McDonald
Observatory 36” (0.9m) Dall Kirkham Review
McDonald observatory in west Texas
boasts some of the darkest skies in the continental US and unlike some of the
observatories I visit for public viewing, it’s still a major hub for
professional research. Their Hobby-Eberly Telescope ties for the 2nd
largest optical telescope in the world at the time of writing. What’s more, the
HET is getting a major upgrade to pursue a cutting-edge study of dark energy.
Perhaps the very largest (just)
telescope available for public viewing is also at McDonald. That instrument is
the 107” Harlan J. Smith telescope. Mostly it’s used for research too, but just
once or twice a year a free night comes up. When it does, they fold a
specially-built optical take-off with an eyepiece into the Coudé focus and let
people take a look on a ‘Special Viewing Night’.
I mention this, because I met a
professional astronomer at the 107”. He’d set up his spectrograph, filled it
with liquid nitrogen and then the clouds had rolled in. We got to talking about
viewing through the giant instrument we were standing underneath and he
confided that another telescope on site actually gives a much better view. That
telescope is the 36”, which lives in a little observatory set just below the
huge (so huge it has a big lift like an office block) building for the 107”.
The 36” was originally built for
research work. It’s not a bought-for-outreach telescope like the RCOS 16” and
20” at Kitt Peak, for example. Nonetheless, it’s only rarely used for science
these days, mostly getting used visually (including by observatory staff, like
the guy I’d met). In between its other duties, it hosts regular public Special
Viewing Nights of its own, so I booked onto one and this review is the result.
Note: Special Viewing Nights at
McDonald all start after dark and the 36” isn’t on the daytime tour, so all the
internal photos were taken in very low red light at high ISO, hence their low
quality.
The 36” observatory is tucked away
below the 107”.
At
A Glance
|
Telescope |
36” DK Cassegrain |
|
Aperture |
0.918m,
36.1” |
|
Focal
Length |
12.27m |
|
Focal
Ratio |
F13.42 |
|
Central
Obstruction (incl. holder/baffle) |
~40% |
|
Length |
3.11m |
|
Weight |
~~1000kg |
Data from McDonald Observatory.
Design
and Build
The 36” Cassegrain is actually quite an
old telescope, though it doesn’t look it. Built in 1956, it was
originally used for research, mostly for photometry. After decades of use, it
was retired and used for training and outreach. However, it was fully restored
a few years ago for a quasar dark matter project by Yale university.
I think I had expected a long, closed
tube on a big old-fashioned German mount. But in fact, the 36” looks like most
large professional telescopes built mid-late century: a short and simple
truss-tube on a massive fork mount. That might be no coincidence, because like
many medium sized professional scopes of that era it was built by a division of
Perkin Elmer, Boller & Chivens.
The observatory building and 6.1m dome
were built by McDonald staff – the dome from leftovers from the 82” Struve dome,
the walls from rough-hewn blocks of local rock. Consequently, the observatory
has a rustic feel, hidden away on the side of mount Locke.
Optics
The 36” was
built as a ‘light bucket’ (their words), for photometry not for wide-field
imaging. As a consequence, it is not a Classical Cassegrain like most
professional instruments of that era, but rather a Dall Kirkham.
The Dall
Kirkham is well known now because Takahashi use it in their venerable line of
‘Mewlon’ telescopes, one of the few mass-produced Cassegrains. Tak’ adopted the
DK for the same reason that McDonald did – it’s cheaper and easier to make, because it uses an elliptical primary and spherical secondary in place of the more complicated parabola/hyperbola employed by the Classical Cassegrain. And though
the DK doesn’t have the well-corrected wide field of a Classical or
Ritchey-Chrétien, it is very sharp on-axis: just what’s required for accurate
photometry and (luckily for the public like me) for visual too!
Despite
being a DK light bucket, the 36” employs a fused silica primary mirror which
should be good for a stable image on cold nights, especially given that it is
15.5cm thick and weighs a whopping 203 kg.
The optical
specs are similar to Takahashi’s DKs – an F3.5 primary and an overall focal
length of 12.27m (F13.4).
