I have previously covered the importance of warping, tuning the Keck primary mirror segments for optimum optical performance. Warping has been my responsibility for some years now. Reading out the settings of the thirty strain gauges on the back of each segment is performed by a test fixture, a computer and a sensitive data acquisition system. Over the last year I have designed, built, and programmed a new test fixture.
The old warping fixture was showing its age. Built in 2000 it has been in use for 16 years. It is the computer that I was most worried about, it has begun to crash randomly, usually at the worst possible time. Replacing the computer has some issues as well, the A/D system uses a parallel interface, something not found on any modern computer. The operating system is Windows XP, while unsupported, at least you can still install and use this old operating system. The software is in an ancient version of LabView. I have no love for LabView, too many bad experiences with it, it crashes too often and the licensing issues are horrible.
As this is the third generation warping test fixture the name of the software is obvious… Warp3
Warping is not much fun. Warping is now on my list of responsibilities. At least I know I am accomplishing something critical to the operation of the telescope.
Warping is a process of tuning the performance of a mirror segment after a segment exchange. A segmented mirror offers large advantages over a monolithic mirror, not least of which is the ability to swap a few segments out for re-coating and refurbishment without the weeks of downtime needed to re-coat a monolithic mirror. Throughout the summer Keck schedules a couple days of SegEx each month, so that at the end of the summer we have a completely clean and re-coated mirror.
Exchanging segments does require some interesting procedures to realign each new segment, each must be warped and the edge sensors tuned. The first few hours of the night after a SegEx is used to evaluate the performance of the newly replaced segments. Using a special alignment camera system the optical figure of each segment can be evaluated and a set of corrections generated to be applied the next day… Warping.
There are six segment types that make up the primary mirror, each with the slightly different curve needed to make up the correct part of the hyperbolic curve. In theory the segments are interchangeable, any type four can be swapped with any other type four. This works… With a little help. It is necessary to adjust the figure of each segment, just slightly, to tune the figure of each segment for its place in the array.
To apply the correct pressure there are small knobs and screws at specific points in the whiffle tree. Each adjustment point also contains a strain gauge, allowing the applied pressure to be measured precisely. A computer and analog interface allows all of the points to be read out and checked against the calculated values.
There are thirty adjusters and strain gauges on the back of each mirror segment. The problem is that you can not simply adjust each one. Adjustment of one point affects all of the nearby points, particularly if the adjustment is large. Typically it is necessary to go around three times before the segment is properly warped. Thirty adjustments becomes ninety. Three segments in a day becomes 270 knobs to turn, 540 over two days, a lot of knobs.
After setup, it takes about an hour to do each segment, an hour of painstaking frustration. the mirror cell is just the right height, too high to sit down and reach the knobs, too low to stand up fully. Working in a jungle gym of frigid steel just makes it worse. A day in the mirror cell is a nice recipe for a tired and sore body.
How careful was I? Did I get all of the points set correctly? The computer is displaying all of the correct numbers. I will not know until the next day, when the night’s performance data is reduced, when we can see the figure of the primary mirror and check the errors.
My first warp is a success, most of the segments show less than 20nm rms error. Next SegEx there are only two segments being exchanged, but Sergey is threatening to have two others re-warped to address some lingering issues. Four? Better than six. Only 360 adjustments to make, more or less.
Postcard from the Summit – SegEx
Postcard from the Summit – Securing the Segment
Postcard from the Summit – Segment Exchange
Postcard from the Summit – Mirror in the Hall
The radio call goes out… “Mirror in the hall, mirror in the hall!” Everyone gets out of the way as a mirror segment is rolled down the central hallway. It is being moved to the coating facility where it will be cleaned, stripped and re-coated with a layer of fresh aluminum.
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.