Arcsecond to None

In the middle of the Pacific Ocean, thousands of miles of thermally stable seas surround the Island of Hawaii. The 13,796-foot Maunakea mountain summit has no nearby ranges to roil the upper atmosphere. For most of the year, this atmosphere is clear, calm and dry, enabling the W. M. Keck Observatory, with its twin 10-meter-mirror telescopes, to observe the Milky Way galaxy and beyond at unparalleled levels.

Now, after the completion of a significant nine-year motion control upgrade project, the Keck Observatory telescopes, standing 30 meters tall each, are offering data and observations with an impressive nanometer precision.

“We are now able to blindly point the telescopes to any star in the sky within Keck’s observable area with an accuracy of 1.0 arcsec,” explained Tomas Krasuski, the Lead Electronics Engineer at Keck Observatory who played an important role in the design and implementation of the telescopes upgrade project, begun in 2009. “That is an accuracy level of one thirty-six hundredth of a one degree.”

Since the Beginning
The twin Keck Observatory telescopes are the most scientifically productive optical and infrared telescopes in the world. Each weighs more than 300 tons and hosts a 10-meter-diameter primary mirror, the largest light-collecting mirrors on the planet.

“We are an extremely important resource for researchers interested in many areas of astronomy and astrophysics – including the discovery of exoplanets; the study of how planets, stars and galaxies form; the nature of black holes; and the chemical composition and evolution of the universe,” Krasuski explained.

When Keck Observatory began science operations in the early ‘90s, it was the first generation of very large groundbased optical/infrared telescopes with segmented primary mirrors. The telescopes worked very well utilizing technology available at that time.

After 20 years, however, some replacement components became hard to find, putting the telescopes at risk of a major failure. Also, the established measurement system included rotary encoders for measurement that were subject to periodic errors. So, following an obsolescence study, the need for renovations became clear.

Launched in 2009, the Keck Observatory’s Telescope Control System Upgrade (TCSU) project set out to not only update the systems but also improve the telescope pointing, tracking and offsetting performance.

Over the next nine years, both the Keck I (K1) and Keck II (K2) telescopes had upgrades done on all major elements including telescope controls, safety systems and rotator and secondary mirror controls. Due to a switching solution between the old and new control systems, the TCSU team was able to complete the specialized upgrade during the day and re-establish operational systems by nighttime.

“TCSU was a complex and challenging project that involved multiple subsystems,” Krasuski explained. “Our team decided to upgrade all at the same time instead of consecutively to reduce the need for regression and repeated cross-compatibility testing.”

At the beginning of the TCSU project, Keck Observatory engineers explained that a significant part of the project was the installation of new telescope azimuth and elevation position encoders based on HEIDENHAIN’s 40-micron grating tape scales. Interpolated to a 10-nanometer resolution with a HEIDENHAIN EIB 749 box, these new ERA 84XX tape encoders promised to provide Keck Observatory with a true 4mas (milliarcseconds) resolution in azimuth and a 1mas resolution in elevation.

“In the end, these new HEIDENHAIN tape encoders performed brilliantly,” Krasuski explained. “Their installation required significant changes to our mechanical infrastructure in order to house them so new designs were developed over time to get it just right.”

Design Strategy
Many telescopes utilize optical encoder tapes by placing the azimuth encoder tape on a dedicated stationary ring which is at or near the diameter of the azimuth bearing. Instead of doing this, the TCSU design team decided to pursue a shorter encoder tape on a dedicated ring located near the center of the telescope axis.

This ring has an O.D. of 1.15 meters that could accommodate an off-the-shelf full circle HEIDENHAIN ERA 8400C encoder tape 3.6 meters long. This was a viable solution on K1 and K2 because of the existing accessory mirror mounting structure below the moving telescope tube. A connection to the telescope could be made here that would rotate an encoder in azimuth.

This ring and tape were fixed to a top flange of a steel support column. The connection from the encoder read head ring to the telescope structure was accomplished with a thin-wall stainless steel bellows tube that transfers rotational motion of the telescope to the read head rings, without lag, wind-up, hysteresis or vibration.

There are four azimuth encoder readheads that now move with the telescope. The read heads are spaced evenly on a flat ring that floats on the larger (fixed) tape ring by means of frictionless air bearings by New Way. The azimuth bearings, tape and read heads are surrounded by a clear sealed air shroud, except for a 0.5mm gap where shield meets top flange. Additionally, two tilt meters were installed.

“The encoders and the tilt meters are the primary reason for achieving the greatly improved pointing and tracking performance,” Krasuski explained.

A HEIDENHAIN optical tape encoder for the elevation axis was also installed on a bar-code surface on the right-side elevation drive sector of each telescope. Here, two optical read heads were installed on the inside of each main yoke structure near the elevation-drive assembly.

The elevation angular motion is measured by these pair of read heads which are carried on an articulating mechanism. A pair of rollers bear against the surface with 1.5N of spring force, and a linear stage provides controlled radial motion. This system allows extremely precise raising and lowering of the telescopes.

Absolute Positioning
The encoder tapes used in the TCSU systems are incremental encoders with center strip markings evenly spaced at 40 microns. They do not code absolute position until initialized using HEIDENAHIN-provided distance-coded reference marks just below the incremental marks.

To initialize position, the encoding software must read three reference marks: Two evenly spaced marks and one mark that is a distinct number of counts from the other two. The distinct count is used to determine the absolute position to a 40-micron accuracy. Once the absolute position is known, then the incremental tape is used as an absolute encoder. In the end, the TCSU project was able to achieve better performance for telescope pointing, open- and closed-loop tracking and repositioning.

“This means we can move these giant telescopes anywhere in the sky with an accuracy of 1.0 arsec,” Krasuski said. “Some field of views have many stars in them, so it is very important for our astronomers, to have our telescopes zero in on their target quickly and accurately.”

Krasuski added that Keck Observatory is now able to track up to an elevation of 89 degrees while meeting their tracking accuracy of 0.050 arcsec RMS (root-meansquare). They can track at this accuracy for an extended period of time, sometimes for six hours or more.

Currently, the TCSU upgrade outperforms the old system by a factor of 2, offering an even more exceptional view of the universe than ever before.

“These installed encoders work brilliantly well and meet the high standards we had heard and expected from HEIDENHAIN,” Krasuski said. “To move these gigantic telescopes to the accuracy of 10 nanometers is absolutely amazing. This is truly where the world of large and small meet.
www.keckobservatory.org
www.heidenhain.ca

Article contributed by HEIDENHAIN