Tube
The tube is
a short semi-open truss type. The massive primary mirror is fully enclosed, the
secondary supported on a four-vane spider with minimal baffling.
Focuser
A
conventional focuser is attached for public viewing, but instruments can also
be attached to the visual back, as recent photos show. Given the restoration
performed for research work by Yale, it’s possible that an electric focuser
moving the secondary is also installed.
Mounting
The huge
grey fork mount looks a lot like the fork for a 16” Meade SCT,
albeit larger. The fork permits a much more compact dome than would a
German equatorial, but even so the operator’s manual cautions to take care not to bump
the shutters when slewing to objects at low altitudes (which explains why Venus
was off the menu).
At the time
of the Yale-funded restoration, the drives were converted to
computer-controlled goto operation and now make the musical slewing noises
familiar from many amateur mounts.
Accessories
The only
eyepiece used during our viewing sessions (the evening is split into two, with
a warm-up in the astronomers’ lodge in between) was a 41mm Tele Vue Panoptic
giving a magnification of 299x.
In
Use – The Night Sky
General
Observing Notes
During our
sessions the mount slewed flawlessly and quickly, with accurate pointing even at
high power. That meant a long list of objects viewed, as many as we got through
at Kitt Peak with a much smaller instrument. This is a massive piece of
machinery so vibes were non-existent, the view through the eyepiece perfectly
steady.
The 41mm
Panoptic gave plenty of eye relief, important in an outreach telescope shared
by those with and without glasses, but even though it has the maximum field of
view for a 2” barrel, that field is still only 0.21° at 299x due to the long focal length (much longer
than a large Dob’ of similar aperture). Some large instruments used visually
benefit from special large-bore eyepieces and diagonals (often made by Siebert)
for larger fields of view.
That high
magnification meant we mainly viewed smaller objects, minus the larger galaxies
and star clusters from a viewing evening on the same sky at Kitt Peak a few days
before. A full list of objects viewed is as follows:
·
Uranus
·
Little
Dumbbell (M76)
·
Almach
(a double star)
·
NGC 604
·
M79
·
M46 / NGC
2438
·
Eskimo
Nebula
·
Cleopatras
Eye Nebula
·
Hubble’s
Variable Nebula (NGC 2261)
·
NGC 2903 (a spiral galaxy)
·
Crab Nebula
(M1)
·
Orion Nebula
(M42)
·
Castor (a multiple star system)
Planets
Though Venus was big and bright, it was just too low to set the 36”
onto, so the only planet we got to view was Uranus, our first target of the
evening.
Although DK’s are known for good planetary views, Uranus through the
36” was a mushy (though bright) ball of bluish light. One look outside
explained why – the stars were twinkling merrily, pretty proof of bad seeing.
The turbulence settled quickly thereafter, but too late for a better look at
Uranus.
Deep
Sky
Given the
absence of planets, my session on the 36” was all about the deep sky. Almost
everything familiar we looked at showed more detail than I’m used to through my
big refractor and previous large Dob’ (a 36” aperture is still much larger than
most Dobsonians and the MCDonald skies darker than most in Europe).
There were
some real highlights amongst the deep-sky feast, though:
NGC 2438 is
a planetary nebula that looks a bit like M57 (the Ring), except that it has
more structure (including a faint outer envelope) and sits embedded in the
dense star cluster M46. A really beautiful and enthralling sight through the
36”.
Hubble’s
Variable Nebula was a new one for me and a strange and impressive sight it made
– a bright, curving cone of nebulosity seeming to flow away from the bright
star at its tip.
The Eskimo
Nebula is one I regularly view and image, but in smaller scopes it can be a bit
disappointing, visually at least. The 36” gave a much more detailed view of
this dramatic planetary nebula: a bright outer envelope containing arcing
shells of gas around its central star. Even its ethereal bluish colour matched
the photos.
Castor is
perhaps the best known multiple star system. But the 36” aperture really made
the colour of Castor C stand out and gave the main components the dazzling brilliance
and spider-spikes of a professional image.
The Crab
Nebula, Messier 1, is another famous deep sky object. Smaller apertures and
more compromised skies show it as just a faint fuzzy. The 36” under McDonald’s
clear, dark sky revealed its outer shape and internal structure to give the
appearance and scale of a long-exposure image.
