The Dunn Solar Telescope

[Doug Kerr]

From many locations in Alamogordo (sadly not from our home), one can see atop a mountain ridge about 10 miles southeast of town a stunning white obelisk:

Douglas A. Kerr - The Dunn Solar Telescope from uptown Alamogordo​

We are in Mescalero Apache country, but this is not a new-age tipi. It is the external tower of the Dunn Solar Telescope, the major instrument at the New Mexico site of the National Solar Observatory, It is located in the tiny town of Sunspot, New Mexico, on Sacramento Peak, about 10 miles southeast of our house in Alamogordo, at an altitude of about 9250 ft MSL.

This unique telescope has a 329 foot long vertical column, under vacuum (to avert disturbance to the image caused by temperature variations in the air that would otherwise be in the column). The primary mirror, at the bottom of the column, is 64 inches in diameter, and has a focal length of 180 feet.

The overall arrangement is shown in this figure:

Schematic of the Dunn Solar Telescope ​

Courtesy of the National Solar Observatory​

The sun's image is formed about five feet above one of six quartz viewing ports located just off the centerline of the column, about 175 feet above the primary mirror. The mirror is slightly tilted to direct the image to the port currently being used. The port cluster is at the center of a 40-foot diameter circular instrument table.

The sun's image is about 20 inches in diameter. Some of the instruments (such as the major spectroscope) are essentially vertical, and receive the solar image directly. Other instruments have horizontal axes, and receive the solar image via a mirror above the port. One port is devoted to an instrument that aims the telescope and periodically confirms the proper tracking of the sun.

The upper part of the column is housed in a pyramidal tower extending 136 feet above the earth, with concrete walls 3' thick. We see it in this picture:

Douglas A. Kerr - The external tower of the Dunn Solar Telescope​

The remainder of the column is in a 228' deep underground "silo". The instrument floor is at essentially ground level.

At the top of the tower (and the top of the telescope column) is a rotatable turret, containing two 44" diameter mirrors in sort of a "periscope" arrangement, looking out through a 30" diameter quartz window at the sun. One of the two mirrors (in the sphere on our right) can be tilted to change the aim of the telescope in altitude. The entire turret is rotated to change the aim in azimuth. We see it here:

Douglas A. Kerr - Turret of the Dunn Solar Telescope​

We think the spoon-shaped member below the altitude sphere is a "pillow" toward which the observation window is directed when the instrument is not in use, so as to protect the surface of the window.

Because of the geometry involved, as the azimuth or altitude are changed, the image of the sun developed by the telescope rotates. This of course would disrupt long photographic exposures or other observations as the sun's position is "tracked".

To compensate for this in a very direct way, the entire telescope proper (including the instrument table and all the observing instruments) rotates (independently of the turret). To allow this, the rotating assembly (weighing about 250 tons) hangs from a mercury float bearing near the top of the tower. It contains 10 tons of mercury. The turret also rotates on its own mercury float bearing. The turret is also under vacuum (in common with the column), and is joined to the column through a mercury rotary seal.

[continued]

 

[Doug Kerr]

[Part 2]

Carla and I today (2012.05.27) visited the National Solar Observatory and the Dunn Solar Telescope (it was Carla's idea - she knew I had been studying the instrument.). We were able to just walk, unaccompanied, into the telescope tower and walk around freely. "Please stay outside of the blue ropes, so as to not go onto the instrument platform. Please do not disturb the scientists that may be working there".

Here we see the scene inside; we see the only person working there at the time (he entered just as we did), at a computer console.

Douglas A. Kerr - Inside the Dunn Solar Telescope​

We can see the small gap between the stationary outer floor (on our left) and the rotating instrument table (on our right). The lighting inside the tower was very low; I shot at ISO 3200. (There are brighter lights that are activated for "group tours", but screw that.)

Here we see the main telescope column as it passes floor level (on the right) and on the left, the instrument that is used to confirm the proper tracking of the sun.

Douglas A. Kerr - The telescope column (r.) and Guider (l.)​

Here we see some of the "optical benches" where special-purpose instruments can be assembled to make particular measurements. They receive solar images from a couple of the floor ports through diagonal mirrors.

