AO – Page 4 – A Darker View

Just a Little Nudge

One of the tasks I have helped with on the K1 AO Laser is aligning the Launch Telescope Assembly (LTA). This is not so much an electrical engineer’s task, but a mechanical one. So how did I end up with the task? Simple, I was present the first time it was done. Since the mechanical engineer who was first responsible has now left the observatory, I get the job by default. Just the way things work around here.

The launch telescope is a small telescope, about 0.5 meter in aperture, that projects the laser into the sky. Mounted behind the secondary mirror of the Keck 1 telescope, it must be precisely aimed to exactly the same spot in the sky the main telescope is aimed.

Launch Telescope Adjustments — Adjusting the mounting of the K1 Launch Telescope

The procedure is not all that difficult. Mount two dial indicators in place, restrain the motion by using wooden wedges or a really big c-clamp, loosen the bolts, make the adjustment, tighten the bolts. No problem, right? The challenge is to move the large assembly just a few thousandths of an inch and have it stay exactly where you want it while re-tightening the bolts.

In practice this adjustment is about one to two hours crouched in the secondary assembly of the Keck 1 telescope. Each time the bolts are tightened the assembly moves about 10-15 thousandths of an inch. Thus I have to guess how much to offset the measurement so it ends up correct when the bolts are tight. It takes anywhere from three to five repetitions to get right sometimes.

This is where I curse the mechanical engineer who dreamed up the mounting for the launch telescope. I look at the dial indicators, shift my stance against the cold steel to stop the cramping, loosen the bolts and try again.

The last adjustment was a mere 0.004″ (four one-thousandths of an inch) to move the pointing about 30 arc-seconds on the sky. I am now only about 10 arc-seconds from the optical axis. Keep in mind that a single arc-second is 1/60th of an arc-minute, which is in turn 1/60th of a degree. Ten arc-seconds is pretty good, but we want closer. Here I go one more time…

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.

Laser Launch Telescope

A big milestone arrived this week. The launch telescope arrived from the manufacturer. It is an impressive instrument itself, a half meter aperture, an extraordinarily short focal length, made to very exacting precisions, even for optics. It is a cassegrain design with an focal ratio of f/1. The primary is coated with a custom coating designed for maximum reflectance at the 589nm sodium D line, where the laser will operate. This gives the primary a notably orange cast that is quite beautiful. The entire telescope is enclosed in an airtight aluminum shell with the optics supported on a carbon fiber frame within.

This is one of the key components in creating a laser for the Keck 1 adaptive optics system. An enormous amount of work has gone into preparing for the laser… modifications to the structure of the Keck 1 telescope, cabling, electrical power and liquid cooling plumbing. An insulated room built on the side of the telescope to house the laser and a safety system to comply with laser safety regulations. All of this in preparation for the arrival of two key components, the laser itself and the launch telescope that will focus this light into a clean beam rising into the night sky over Mauna Kea.

The entire assembly will mount behind the secondary of K1. This is different than the current laser in Keck 2 that is emitted from a launch telescope along the side of the main telescope. Having the laser on the side creates some problems for the AO wavefront controller, the artificial guidestar can be elongated by parallax when seen from the other side of a ten meter telescope. Having the laser launched from the center of the main telescope is a far more optimal solution. But doing that takes a far more difficult to design and build launch telescope as it has to be extraordinarily compact.

We expect delivery of the laser itself in May. Our laser engineer has been working with the laser manufacturer to insure it meets all specifications. Reports indicate it is not only meeting those specifications, but surpassing them. When we take delivery there will be a period of testing before the equipment is trucked to the mountain for installation in its final position.

The coming week will be fun, need to string a new set of cables to the Keck 1 secondary mirror. This means 150 feet of cabling from the Nasmyth Deck, up the tubular structure of the telescope and across the spider to the secondary. On a ten meter telescope this means a lot of high work from the personnel bucket of the jib crane among the girders of the telescope. This is going to be fun!