I’d love to try the 82” yet again on a night when more planets are up.

Special Viewing Nights on the 82” at McDonald are a fantastic experience (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.