A Co-Phasing technique for segmented mirror telescopes

Abstract

We are in an era where astronomy had taken steps to build giant telescope which have optics of size more than 10m, we can commonly call them ELTs or Extremely Large Telescopes. Due to various reasons like cost, ease of maintenance we need to go for segmented mirrors in their design. But with segmentation new problems arises which need to be taken care of. In this project we are addressing the co-phasing problem of the segmented mirror telescope. This is the problem we need to face in the final stage of aligning the telescope, to make it act like a single monolithic mirror. This is a very important problem as, without Co-phasing the resolution to which telescope was indented to, can never be achieved. Our aim is to study and understand co-phasing of segmented telescope. We use Keck co-phasing technique for the same. We chose this technique as it a working technique suitable for 8 to 10m class ground based telescope. Also due to ease of implementation In this project we will be discussing about Co-phasing technique of Keck alignment and phasing system. Mathematical background is derived. We have simulated the technique in MATLAB. Cross checking of the principle with ZEMAX is done. Preliminary experimental design in ZEMAX and experiments are done. MATLAB simulation gives good correlating results with Keck system. ZEMAX design and experiments are agreeing with the possibly of the system. In KECK alignment and phasing system two techniques are used, narrow band and broad band. Both are simulated in MATLAB. Narrowband contour plots generated numerically, and shown by KECK system are same. In broad band technique the coherence curve is derived, and simulated in MATLAB. We were able to get same range and type of coherence curve. The narrow band and broadband technique based on the principle is simulated in ZEMAX. Both MATLAB and ZEMAX are giving same result for narrowband. Due to the absence of the filter coating data files from the manufacturer, we were not able to design the broad band technique system in ZEMAX. But it is simulated by applying equal weightage for all wavelengths and Gaussian weightage for all wavelengths. Both methods gave approximately the same results, as contour plots which also matched with the simulated contours in MATLAB. So this can be used as an approximate model to make the preliminary ZEMAX design. Optical system designed in ZEMAX to check possibility of preliminary experiment is showing very good feasibility for narrowband and broadband. In the Actual experiment done the system works well for narrowband. Broadband system need to be improved.