One of the miscellaneous systems in the observatory that I have inherited is the weather station. A critical set of gear that has been neglected far too long. Neglected to the point our telescope operators had been complaining, loudly, about a system that frequently gives erroneous data or provides wildly oscillating readings.
The weather station is critical in protecting the all important optical surfaces of the telescope. The mirrors that gather light from distant galaxies depend on a thin coating of aluminum that is easily damaged. Snow, ice, fog or even simple dew can damage the coating and require the mirror segments to go through a laborious re-coating process. Thus the operators monitor the weather closely, when fog and humidity roll in, alarms go off, and the great shutters are closed to protect the telescope.
The first part I have replaced is the humidity and dew point sensor. In many ways the most important part of the system. The new unit is a modern sensor with a direct ethernet interface, simple to link into the observatory network. This is the same sensor used by the National Weather Service in their remote weather stations. All I had to do was spend a little money, and spend a day hanging off the weather tower on the observatory roof installing it. It was a beautiful, warm, sunny day up there, I got the job done, and got a slight sunburn in the process.
I had expected it to be a day of 20mph winds and freezing temperatures. What I got was a balmy 11°C (52°F) and just a gentle breeze. All for the good as I planned to spend several hours hanging off the weather mast installing wiring to improve the new dew point sensor. A cold wind can quickly turn the roof of the observatory into a miserable place.
The original sensor housing had proved to be vulnerable to heavy icing. The new housing should be more resilient, as well as providing better daytime temperature readings. This is due to changing to a different shelter design that uses a fan to move air through the housing past the sensor. I also modified the housing with the addition of a heating element to allow de-icing.
To make the heating element I needed heavy nichrome wire. Not having any on hand I took a trip to the thrift shop. There I bought a used toaster for a couple dollars and spent an hour dismantling the toaster to remove the heating elements. I took the wire and wrapped it through the interior of the instrument housing, creating a heating element that should work quite nicely with a 12V supply, gently warming the housing and melting any ice.
A beautiful day on the summit, nice to spend a few hours atop the roof, hanging in a safety harness from the weather mast. I even remembered to put on some sunscreen to avoid frying in the high altitude sunlight. A new cable pulled through the conduit, the instrument shelter replaced, a little further wiring inside and the job was done. I will have to await another round of bad weather to see if the changes work, but given the trend this winter, I will not have to wait long.
Until the phone rings… The caller ID shows K1 Control.
Noooo!! It is Christmas Day!
What is wrong now? Time? 4 pm, daycrew should be doing final checks before releasing the telescope, just the time things usually go wrong. ACS?!? It has been creating a lot of trouble lately. Autofill? My usual problem child… No, I checked that already today. HIRES? Tonight’s instrument that I really know nothing about. Nothing to do but answer the phone…
Just Robert calling to say everything is fine and Merry Christmas.
I am waiting for the Moon to leave the evening sky before shooting the comet again. In the meantime I am processing more of the material obtained earlier in the month. In this case a photo of Comet 103P/Hartley 2 taken October 6th with Keck 2 and DEIMOS. The image marks the first time I have attempted to take and process an image with a 10m telescope. Just a wee bit larger than the 76mm refractor I usually use to take astrophotos!
The image is notable for its complete lack of any interesting structure. There are no jets, shells or other inner coma detail visible. The tail is simply a general brightening to the southwest (lower right in this image).
The comet is moving very quickly across the sky, even more so with the high magnification lent by a large telescope. Even short exposures turn the stars into long streaks. In this case multicolor streaks as the camera cycles through the filters needed for a color image.
A telescope relies on the quality of the primary mirror. The shape must be exquisite perfection, with errors measured in millionths of a meter. The reflective coating must also perform to high standards, reflecting well over 90% of the light across a wide region of the spectrum.
Keck observatory carefully monitors each primary mirror to insure it is performing accurately. Instruments can detect small variations in the shape, indicating where there may be trouble in the support structure and active positioning of the segments. The coating is tested for reflectivity, to insure as much precious starlight goes to the instrument as possible.
Keck uses pure aluminum to coat the surface of each mirror segment, chosen for its excellent reflectivity in the visible and infrared parts of the spectrum. It takes only 20.5 grams of aluminum to coat an entire Keck primary mirror. This thin layer of aluminum degrades with time, losing several percent of it’s reflectivity each year. Eventually it must be replaced.
Re-coating a mirror is a painstaking process of stripping the old coating, carefully cleaning the mirror, the placing the mirror in a vacuum chamber to deposit a new metal coating onto the glass. The process takes about a week per segment, with one full time technician dedicated to the task, with a little help to handle some of the more intense parts of the process.
