Development of High Resolution System for Stellar Imaging

Abstract

Adaptive Optics (AO) technology is a part of ground-based astronomical observatories around the world. It enables the telescopes to attain near

diffraction-limited resolution. We have designed, developed and tested a tip- tilt image motion compensation system for 1.3m J.C. Bhattacharya (JCB)

telescope at Kavalur. This thesis includes the measurement of turbulence parameters, design development and demonstration of a tip-tilt instrument on 1.3 m telescope at Vainu Bappu Observatory, Kavalur.

Measurement of atmospheric turbulence parameters on site of the tele- scope is essential prior to the development of an AO system. The atmospheric

turbulence parameters namely, atmospheric seeing, the tilt-anisoplanatic an- gle (θ0) and the coherence time (τ0), were measured under various sky con- ditions, at Vainu Bappu Observatory in Kavalur. Bursts of short exposure

images of selected stars were recorded with a high-speed, frame-transfer CCD

mounted on the Cassegrain focus of a newly commissioned 1.3 m JCB tele- scope. The estimated median seeing is ≈ 1.8500 at wavelength of ∼ 600 nm,

the image motion correlation between different pairs of stars is ∼ 44% for θ0 ≈ 3600 and mean τ0 is ≈ 2.4 ms. The optical model of the instrument was designed in ZEMAX ray-tracing software. The diffraction-limited field of view (FOV) of the instrument is 1 0 × 1 0 with a wavelength range of 0.48 - 0.7 μm. A telescope interface unit was designed in AutoCAD and fabricated to house all the sub-components of the system. Control software with graphical user interface was developed

in LabView software. In closed-loop operation the control software can op- erate at loop frequency up to 300 Hz. To characterize its performance, the

instrument was thoroughly tested in the laboratory by simulating the image motion data obtained the telescope.

The instrument was commissioned on the telescope to analyze its per- formance in real-time. The tilt corrected images have shown up to ≈ 57%

improvement in image resolution and corresponding peak intensity increased by a factor of ≈ 2.8. A closed-loop correction bandwidth of ≈ 26 Hz has been achieved with on-sky tests and the root mean square motion of the star image

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has been reduced by a factor of ∼ 14. These results are not only consistent

with theoretical and numerical predictions of image quality improvement ex- pected from a real-time control system but also consistent with the reports

of performance of similar systems elsewhere in the world.