A Few Grams of Aluminum

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.

Keck 2
Looking into the optics of the Keck 2 telescope
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…
2.70g/cm³

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.

Mercury in the Sunset

Mercury
A crescent Mercury, 16Apr2010
Earlier this month, as Mercury was slipping back into the glare of the Sun, I had an opportunity to shoot some webcam material of the planet in hopes of getting an image of the crescent shape. The resulting image does not look like much, but I have to think it really isn’t all that bad.

The photo does represent Mercury fairly well, at least the normal view you get in a telescope. As the innermost planet does not get very far from the Sun, it is typically seen quite low on the horizon. This leads to poor views seen through a great deal of atmospheric distortion.

What the photo does not show is the chromatic distortion, this was corrected during processing of the photo. The atmosphere will also break up the color, refracting the light when an object is low on the horizon. The processing software allows realigning the color planes, correcting much of the effect.

A Smoother Saddle Road

It has been a week since the paving machine began it’s slow work. Gone is the patchwork of pavement, a road seemingly built by many years of repair crews, so many patches that little remained of whatever pavement originally existed. Bit by bit the ragged road we have bounced over for many years is being covered by a smooth surface.

Paving Saddle
Crews laying asphalt on Saddle Road near MP40 in 2009
The machine has reached milepost 48, a half mile more than that in the Kona bound lane. The first layer mostly completed by the county crews. From there to the district line the lanes are pleasant and smooth drive, such a contrast from the old pavement. This latest segment leads to the section that was paved last year, from MP48 to the rebuilt sections across PTA, the road is nothing like the rough experience of Saddle Road past. The only rough section remaining is the few miles from MP48 to the western terminus at Mamalahoa highway.

While making a pass in each lane, the crews left about a foot in the middle unpaved for now, keeping the center line exposed, and creating a road a few inches wider. Breaking with tradition, no one drives the center of the road in the repaved section, avoiding the small trough created between the lanes.

The infamous Saddle Road of fable and legend is vanishing, repaved or completely rebuilt. Those of us who drive it regularly enjoy the new smooth ride, but in some ways we also mourn the disappearance of the real Saddle Road.

A Box ‘O Broken Telrads

The Telrad finder is one of the most useful telescope accessories ever invented. A set of glowing red rings showing you, at a glance, exactly where your telescope is pointed in the sky. I have one on each of my telescopes. The Mauna Kea VIS also equips each telescope with one of these simple devices.

They do not work so well after hitting the ground a few times.

As I have mentioned in the past, the equipment at the Mauna Kea VIS gets used hard. It is setup every single night of the year. Thousands upon thousands of people use these telescopes to see the wonders of the night sky, the first time for many. The wear and accidental damage in the darkness takes a toll.

Broken Telrads
A box full of broken Telrad finders awaiting repair
When Deb and I were last at the VIS we spent the day cleaning eyepieces and making other repairs to the ‘scopes. One of the things I found in the storeroom was a small pile of broken Telrads. Some were missing windows, many had broken battery holders, mirrors were missing and reticle holders hanging loose. Many had been patched back together with tape or hot glue, attempts to keep them working for another night.

Quite a few had reticles that were missing or melted by exposure to sunlight. The lens that focuses the reticle’s ring pattern, projecting it into the sky, will also focus sunlight on the reticle, quickly melting the thin film if a Telrad is left in the sun.

Gathering up partial and scattered parts I collected a box of finders that I can work on later. It made quite a pathetic sight, a box of broken Telrads. A couple evenings later, five of the Telrads are now rebuilt and ready to return to duty. Four more are awaiting replacement reticles before I can call them completed. I will take them back up next time we are on the mountain, but I expect we will find something else that needs to be fixed.

What can go wrong?

I mentioned in another recent post that astrophotography is an art of details. Dozens of little issues must be dealt with, failure to properly address even one item, and hours of work can be lost. Each technical issue must be understood, and a solution found through technique and experience.

The saving grace of modern astrophotography is that once all the equipment has been set up and the details under control, the process becomes automatic. The computer, telescope mount, and camera operating for long periods of time, often hours, with no human intervention. One of the critical functions to long exposure photography is an autoguider, a small, automated secondary camera that keeps the telescope and mount pointed at exactly the same point in the sky for the entire exposure.