An advantage of a segmented telescope is that individual segments may be swapped in a single day. Telescopes utilizing monolithic mirrors must shut down for weeks to remove the primary mirror, strip clean and re-coat. With spare segments available the maintenance crew can perform the task of re-coating on a reasonable schedule, without taking the telescope off sky for an extended period.
At Keck there is a special storage facility for segments awaiting re-coating and those that are ready for installation back into the telescope. The process is continuous, once the last segment is finished, it is time to start the rotation again.
The first step in replacing the old coating is to chemically strip the old coating. This is done in a special bay used only for this purpose. An acid solution dissolves the aluminum revealing the glass below. The mirror is the extensively cleaned to remove any remaining contamination. If the mirror surface is not perfectly clean, the new aluminum coating will not adhere properly. All of the chemicals used are caught in a closed system for proper disposal off the mountain.
Once cleaned the mirror is moves to a large vacuum chamber where the new coating will be deposited. Here the mirror is positioned face downwards. With the cover reinstalled on the chamber it will take most of a day to pump out the air and ready the chamber for coating.
Glow discharge is a method of cleaning a surface prior to vacuum coating it. A high electrical charge is placed on an electrode just below the mirror in a partial vacuum. The result is something like creating a storm of electrons to blow any remaining impurities off the surface of the mirror. It is also a very beautiful process, looking through the ports one can see a brilliant violet haze around the electrode with sparks flickering along it’s length.
The final step is to vaporize the aluminum itself. In the bottom of the coating chamber are arranged a number of electrodes, each made of pure aluminum. By electrically heating these electrodes a few ounces of metal is vaporized. In the vacuum this aluminum forms a cloud of metal that coats everything in the chamber, including the mirror segment positioned above the electrodes. An instrument measures the buildup of the layer and shuts off the current when the deposited layer reaches the desired thickness of 100nm.
The coating process takes only a few minutes once the electrodes are turned on. Peering in through the small view port a cheery red glow is seen from each of the electrodes at the bottom of the chamber. The view only lasts a few moments as the cloud of vaporized aluminum soon reaches the view port and the glow fades as the window is covered by a layer of deposited aluminum along with the mirror segment.
What emerges from the chamber is a mirror with a beautiful, reflective metal coating. A few tests will be performed to insure the coating meets specification. If all is well the mirror segment will be prepared for installation in the telescope. It will await another segment exchange when it will replace another segment that has become dull with years of exposure to the elements. That segment will then receive it’s turn in the coating chamber.
It is an observation I have made before, but one that continually amazes me… Each Keck telescope consists of three hundred tons of steel and glass, with one simple purpose, to hold a few grams of aluminum in the perfect shape necessary to collect the light from distant stars and galaxies.
Each segment of the primary mirror is covered with a very thin coating of pure aluminum, about 100nm thick, this is 1/10,000 of a millimeter or 0.000004 inches. Aluminum is used in the Keck telescope as it reflects over 92% of the light across a wide wavelength range extending from the UV well into the infrared.
The layer is just thick enough to reflect nearly all of the light, any thinner and too much light would penetrate the mirror, any thicker and small variations in the coating would begin to distort the shape of the mirror.
How much aluminum?
Density of Al…
Area of a Keck Primary…
36 x 2.598 x (0.9m)² = 75.75m²
Mass of Al…
2.70g/cm³ x 75.75m² x 100nm x 1,000,000cm³/m³ = 20.45g
20.45g = 0.71oz (if you prefer imperial)
Just how much aluminum is really on each Keck primary mirror? Simple enough to calculate… just multiply the surface area of thirty six hexagons by the thickness of the aluminum layer to figure the total mass of metal used.
The figures are found in the sidebar, and the answer is surprisingly little, about 20.5g. In comparison, an empty 12oz soda can weighs about 15g, thus it take a bit more than one soda can of aluminum to cover the Keck’s 10 meter primary mirror.
There is much more to a telescope than just one simple layer of aluminum. But that one component is critical. It is the mirror that gets a great deal of the attention. The primary mirror is what gives a large observatory the ability to capture light from the earliest eras of the universe, billions of years in the past.
Yellow light, specifically light at 589nm, the yellow glow of excited neutral sodium. A color of light familiar to anyone who has stood under the soft glow of low pressure sodium streetlights. A laser shining at 589nm, aimed high into the atmosphere, will encounter a layer of sodium atoms at an altitude of 90km (60miles). When the yellow light strikes this sodium it will excite the atoms and cause them to glow, creating a dot of light, an artificial ‘star’ in the sky.
