The University of Tokyo Atacama Observatory 6.5-m telescope: screen camera system for the telescope mirror alignment

Measurement of the alignment error of the telescope mirrors is an essential and demanding task in the telescope assembly phase. One method is to examine the aberration over the whole telescope field of view from sky images with stars, but there are complicated issues in the case of large telescopes. The focal plane of the University of Tokyo Atacama Observatory (TAO) 6.5-m telescope has a large diameter of 546mm and a field curvature. Therefore, many imaging sensors must be arrayed on the curved focal plane. We propose a concept of a screen camera for the TAO 6.5-m telescope. To lower the cost, we accept the degradation of the spatial resolution up to ∼2 arcseconds and the decrease in optical throughput. This system consists of a transparent screen, a camera lens, and a CMOS sensor, and it obtains sky images through the telescope on the screen. The transparent spherical screen with one side sanded is placed at the telescope focal plane. A CMOS sensor with a commercially available camera lens and filters is placed at about 1.5 meters from the screen and captures the starry sky on the screen. The pixel scale on the CMOS sensor is calculated to be 0.31 arcseconds, and the estimated limiting magnitude is about 13 in a 10-second exposure at a 10σ level. After the telescope mirror alignment, the screen camera will provide focused sky images in the whole field of view, 25 arcminutes diameter.