Development of Lunar Scintillometer for Probing the Ground Layer Turbulence. Ravinder K. Banyal. Indian Institute of Astrophysics Bangalore

Transcription

1 Development of Lunar Scintillometer for Probing the Ground Layer Turbulence Ravinder K. Banyal Indian Institute of Astrophysics Bangalore Indo-US meeting on Adaptive optics with moderate-sized Telescopes IUCAA, 22nd-25th August,

2 Talk Plan Introduction Relationship between seeing and scintillation Principle of Lunar Scintillometer Hardware components Photodiode array & electronics Tracking system Webcam & DAQ system Software Mechanical Design Testing 2

3 Introduction Atmospheric turbulence responsible for scintillation effects Turbulence in upper atmosphere: Stellar scintillations Turbulence in lower atmosphere: Weak scintillations from planets, moon and sun. Ground layer (GL) turbulence & overall seeing Measuring the Optical Turbulence Profile(OTP) in GL is important: for new site testing and monitoring to predict the performance of ground layer AO and interferometry determining the optimum height of the telescope enclosure resolving discrepancy between DIMM seeing and telescope resolution Lunar Scintillometer (LS):Simple and accurate way of determining the GL turbulence strength and its vertical distribution 3

4 Scintillations and Seeing v Solar scintillations are related to atmospheric seeing (Codona 1986, Seykora 1993) v Explanation by J.M. Becker (1993) v Photodiode array to measure the refractive index fluctuations along the line of sight (Becker et al 1996, 2001) v Used extensively in day-time site testing (e.g. SHABAR) v Hickson & Lanzetta (2004), and Tokovinin (2008,2010) night time astronomy using LS. 4

5 Principle of Operation q LS: A linear array of photodiodes correlated Intensity fluctuations in the light path are recorded q Cone extending from two detector overlap at altitude z q Correlated intensity fluctuations at two detectors (z > r/θ 100r) q Independent baselines using an array of detector q Vertical structure of the turbulence from covariance measurements uncorrelated z θ r q 5

6 Covariances between each pair of detector signals 1 N (ζ i ζ Bij = N 1 m= 0 ) j m Each baseline has peak sensitivity to turbulence originating in a certain altitude range. covariance is related to the distribution of the R.I. structure constant Cn2(z) along the line of sight W(r,z): Weighting function Wavelength independent moon phase, sensor area and shape 6

7 2 Cn Restoration Recovering the Cn2 profile from the measured covariance OTP restoration Linear Model Fitting 7

8 Typical LS signals Covariance signal (Paranal, Feb 2009) 8 Tokovinin, MNRAS (2010)

9 Lunar Scintillometer Hardware Six channel photo-detector array & electronics DAQ Card (8 channel, 14-bit ADC, NI) A stable moon tracking system ( few degree accuracy) with computer control ( e.g. MEADE LX200 ACF Telescope without optics) A modified (Logitech HD C510) webcam for guiding PC/Laptop 9

10 Si Photodiodes (FDS1010 Thorlabs) Wavelength Range: nm Active area = 9.7 x 9.7 mm NEP = 5.5x10-14 W/ nm Responsibility = 0.65A/W Linear array of 6-photodetector 15 non-redundant baseliines Shortest baseline 2cm Largest baseline 40 cm 2cm 10 40cm

11 Electronics - Relative flux fluctuation caused by scintillation: I/I 10-4 High Signal to Noise Ratio Dark current under bias conditions ~ 600nA Full moon photocurrent 90nA (~ 1V) Electronics and photon noise < 1% Schematic for single channel LS (Tokovinin A. et al, MNRS (2010) 11

12 In-house fabricated 2-Channel PCB 12

13 DATA ACQUISITION Data to be acquired from 6 photodetector sensor modules with NI-USB 6210 DAQmx device 250 ks/s, 16 bit resolution, 16 analog, 16 digital channels Required data acquisition parameters: 1. Continuous acquisition: With adequate buffer size to safely acquire continuously 2. Sampling Rate: 500 samples/sec from each channel 3. Measurement range: range over which input signal is expected to vary LabVIEW Implementation: DAQmx VI function library used for acquiring data from the detectors For real time analysis - Probability and Statistics functions used For saving data - TDMS file functions used 13

14 Logitech HD C510 webcam for moon guiding 14

15 Software Development for LS: GUI, Telescope control and Data Acquisition Requirements of the Application: Motion Control of small telescope Acquisition of Data received from the photodetectors of Lunar Scintillometer Displaying data in real time, Some preliminary data analysis Storage of data acquired in disk, for later analysis WEBCAM CONTROL Software platform used : LabVIEW 15

16 Software Development for LS Why LabVIEW? Intuitive graphical programming Faster Development Extensive collection of function libraries DAQ, Serial communication Convenient compatibility with NI-DAQ device Two important considerations: Parallel Execution of Multiple Tasks Simultaneous Tasks in Parallel Loops LabVIEW s inherent multi-threading capability Priority between multiple tasks Appropriate Execution System Data Acquisition Execution System Chosen User Interface Interaction: Requirement of a Graphical User Interface 16 Standard State Machine Template - Favourable for building GUI 2 Standard State Machines used Serial Communication, Data Acquisition

17 Software Graphics User Interface 17

18 Meade Telescope Motion Control through Serial Port Communication Meade Telescope Serial Command Protocol: Alphanumeric 2 to 3character strings preceded by a colon, succeeded by # termination character LabVIEW VISA (Virtual Instrument Software Architecture) function libraries for Serial (RS232) Port communication used MEADE LX200 ACF (WITH GPS) 18

19 Telescope Motion Control Standard state machine : An Event structure wrapped in a while loop Each button press event handled by a case of the Event Structure Each event case contains appropriate code for the required function 19

20 Mechanical Design -3D View Detector locations Circular aperture Web-cam DAQ baffles 20

21 Mechanical Design Drawing 21

22 TESTS A prototype of Lunar Scintillometer was built using two Si PIN photodetectors PDA36A (from Thor Labs) to test instrument functionality and developed computer software Specifications LABORATORY TEST Active area 3.6X3.6m m Gain range 70db Peak Response 0.65A/W at 970nm Mechanical structure: Optical tubing to limit FOV to 10 deg Mechanical arrangement for tip-tilt for each sensor 22

23 Observing conditions during the tests Date/time 17 April 2011, 00:00am02:00am Position of moon in sky at start of test: (AZ, ALT) 217ᵒ11, 64ᵒ 11 Phase of moon Full moon (99% illuminated) Current moon angular diameter 0.48ᵒ Measurements taken at 1000 samples/second: Moon measurements approx. 24 min ( samples) Sky measurements approx. 5min ( samples) Dark measurements approx. 4min ( samples) 23

24 RESULTS FROM PRELIMINARY TESTS WITH 2-CHANNEL LUSCI PROTOTYPE SIGNALS FROM CHANNEL 1 & 2 SKY AND DARK MEASUREMENTS 24

25 Team Members Ms Anusha Kalyan Dr Padmakar Parihar & Dr Ravinder K Banyal 25