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 23rd 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.