NGC 2903 is
a smallish but beautiful spiral galaxy with wide-open arms. I was surprised to
clearly make out its curving arms with my own eyes and perhaps a hint of
brighter star-forming regions too. It was perhaps the best view of a spiral
galaxy I’ve ever had: quite different from the faint fuzzy blob galaxies
usually are visually. I went all selfish and lingered at the eyepiece over this
one.
Viewing (not
me!) through the McDonald 36”.
The Broader
36” 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, this section is where I describe it;
skip it if you’re only interested in the telescope and observing.
Special
Viewing Nights are currently available on three of McDonald’s telescopes: this
36”, the 82” Otto Struve telescope and the 107” Harlan J. Smith.
By far the
most desirable 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”). 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.
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 described here is the
cheapest, the most frequently available and probably covers the most objects
due its swift goto slewing and pointing.
The Struve
82” (top) and Harlan J. Smith 107” (bottom) are also occasionally available for
Special Viewing Nights.
All Special
Viewing Nights start 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 probably 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.
Unfortunately,
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. During the day, it’s possible to drive up onto either of
the observatory peaks, park and look around (the HET has its own free
interpretative centre), but that’s off-limits after dark and they bus you up for
the viewing. 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.
The road up
to McDonald from Fort Davis in the south. The 36” dome is just visible below
the 107” on the right.
The 36”
lies off a service road that winds behind and below the looming
white bulk of the 107” observatory and past the astronomers’ lodge where you
can go for warm-up breaks and the loo. The road doesn’t seem much used and I
took lots of photos from it later (though it is totally dark – don’t lose your
tripod like I nearly did!)
If you bring
binoculars (they recommend it) to enjoy the star-filled sky whilst waiting your
turn at the eyepiece, there is a small terrace area outside the observatory
door and a nice broad wall to lie on too.
The little
36” dome is right below the 107”, at the bottom of a steep slope. The
operator’s manual cautions of possible ice falls from the big dome looming
overhead. Inside the dome it’s a little cramped, but there are enough chairs to
accommodate groups of 10-15. TIP: I took one of two chairs left of the door
(most are to the right), a good location if you want to keep popping out to
enjoy McDonald’s breathtakingly dark skies.
It does get
cold, so dress up nice and warm, but I found the closed dome a more comfortable
environment than the open roll-off observatory at Kitt Peak, for example.
Our guide
and telescope operator – a keen local amateur astronomer - did a great job both
of operating the scope and of interpretation. He showed us on-screen images of
every object we looked at, carefully selected from his own image collection
(taken through his C14) and from ones found online to match what we would see
through the 36”. He gave a brief talk about each one in advance and then giving
another quick description of the object as each person stepped up to the
eyepiece.
After many years of viewing
faint fuzzies through small scopes, I found the DSO viewing really enthralling because so much
normally-photographic structure was visible and yet we were able to cover many
more objects than most large scopes I’ve visited.
Others expected Hubble images in rich
colour and seemed genuinely disappointed not to get them. One guy told me
ruefully that he intended to stick to the NASA channel on his 50” LED TV in
future. Another, who’d been bought the experience as a birthday gift by his
wife, seemed really disappointed too. As usual with such evenings, I was the
only other amateur astronomer.
Enjoying
binocular views of the dark sky in between turns at the eyepiece.
Summary
The McDonald
36” Special Viewing Night was one of my best public observatory experiences so
far and it really is cheap for what you get to see. The organisation, telescope
and interpretation were hard to fault.
McDonald’s
36” DK may have been built for photometry and lack the wide corrected field
that a modern Ritchey Chrétien would have, but it gave some really interesting
views of a big range of smaller DSOs and doubles. I’m not sure how it would
perform on planets though, because only Uranus was available and we viewed it
first in unstable dusk seeing.
The McDonald 36” DK makes an excellent
outreach tool, with its combination of relatively large aperture and fast,
accurate pointing. If you’re an amateur astronomer interested in the deep sky,
a Special Viewing Night on it is an outstanding experience not to be missed.