An artificial star, a useful thing if you want to analyze the distortion caused by the atmosphere. If you can understand these distortions you can use the information to correct the images of an instrument looking though the atmosphere, creating sharp views of stars and galaxies, views vastly better than were possible before the advent of adaptive optics. Such system are now routinely used on large telescopes across the globe to allow a clear view of the universe we live in.
Adaptive optics systems are amazingly complex instruments. Hundreds of filters, lenses, mirrors and other optical surfaces interact with dozens of motorized stages and half a dozen cameras. Controlling the system are a horde of computers, some of which are specialized machines with impressive processing power. Everything must work in concert, the failure of one element can bring the whole system down.
A laser is not necessary for an operating AO system, but without it there is 70% of the sky that can not be observed, making a laser highly desirable. While the K2 AO laser has been operating for several years, Keck Observatory has never had a laser on the Keck 1 telescope.
Waimea was living up to reputation with a gusty wind and blowing mist. But this did little to dampen the Waimea Planet Walk sponsored by Keck and CFHT Observatories. A steady stream of parents and kids walked the length of the main street to visit the booths representing a scale model of the solar system.
The Sun and the inner solar system… Mercury, Venus Earth and Mars, all occupied the lawn directly in front of the Keck lobby. Each location was measured properly to achieve the correct position, each booth had a scale model of the planet to correctly interpret the scale of the entire solar system. This meant the Sun was about 8 inches (20cm) in diameter and the Earth a small dot mounted to a piece of wood. In the far corner of the Keck lawn, several hundred feet from the Sun, sat Jupiter represented by a 1/2-inch (1cm) ball bearing.
To visit the remainder of the solar system it was necessary to walk down Mamalahoa highway. Saturn sat in the upper corner of the KTA parking lot, beside the historic cemetery. Uranus was located in the lower corner of the KTA parking lot. Neptune in the park across the street from the library. At the end of the walk, Pluto and the rest of the TNO’s in the lawn of CFHT.
Everyone, volunteers and guests, seemed to be having a great time learning. All up and down the street walked parents and kids from station to station. It is always interesting to see the entire solar system represented to scale like this, even if you have seen it before. The experience is the first step to seeing just how big space is…
It started when we arrived at the morning rendezvous and noted the number of vehicles waiting. Transportation sets up up as many vehicles as necessary based on the ride board, usually two or three vehicles are sitting by the door waiting to transport our crew to the summit. That morning there were five, and we all knew from the schedule that many more would be leaving later in the day. This was going to be a busy and crowded day.
At Hale Pohaku we were met by a film crew. Documentary film crews are an occupational hazard at Keck. I have appeared in more than one show. Not usually a problem, this day the crew would be yet one more complication.
For myself, things started to go bad with an email message, trouble with a key piece of equipment in AO. At the heart of the adaptive optics system is a thin flexible mirror that can change shape to correct the light, the DM or deformable mirror. In order to monitor this mirror a WYKO interferometer is used to image its surface. This device shines laser light at the mirror, the return light is interfered with itself, allowing the surface shape to be analyzed with incredible accuracy. This is used to calibrate the AO system at the beginning of each night. Gone was my plan of a simple day doing some documentation checks to prepare for some upcoming modifications to the system.
For nearly a decade, Cal-Berkeley astronomer Geoff Marcy and his colleagues have been using the W. M. Keck telescopes to discover giant planets orbiting distant stars. Now, with the successful launch of NASA’s Kepler mission, they will be using Keck I’s ten-meter astronomical eye to discover distant Earths. Kepler will pick out Earth-like candidates. Keck will then zero in on them and determine, with certainty, if they are at all similar to our home planet.
“Keck and NASA have a long-standing partnership to push astronomy research to its fullest potential. This Keck-Kepler collaboration gives that partnership a compelling new scientific focus,” said Taft Armandroff, the Director of Keck Observatory headquartered in Kamuela, HI.
Kepler was launched from NASA’s Kennedy Space Center last Friday. Aboard the spacecraft is an 84-megapixel camera that will focus on a single region of the sky and snap repeated images of 100,000 stars looking for those that dim periodically. By studying the stars’ episodic decreases in starlight, astronomers will be able to determine the diameter of the object that passes in front of the star, blocks its light and causes the dimming.
“Kepler does not tell astronomers with certainty if the object taking a bite out of the starlight is a planet or another star. That is where Keck plays a crucial role to the Kepler mission,” said Marcy, a frequent Keck user and Kepler mission co-investigator. He, along with a large international planet-hunting team, has discovered nearly half of the 300-plus known planets outside the Solar System.