<title>Displacement sensors for the primary mirror of the W. M. Keck telescope</title>

Abstract

The Primary Mirror of the Keck Observatory Telescope is made up of an array of 36 hexagonal mirror segments underactive control. The measurement of the relative orientations of the mirror segments is fundamental to their control. The mechanical and electronic design of the sensors used to measure these relative positions is described along with the performance of the sensors under a variety of tests. In use, the sensors will measure relative positions with a resolution of a few nanometers. This resolution and the low noise, drift and thermal sensitivity of the sensors are adequate to stabilize the primary mirror figure to the precision required for optical and infrared astronomy.

ULTRAsim: an integrated modeling environment for large, segmented telescopes with ultra-stable wavefronts

“…The Keck telescope utilities plates with capacitor plates on either side of them to actively sense disturbances, correcting for them with actuators to maintain an error of 4 nm or less over the surface. [18][19][20] The planned Thirty Meter Telescope (TMT) will also use capacitive edge sensors in a different configuration from Keck, operating in a three plate design. 21 Adding capacitive edge sensors to ULTRAsim will enable a broader range of trade studies on potential mission architectures.…”

The Edge Sensor of Segmented Mirror Based on Fringes of Equal Thickness

“…Thirty Meter Telescope (TMT) (Mast et al 2006), Keck telescope (Jared et al 1990), and Gran Telescopio Canarias (GTC) telescope use similar capacitive edge sensors (Lefort & Castro 2008); Extremely Large Telescope (ELT) will uses inductive edge sensors (Wasmeier et al 2014). The parameters of each edge sensor are as follows: the capacitive sensor of the Keck telescope has a measurement accuracy of 3 nm, a range of ±12 μm, with a temperature drift of 2 nm • C −1 and a time drift of 3.2 nm per week (Minor et al 1990); the TMT telescope's edge sensor was improved from the Keck one with a measurement accuracy of 1 nm, but still has a temperature drift (Mast et al 2006); the inductive edge sensor for ELT has a measurement accuracy of 1 nm, a range of 400 μm, and a temperature drift of 1.32 nm • C −1 (Rozière et al 2008). The measurement accuracy of such electrical sensors is greatly affected by the environment, such as temperature drift and time drift, which are challenging to eliminate, and have poor antielectromagnetic interference ability.…”

New phasing algorithm for large segmented telescope mirrors

“…For instance, the precision of the sensor used in Keck telescopes is 3 nm 1sb, its measurement noise is 1-nm rms, and its temporal drift is 3.2 nm/week. 1 They are installed under the segments to avoid blocking the lightcollection area.…”