Development of instruments for astronomical site characterization and their application

Abstract

Before setting up any large observational facility, selection of a suitable site is an important task. It requires a variety of site survey instruments, which helps to carry out a detailed comparative study of candidate sites and choose the best one. There are a number of parameters used, to judge the suitability of a site for installation of a large telescope. Few of these parameters are meteorological in nature like Winds speed, Temperature, Ground-level humidity, Precipitable water vapor, etc. and can be easily obtained by installing commercially available weather stations. Whereas, several other key parameters can only be derived by making use of specifically designed site survey devices and these parameters are Cloud cover, overall atmospheric seeing, Turbulence profiles, Atmospheric transparency, Sky brightness, etc. Most of these site survey instruments are indigenously developed devices and cannot be procured commercially. In order to search a few potential sites for the India’s future large telescope project: - National Large Optical Telescope (NLOT), we have initiated a site survey program and started indigenously developing various site characterization instruments. Under this Ph.D. thesis, we have developed two innovative instruments. The first one is an automated extinction monitor, a device which measure atmospheric transparency very precisely. Other device is a scanning cloud monitor, uses IR sensor to measure excess radiation reflected from the cloud. In addition to above two works, the effort has also been made to develop a sturdy telescope for the MASS-DIMM instrument. Automated Extinction Monitor (AEM) is a unique device for measurement of the atmospheric extinction and sky brightness in automatic mode, it can also detect the presence of thin clouds. There are hardly any observatories around the globe which have got such a dedicated instrument for the continuous measurement of

atmospheric transparency. AEM is a small equatorial mount telescope with provision to change the polar angle depending upon the latitude of the place. We make use of a telephoto lens as main telescope optics, which gives the advantage of wide field and short focal length. The wide field allows the instrument to observe several hundred stars in a single frame, which in turn increases the measuring accuracy of the instrument. Short focal length also helps in reducing the overall size of the instrument. A Peltier cooled large format but very compact CCD with inbuilt shutter from Apogee is used as an imager. The telescope control hardware is developed around a high end microcontroller having capability to work in networked environment. All the software required, to control the telescope, control of CCD and data analysis have been developed under Linux platform. We have developed a fully automatic instrument, which does not require any operator. AEM is protected by a motorized enclosure, which has been designed and fabricated in-house. Instrument was installed at IAO Hanle in November 2012, and several improvements has been made in the instrument, and the instrument is in continuous operation. So far we have collected more than 150 nights data, and out of that about 92 nights data has been analyzed and reported in this thesis. Extinction measured using AEM as well as 2m HCT telescope matches very well. Device is also very sensitive to measure very small changes in the sky transparency over the night. AEM can also very precisely measure sky brightness which again very well matches with HCT data. As a trail run, we looked at the possibility of using part of the spare time of AEM in variability study of bright variable stars. The information related to the clarity of sky can be either obtained by making use of a device like All Sky CCD Camera or else by using NIR sensors. The CCD-based detectors are found to be not very sensitive during night time and any automatic algorithm to investigate images for clarity of sky at nights are subjected to large errors. Whereas, manual handling of a large amount of imagery data collected over

longer run from different sites is indeed a cumbersome task. To avoid this problem, a scanning cloud monitor which uses an array of thermopile sensor can be a better choice and has been developed under the present thesis. All Sky Scanning Cloud Monitor is a unique instrument for determining the cloud coverage of the sky. Instrument operates on the principle of detection of IR radiations reflected form the clouds. The detecting element (sensor) is a thermopile based sensor with an internal thermistor for ambient temperature compensation. This type of sensor has a small finite field which can cover only a small portion of the sky. To overcome this difficulty we have used an array of the sensors mounted on a circular arc. This arc along with sensors is rotated in azimuth to cover the whole 360 degrees of the sky. The mechanical design and fabrication, sensor electronics, instrument controller and the data acquisition system have been designed and developed in-house. The software required to operate instrument as well as for data acquisition and analysis has also been developed. This instrument was installed at IAO Hanle in December 2015. Altogether we have collected data for 65 nights in three months of instrument operation. The data has been analyzed and also percentage cloud cover is computed. From distribution, we get that nearly 70% of the time sky is clear (cloud cover less than 10%). Whereas, fully cloud time is not more than 3%. Remaining time which is about 27% of total time is found be partially clear. We also cross-checked these finding with CONCAM all-sky data and confirmed that our scanning cloud monitor provides reliable results. A MASS-DIMM (Multi-Aperture Scintillation Sensor-Differential Image Motion Monitor) has been acquired from TMT Project USA, which gives turbulence profile of the atmosphere. One of the prime requirement of this instrument is that it needs very stable telescope which can work in open and windy condition. We are developing a very stable telescope which can operate in windy conditions without degrading the image quality. For doing this, we have explored all aspects of the telescope design

and have chosen the approach which is best suited for our requirement. The telescope is an Alt-Az (Altitude-Azimuth) mount telescope. To minimize the cross section area and hence dynamic wind loading on the telescope, a carbon fiber truss based 16-inch f/8 Ritchey-Chretien OTA from TPO USA is used. Another measure taken is to make use of Direct Drive Technology, which removes any intermediate reduction system (gear, timer belts, friction drive, etc.), this makes the telescope electromechanically stiff and very responsive. We have also explored the possibility of designing the motor in-house, and an initial prototype has been made. An effort has also been made to develop the drive electronics for the direct drive motor. At present, the telescope design and analysis has been completed, and it is going through manufacturing/assembling phase.