An Image Auto Guider System for Kodaikanal Tower Tunnel Telescope

Abstract

The Kodaikanal Tower Tunnel Telescope (KTT) is a three-mirror Coelostat-based Solar telescope. An inherent variable image drift is present in the KTT which is detrimental to observations particularly to slit-based spectroscopic and spectropolarimetric observations. An image Auto Guider system has recently been installed at KTT to arrest the image drift based on sensing the image limb. The image’s limb position on the image plane is sensed using a two-segment photodiode sensor in North–South and East–West directions. Feedback is given to the second mirror of Coelostat, which tilts, causing the image to move on the image plane. This paper discusses instrumental setup, calibration, and testing of the Auto Guider system at KTT. The Auto Guider system is robust and capable of arresting image drifts up to 55arcsecmin−1 , which is much higher than the typically observed drifts (maximum 7arcsecmin−1 ) at KTT with a precision of ±∼0.9 arcsec.