The installation of this new design with optical tape encoders on both telescopes began in 2014 and 2016. 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 read heads that now move with the telescope (Figure 3). 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 0.5-mm gap where shield meets top flange. Additionally, two tilt meters were installed in this area. “The encoders and the tilt meters are the primary reason for achieving the greatly improved pointing and tracking performance,” said Krasuski.
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 (Figure 4). Here, two optical read heads were installed on the inside of each main yoke structure near the elevation drive assembly for this. The tape is installed with distance-coded reference marks toward the primary mirror side of the telescope.
The elevation angular motion is measured by the pair of read heads that is carried on an articulating mechanism. A pair of rollers bears against the surface with 1.5 N of spring force, and a linear stage provides controlled radial motion. This system allows extremely precise raising and lowering of the telescopes (Figure 5).
Absolute Positioning Using Incremental Encoders
The HEIDENHAIN 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 HEIDENHAIN-provided distance-coded reference marks just below the incremental marks (Figure 6).
The encoding software must read three reference marks to initialize position. The three reference marks will contain 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, or angle, on the tape to a 40-micron accuracy. Once the absolute position is known, i.e. the reference, then the incremental tape is used as an absolute encoder. “We found that this works great,” said Krasuski.
In the end, the TCSU project was able to achieve better performance for the key requirements: telescope pointing, open-loop and closed-loop tracking, and repositioning (offsets).
“This means we can move these giant telescopes anywhere in the sky and have confidence that we’re pointing the telescope on our chosen star with an accuracy of 1.0 arsec,” said Krasuski. “Some fields of view have many stars in them, so it is very important for our astronomers, such as planet hunters, to have our telescopes zero in on their target quickly and accurately. This also helps in tracking.”
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-mean-square). They can track at this accuracy for an extended period of time. “Sometimes a researcher will track a single star for six hours or more,” he said.
Lastly, repositioning or offsetting refers to small moves of the telescope for the purpose of fine alignment, dithered observing, or nodding. At this time, the TCSU implementation outperforms the old system by a factor of 2, translating to improved times and efficiencies. “We are able to reposition the telescope while tracking a star by 1, 5, and 10 arcsec and still meet our accuracy requirements in <1 second, which is important for observing because scientists frequently need to do this during data collection,” explained Krasuski.
Keck Observatory now offers 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 HEIDEN-HAIN. This is truly where the world of large and small meet,” said Krasuski. “To move these gigantic telescopes to the accuracy of 10 nanometers is absolutely amazing.”
This article was contributed by HEIDENHAIN Corp., Schaumburg, IL. For more information, visit here.