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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 its parabolic primary is paired with an elliptical secondary in place of the more complicated hyperbolic secondary employed by other Cassegrains. 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.

 

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