Douglas A. Kerr - Optical benches​

It was a lovely visit, The trip was about a 35-mile drive through stunning mountain scenery. The temperature at the Observatory was 62° F (about 95° F in Alamogordo, albeit with lovely low humidity), with an brisk breeze singing through the incredibly tall pine trees, whose aroma permeated the whole establishment.

Best regards,

Doug

 

[Tom Robbins]

Thanks for a fantastic essay on an incredible place, Doug. The photos you included were orders of magnitude better than others I found with an internet search of the Dunn Telescope.

 

[Doug Kerr]

Hi, Tom,

Thanks for a fantastic essay on an incredible place, Doug. The photos you included were orders of magnitude better than others I found with an internet search of the Dunn Telescope.

Thank you so much.

Best regards,

Doug

 

[Doug Kerr]

A plus while visiting the Notional Solar Observatory at Sacramento Peak, New Mexico was that we saw our first robin of the season (we have many birds in our back yard, but had not yet seen a robin).

He hopped up on a stump and hit his marks. "This is picture."

Douglas A. Kerr - Robin at 9250 ft MSL​

Best regards,

Doug

 

[Bob Latham]

A fascinating insight into the place Doug. The turret looks far more nautical than scientific...I can imagine it atop the bridge on some oil exploration vessel.

Bob

 

[Doug Kerr]

Hi, Bob,

A fascinating insight into the place Doug. The turret looks far more nautical than scientific...I can imagine it atop the bridge on some oil exploration vessel.

Oh, indeed, or atop a submarine in a non-futuristic fantasy drama.

But it is a wondrous thing indeed.

Best regards,

Doug

 

[Doug Kerr]

Although the instruments at the National Solar Observatory at Sacramento Peak, New Mexico are extremely sophisticated and built with great precision, there is still a wonderful air of straightforward, "head-on" design and robust construction to the whole place.

A good example is the concept of the Dunn Solar Telescope itself. In order to nullify the rotation of the sun's image that occurs when the sun is tracked using the azimuth-elevation mount of the Dunn's intake turret, the solution was to rotate the entire telescope, including the 4-foot diameter floor on which many of the instruments are deployed - 250 tons altogether.

And the Dunn's construction was overseen by the U.S. Army Corps of Engineers (the facility was in fact originally sponsored by the U.S. Air Force).

Another wonderful example comes from the observatory's earlier days. A "dome" was needed to house one of the earlier refracting telescopes. The staff ordered a corrugated iron circular grain bin from Sears, Roebuck (shipped knocked-down), mounted it on a turntable so it could revolve, and added a system of sliding doors to expose the slot through which the telescope would view the sky.

Here we see the "Grain Bin Dome", as it is known today:

Douglas A. Kerr: The Grain Bin Dome at Sacramento Peak​

Now, moving into a quite different context, we see this same motif of straightforward design and robust construction amply followed in the flush control for the urinal in the men's rest room at the observatory's Visitor Center:

Douglas A. Kerr: Press​

Best regards,

Doug

 

[Bob Latham]

Now, moving into a quite different context, we see this same motif of straightforward design and robust construction amply followed in the flush control for the urinal in the men's rest room at the observatory's Visitor Center:

Douglas A. Kerr: Press​

An item of questionable beauty Doug, although perhaps appealing at the end of a longish journey. Should you plan to return in the future then I'd suggest employing a TS-E24 shifted down (from the same vantage point) to eliminate the distorted perspective and highlight the porcelain so that we can see it through the designer's eyes.

Bob

 

[Doug Kerr]

Hi, Bob,

An item of questionable beauty Doug, although perhaps appealing at the end of a longish journey. Should you plan to return in the future then I'd suggest employing a TS-E24 shifted down (from the same vantage point) to eliminate the distorted perspective and highlight the porcelain so that we can see it through the designer's eyes.

An excellent recommendation. I could perhaps have borrowed one of Will Thompson's set of 19 TS-E lenses.

Best regards,

Doug