Guiding Error
An exposure of the Christmas Tree Cluster is guided into oblivion when the ST-4 autoguider loses the star
Last Sunday all was going well, several hours of exposures looked pretty good, at least on the little LCD display of the camera. The astrophoto gear had setup smoothly, all systems checked out and running well in just a few minutes after rolling the rig out of the garage. Mental checklist complete I hit the button to start exposures and left the gear to do it’s thing.

The first hint of trouble was the display on the autoguider. I did not see this until I went to stop the series of exposures I had hoped was complete. The guider was displaying large guide errors each cycle, where I should see zeros, or at least small numbers, it had E’s, a bad sign with the venerable ST-4 autoguider.

Hoping for the best, but fearing the worst, I rolled the ‘scope back into the garage and setup the camera to take calibration frames. I would discover the truth later, after I got a few hours of sleep.

When I downloaded the memory card the extent of the problem becomes apparent. Much of the last sequence of photos is ruined. The guider clearly lost the star somewhere into the third exposure, reason unknown. What resulted was twelve exposures with ziz-zag star trails across them, well over an hour and a half of wasted exposure time.

All of the evening’s early exposures were fine, only the last sequence was ruined. Thus, the evening was not a total loss, and I do have some new material to process. Now to figure out what went wrong with the autoguider, probably just some small detail I missed.

A Beam of Yellow to Pierce the Sky

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.

Laser Optics
A sodium yellow laser propagating through the optics of the Beam Transfer Optical Bench

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.

First laser Light
The Keck 1 Laser piercing the sky for the first time on 28 Jan 2010

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.

Continue reading “A Beam of Yellow to Pierce the Sky”

Zodiacal Light

A strange glow visible long after sunset, or well before dawn, along the path of the Sun in the sky. Called the “False Dawn” by Muhammad in Islamic texts and by other classical sources, this glow is often confused with the light of dawn, or simply overlooked by many.

Zodiacal Light
False dawn, actually zodiacal light, rising over Mauna Kea
Once thought to be the extended atmosphere of the Sun, the zodiacal light was recognized as something interesting by early scientists. The real reason for the glow was not understood until described by Nicolas Fatio de Duillier in 1684.

The answer is simply dust.

Space is not only very big and mostly empty, it can also be dirty. Dust from comets, dust from asteroid collisions, dust floating in orbit about the Sun, reflecting sunlight and glowing across the sky. Because this dust lies in the plane of the solar system the glow is visible as a band along the ecliptic and through the constellations of the zodiac. The dust is thickest, and the glow brightest nearer the Sun, thus the glowing dust is best seen when the Sun is just out of sight, after dusk, or before dawn. The light is simply reflected light. Looking at the spectrum of the zodiacal light shows it to be sunlight, with the same spectral features.

The zodiacal light can be quite bright, clearly visible to any who look in the hour after dusk, or before dawn. It can be bright enough to be a nuisance to amateur astronomers attempting to view through their telescopes. At the same time it is a sign of clear dark skies, as it is otherwise hidden by the glow of artificial light. The slightest amount of moonlight or light pollution and this glow disappears from sight.

The effect can be seen completely across the sky, along the ecliptic, as a very faint glowing band when viewed from the darkest of places. Directly opposite the Sun in the sky is a brighter spot in the glow, the gegenschein or counterglow. A form of glory, the dust reflects sunlight back in the direction it came.

The zodiacal band and the gegenschein are both visible from Mauna Kea on a dark night. From this dark place the most subtle of astronomical spectacles can be appreciated.

Lunar Halo

A lunar halo may be quite pretty, but along with pretty sunsets, it indicates lousy sky conditions for astronomy.

In this case a high layer of thin cirrus, consisting of small ice crystals, cover the sky. The crystal shape is just right to bend the light at a specific angle, creating a halo 22° from the light source. A faint hint of color can be seen at the inner edge of the halo.

Lunar Halo
Lunar 22° halo over Mauna Kea

Meteor Fireballs

Thanks to all of those folks who posted comments about meteor fireballs. As I mentioned, these sort of events are not all that uncommon, and occur regularly across the globe. What is unusual is good photos of the event. This did happen this week, a photographer had a good camera at the ready when a similar fireball came in over Groningen, Netherlands on October 13th. Great photos of an event much like the one we saw last week over Hualālai. Check out his page for the full set of photos including the final breakup, the website is in Dutch, but a Google translation will let you follow the description.

Mikaelyan Meteor
A meteor breaking up over Groningen, Netherlands, 18:57, 13 Oct 2009, photo by Author AndrewPosted on Categories Meteor ShowersTags , Leave a comment on Meteor Fireballs