McDonald Observatory 107” Harlan J. Smith Telescope Review

McDonald observatory was founded on 23^rd November 1932 and within a year work began on the 82” reflector which is still operational. The 82” made a number of major discoveries, but by the mid 60’s it was starting to get tired and outdated; McDonald needed a big new telescope to do cutting edge research and stay relevant.

Fortuitously, NASA approached McDonald at that time. They wanted to use the 82” in order to survey the Solar System before launching probes into it. The director of McDonald – Texan astronomer Harlan J. Smith, after whom the 107” was renamed in 1995 – told NASA that the 82” was booked up. But he seized the opportunity and offered them a deal – fund a new telescope and McDonald would provide the dome and infrastructure.

When it was completed in 1969, the 107” was the third largest telescope in the world after the 200” at Palomar and the 120” at Lick. Just a year later in 1970 it became famous for a very unusual and unfortunate reason: a deranged employee shot up the mirror with a handgun. Amazingly, the 107” survived this ordeal and remains one of the largest fully-functional telescopes in the continental U.S. The 107” has another claim to fame: it was used to fire a laser at the Moon as part of the Apollo 11 laser ranging experiment.

Although the 107” is used most nights for research, mainly spectroscopy, a few times a year (McDonald say 9-10, but not this year anyway!) around full Moon they organise a public Special Viewing Night on the telescope. So I booked myself on one: rude not to, really.

This is my review of that experience and the instrument itself. Unfortunately, a cloudy sky prevented any viewing during my evening on the 107”. But at least I got to see the telescope in action, see an instrument package (the VIRUS spectrograph) attached and running and talk to the astronomer using it about his research. I was able to see the control room in operation, too. I also got lots of photos! Hopefully, I’ll go back and update this review with some actual observing.

This review is two in one. It’s a review of the telescope and using it in my usual format. It’s also a review of the Special Viewing Night experience, with its own separate section below.

At A Glance

Telescope	107” Ritchey-Chrétien
Aperture	107” (2.7m)
Focal Length	23.91m/47.98m/88.43
Focal Ratio	F8.8 RC/F17.7 Cass/F32.5 Coudé
Central Obstruction (incl. cage/baffle)	? 40% ?
Length	9.75m (32 ft)
Weight	190 tons

Data from McDonald Obs./Univ. of Texas.

Design and Build

Though it was built thirty years later, it’s interesting that the 107” carries over a lot of design elements from the 82”, including the massive cross-axis equatorial yoke mount at a time when simple fork mounts were becoming common. The 82” has a semi-enclosed design and the 107” unusually extends that so almost the whole tube is closed.

Optics

Like most modern professional reflectors, the 107” employs the Ritchey-Chrétien optical design, whereas the older 82” at McDonald is a Classical Cassegrain. The RC is widely employed because it allows a shorter focal length for a wider field, but free of the coma and astigmatism that distort stars off axis in many designs. In this case the RC focus of F8.8 is indeed quite a bit faster than the F13.7 of the fastest (Classical Cassegrain) focus for the 82”.

However, that F8.8 focus, situated in the usual place at the back of the mirror, is one of three different foci available. An F17.7 Cassegrain angle focus is accessed through the plate seen on the side of the telescope. A further F32.5 Coudé focus directs light down inside the RA axis of mount and into a room beneath. Changing focus involves swapping out the secondary mirror (and thus the whole truss-tube assembly at the top of the telescope) for one with a different figure.

The primary mirror has an interesting mechanism at the back, comprising numerous pendula with small black weights (see photo). These pendula differentially change the pressure on the mirror in different orientations, reducing deformation. It’s a job done by computer-controlled actuators in more modern scopes.

The primary was cast using a then-new fabrication technique. It is made, not of poured and moulded glass, but by putting chunks of silica into a mould and heating them to melting. This makes the optic more robust and less temperature sensitive. Good decision, because it doubtless saved the mirror …

In 1970 an employee suffered a breakdown and having taken a shot at his boss, unloaded his pistol into the 107” mirror. The 9mm pistol shots (eight, I believe) fired into it didn’t break the mirror, just dented pits into it. Bored and blacked out, they have no effect other than to reduce its effective area by 1% or so.

System of pendula around the Ritchey-Chrétien focus to offset changing stresses on the mirror at different orientations.

Interchangeable secondary mirrors and cages allow for the three different foci.

Tube

The 107” has a fully-enclosed tube, a real rarity in a large telescope of the modern era. Only one of the short swappable secondary mirror assemblies (the one for the Coudé focus, I think) is of truss-tube construction.

I can’t find any information as to why the 107” has a closed tube. Usually, open tubes allow quick thermal equilibration. But perhaps it’s because NASA wanted it for planetary imaging at high image scale and the tube helps to block air currents moving through the dome and slit.

