高い天体追尾精度を実現する望遠鏡駆動制御技術

Abstract

すばる望遠鏡の駆動制御系の目的は、望遠鏡の光軸を観測天体に精度良く向け、波面収差が最小になるように複数の反射鏡の位置や形状を制御して、高精度で天体を追尾し、要求される星像の分解能を達成することにある。波面の傾きや収差は、次のような互いに独立なZernikeモードに展開することができる。(1)A11、B11:波面の傾き(Tilt)、(2)A31、B31:コマ収差(Coma)、(3)A20:焦点ずれ(Defocus)、(3)A22:非点収差(Astigmatism)、(4)A40:球面収差(Spherical aberration)など。これらの値はシャック・ハルトマン鏡面検査装置によって検出するが、制御系では次の方法によって星像の分解能を高めている。(1)望遠鏡架台駆動制御系は光軸を観測天体に向けるが、これは波面の傾きを制御することになるので、A11/B11は主鏡制御には用いていない。高周波数領域では振動副鏡制御系が波面の傾きを制御する役割を持っている。(2)主鏡能動支持制御系は、直径8.3mの主鏡の形状制御を担い、非点収差A22、球面収差A40を含む高次のZernike収差を補正する制御系を構成する。(3)側鏡駆動制御系は、主に焦点ずれA20とコマ収差A31/B31を補正する機能を持っている。このようにZernikeモードの実測値を分配する制御系の構成により、0.2arcsecという優れた星像の分解能を達成した。

The purpose of the driving control systems of the Subaru astronomical telescope are to acquire a requisite resolution of star image, by facing the optical axis toward an observed celestial body with high precision, by controlling the positions and configurations of several reflecting mirrors to have a wavefront aberration minimize, and then by tracking the celestial body in a highly precise way. A tilt and an aberration of wavefront can be expanded into the following each independent Zernike mode: A11 and B11 as tilt; A31 and B31 as coma; A20 as defocus; A22 as astigmatism; and A40 as spherical aberration. These values are detectable using Shack-Haltman mirror surface inspection device. And further, in the control system, the resolution of star image has been improved in the following methods: (1) the driving control systems of telescope holder device are to have the optic axis face toward an observed celestial body, however, since this comes to control a tilt of wavefront, A11/B11 is not to be used for controlling the primary mirror. In radio frequency region, an auxiliary oscillation mirror control system plays a role to control the tilt of wavefront; (2) the active primary mirror support control system plays a role to control the geometrical shape of the primary mirror with diameter of 8.3 m, and composes the control systems to correct the high order Zernike aberrations including astigmatism A22 and spherical aberration A40; and (3) auxiliary mirror driving control system has a function mainly to correct defocus A20 and coma A31/B31. As mentioned above, the resolution of star image with an excellent value of 0.2 arcsec has been achieved by the composition of the control systems to share the measured value of Zernike mode.