Focuser

Although the 107” is too modern to have been designed for visual use, they have modified it by adding a (probably unique) mechanism in the control room. That mechanism tilts a beam tube into a manhole in the control room floor that taps into the light travelling down to the Coudé focus room below. So viewing at the 107” doesn’t involve climbing a long stepladder in the cold and dark like every other big observatory telescope used visually. Instead, visitors on Special Viewing Nights line up at the eyepiece in the warm control room.

There is a problem, though: magnification. The Coudé focus is F32.54, giving a gigantic focal length of 88.43m. Let’s crunch the magnification numbers on that.

The largest available eyepiece that I am aware of is the 100mm 4” eyepiece made by Siebert and used at Mount Wilson (and perhaps a few other large instruments as well). Such an eyepiece would yield a magnification of 884x on the 107”. This is about the maximum power ever used on the Mt Wilson 60” and I found it of little utility, even on a night of stunningly good seeing (the best I’ve ever experienced anywhere). I much preferred the views at 400-500x.

Unfortunately, the 107” isn’t equipped with a giant 4” focuser, so the longest focal length eyepiece likely to be available is Tele Vue’s standard 55mm Plossl (the default eyepiece in every large instrument I’ve visited). For context, that 55mm eyepiece gives a magnification of 436x in the 60” at Mt Wilson. Here on the 107” it gives a magnification of … 1607x!!

Meanwhile Tele Vue themselves recommend a maximum magnification of 350x on any telescope.

The Coudé focus beam tube can be tapped into by a visual adapter in the control room – seen folded up and out of the way here.

Mounting

The giant cross-axis yoke mount was built by a Perkins Elmer division and though it is of the same basic design to the 82”, it is much more massive with a heavy RA axis and giant counterweight. It’s all in a pleasing cream finish that reminds me of an Astro Physics mount compared to the dark grey of the 82”.

We were given a demo’ of slewing and the telescope slews rapidly and quietly using a hand panel. Though there is a separate control room with lots of screens for the instruments, the scope and dome can be controlled from a console on the observing floor.

The mount allows the telescope to be laid flat on the floor for cleaning the mirror every fortnight with CO₂ snow blown from a gas bottle and every two years for removal of the mirror for re-aluminising. The latter is a major operation, involving a big crane and blokes in hard hats. The mirror is moved into a cradle and lowered through a huge trapdoor into a giant vacuum chamber beneath.

The giant cross-axis yoke mount and huge counterweight.

This group taking the day tour gives more sense of scale.

Accessories - The VIRUS-W Mitchell Spectrograph

The 107” can be fitted with several different instruments, including at least three types of spectrograph, at all three different foci. On my visit, the ‘VIRUS-W’ spectrograph was fitted to one of the four angle Cassegrain foci arrayed around the tube.

The VIRUS-W is a strange black device shaped like a segment of Toblerone. It was sitting in a blue gimble off the side of the telescope that keeps it vertical and allows it to be calibrated only once a night.

The VIRUS-W is one of the widest field spectrographs available at about 1x2 arcmins. It employs 267 fibre optics cables to take spectra of multiple stars simultaneously and due to high spectral resolution can determine the motion of stars in nearby galaxies to a precision of a few kilometres per second. On the night I visited, this was what the professional astronomer would have been doing had the clouds not rolled in. He was studying the distribution of stellar velocities in galaxies at medium distances to see how their gas distribution varied. A minor coincidence is that VIRUS-W saw first light on spiral galaxy NGC 6903 – my favourite object from my previous viewing night on the McDonald 36”.

The spectrograph was already filled with liquid nitrogen and whirring away, ready for action after we left. Unfortunately, the same cloud cover that prevented our viewing was also destined to stop the VIRUS and when we emerged from the control room later it had already been dismantled and the telescope’s secondary focus cage swapped back over to Coudé configuration (I know this because the cage was a solid one when we arrived, but had reverted to truss-tube later).

107” with VIRUS-W attached at the Cassegrain elbow focus. Compare the solid-tube Cassegrain secondary assembly with the title photo.

VIRUS-W Mitchell spectrograph attached to one of the F17.7 Cassegrain angle foci. The old guidescope with eyepiece and Telrad finder is less cutting edge!

In Use – Astrophotography

I assume the NASA planetary survey work for which the 107” was funded was done using plate photography, but I can’t find any information or surviving images, apart from a low-power image of the Moon taken through the guiding eyepiece when the 107” was being used to project a laser beam onto reflectors left by Apollo.

The 107” is largely used for spectroscopy now and its description on the McDonald and U.T. websites doesn’t list any photographic (obviously CCD these days) instruments.

In Use – The Night Sky

General Observing Notes

Dusk fell to complete cloud cover on my evening with the 107”. However, the outreach staff explained the unique experience I was missing.

Due to the insanely high magnification, only two very small objects, usually planetary nebulae, are viewed; and that by the light of the full Moon (viewing nights are scheduled at full Moon to minimise impact on the research work, which is over-subscribed as it is). The viewing takes place over an hour, not the full-evening you get on the 36” and 82” Special Viewing Nights.

The outreach staff all said that the views are actually pretty uninspiring due to the extreme magnification necessitated by the very long Coudé focal length (see above). The astronomer working with the VIRUS spectrograph confided that even the views through the ‘little’ 36” are generally much better. But that’s not to say I won’t be going back for another shot at the viewing: I most certainly will if I can!

McDonald under disappointingly cloudy skies for my Special Viewing Night on the 107”, whose dome is the left of the distant pair atop the hill.

The Broader 107” Special Viewing Night Experience

The other two instruments available for Special Viewing Nights at McDonald – the 36” Dall Kirkham and the 82” Struve Classical Cassegrain – are all about the observing. Even on a clear night, the 107” really isn’t. Just two objects are viewed and viewing only takes an hour of the three hour program. And as explained, the viewing experience isn’t actually that great. No, the 107” Special Viewing Night is mostly about seeing the telescope in operation and a talk by the professional astronomer using it.

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.

The bus dropped us by the entrance to the huge building for the 107” and we traipsed inside. There, we found not the carpeted lobby of the 82” (once the reception for the whole observatory), but a dark and cavernous industrial space humming with machinery and containing a mass of pipes and tanks.

Getting up to the observing floor was different too. In the 82” dome, it’s a series of narrow staircases past doorways to offices. In the 100” dome at Mount Wilson, it’s flight after flight of industrial-grey metal stairs. Here, it’s a big lift – all brushed stainless - like the one in an office block.

There was a bit of an X-Files moment when someone asked, ‘why do the lift buttons go from floor 3 to floor 5 with no floor 4?’ The answer is that Floor 4 is where a spectrograph, one of the world’s most sensitive, lives and there’s no general access to it.

Walking out into the gloom of night on the observing floor with the huge counterweight looming overhead was a singular experience. The hum of machinery filled the space – the VIRUS-W spectrograph was filled with liquid nitrogen and ready to go, the astronomer who would have been doing research with it later ready to greet us.

The astronomer was a friendly chap who answered all of our dumb questions politely and modestly. Turned out he was a member of staff, unlike most who book time on the 107” from other institutions. Though a science manager on the Hobby Eberly telescope, he explained that his job comes with a week a month of research time. So he was fairly relaxed about losing a night’s work to the weather, unlike visiting astronomers who have to reapply from scratch if the clouds thwart them.

Having explained the telescope and instruments and demonstrated slewing both the scope and the dome, he led us into the control room, which is situated at the south end of the RA axis and so right above the Coudé focus. The control room contains multiple screens, including four large monitors with numerous windows displaying different information about the telescope and instruments. In addition to video monitors and star maps, he showed us weather radar displays and a grid showing the positions of the VIRUS-W fibre optic cables overlaid on his target galaxy. On the far right were the multiple lines of a spectrum, the only one he would record that night – a test spectrum taken at the start of every session.

Finally, he explained the visual feed that would have been swapped into the Coudé focus light path (see above for a detailed description) … if only it had been clear.

Eventually it was time to go and they ushered us back into the lift in groups. I went last, enjoying the view of the 107” towering into the darkness. Then it was the short bus ride back to the visitor centre and the red-eye drive back to my motel down winding Highway 118. I saw loads of wildlife on my way back; too much, including a magnificent Great Grey Owl taking off from the road just ahead on vast but silent wings.

107” dome shutters open at dusk on a previous clear evening at McDonald.

Summary

If you’re going to book onto one of McDonald’s Special Viewing Nights and then have it clouded out as I did, it is paradoxically the largest, this 107”, that you’d choose. That’s because the larger part of the evening (two out of the three hours) is spent meeting a professional astronomer working with the telescope and seeing a spectrograph in action. In contrast the two smaller instruments you can book for viewing, the 36” and 82”, allow you to spend almost the whole session at the eyepiece with limited interpretation.

Even so, the 107” experience is very worthwhile, especially of you are interested in a behind-the-scenes glimpse of how astronomical Big Science actually gets done. It’s also a little cheaper than the 82” viewing night.

Seeing the control room screens, complete with the control interface and real data from the spectrograph, was a highlight for me. So too was seeing the huge telescope and mount towering into the gloom of night and fully fitted out for a night of data gathering – much more atmospheric than the day tour.

A Special Viewing Night on the 107” at McDonald is a different experience from the 82” or 36”, with the emphasis less on viewing and more on interpretation of the instrument’s professional work. It’s a most interesting experience nonetheless.

The 107” dome and observatory building towers into a starry sky – how I wish it had been on my Special Viewing Night!