How to Get the Final Lightcurve from Occultation Image Sequences or Lessions, which have to be learnt one day
International Occultation Timing Association * European Section * How to Get the Final Lightcurve from Occultation Image Sequences or Lessions, which have to be learnt one day Wolfgang Beisker International Occultation Timing Associatio, European Section München, [email protected] International Occultation Timing Association * European Section * The measurement of the ocultation is not the only thing, what we have to do! Dark Frames Flat Fields Reference Images And what we have to do in the lab before? Linearity (Gamma factor) What are reference images for? International Occultation Timing Association * European Section * Correction of linearity curve for ~ 5000 images Therefore: software required with batch proc. Best: Image format FITS Which software??? MIDAS: This package can do all the job IRIS: Arithmetics is problematic, if in batch mode International Occultation Timing Association * European Section * Recording of images: IOC : Special movie files, can be converted to FITS files using mov2fits software Toucam (IOClight), Videocamera, Logitech etc using QASTROCAM Direct FITS files, with timings! using IRIS, GIOTTO,... if speed is necessary, only AVI files Conversion to FITS by IRIS, AVIEDIT etc. International Occultation Timing Association * European Section * IMPORTANT: At the end, we MUST have a sequence of FITS images... IOC IRIS TC001.fit TC002.fit TC1.fit TC2.fit TC009.fit TC010.fit TC011.fit TC9.fit TC10.fit TC11.fit TC099.fit TC100.fit TC101.fit TC99.fit TC100.fit TC101.fit Both sequenes work, IRIS allows conversion from IOC mode to IRIS mode and for MIDAS it can be programmed anyway International Occultation Timing Association * European Section * 3com home connect toucam default 225 225 200 200 175 1.0 175 150 150 125 1.45 125 100 100 75 75 50 50 25 25 0 0 0.0 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.7 0.7 0.8 5 5 5 5 5 5 5 5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Intensity vs. Expotime 120 110 Raw mode toucam (Etiennne Bonduelle...) 100 90 80 70 1.0 60 50 40 30 20 10 0 0 25 50 75 100 125 150 175 200 Exp. Time [msec] International Occultation Timing Association * European Section * Example: DEFINE/PARAM P1 ? IMA "Enter root_name for input frames: " DEFINE/PARAM P2 ? IMA "Enter root_name for output frames: " DEFINE/MAXPAR 2 ! max. 2 parameters expected SET/FORMAT I4 ! WRITE/KEYWORD N/I/1/1 0 ! DO N = 1 9999 ! default increment is 1 COMPUTE/IMAGE res = {P1}{N}.fit ** 1.45 ! gamma correction copy/II res res16 I2 LOAD/IMAGE res16 ! display the result frame OUTDISK/fits res16 {P2}{N}.fit ! convert to Fits ENDDO International Occultation Timing Association * European Section * Bias image... just to be complete It gives the offset of the elctronic amplifier of the CCD camera without any light or thermal electrons from the chip. It can be recorded with an exposure time of zero and no light present. For simplification we treat it just with the dark frame and seperately, as it would be required in a very rigorously evaluation International Occultation Timing Association * European Section * Dark frames... it seems to be an easy job... but Temperature must be the same Exposure time must be the same Gain must be the same as well as all other parameters CCD chip must be really dark, no light at all! Therefore: The darkframe has to be recorded immediately before or after the occultation! AND: It should be checked on site, that the darkframe intensity level is lower or equal as compared to the occultation images. International Occultation Timing Association * European Section * Flat field: Its the most difficult of all calibration images! The entrance aperture of the telescope has to be homogenously illuminated. The intensity level has to be sufficiently high, about 2/3 of full well capacity or maximum ADU value. Sky flat at dusk or dawn Dome flat Artificial light source with a white screen But beware of saturation! Sometimes its hard to recognize! And: Remember to get an appropriate dark frame! International Occultation Timing Association * European Section * All you need is ... Occultation movie itself Dark frame for the occultation movie Flat field Dark frame for the flat field Calibration images with full set of flats and darks... International Occultation Timing Association * European Section * Choosing the right color filters: If star and occulting body have different colors, we can use this, to enhance contrast. As we know: Most stars are red stars compared to solar system bodies, therefore a red to near infrared wavelength range gives a better contrast. Specially important for faint objects, i.e. Pluto occultations ec. . International Occultation Timing Association * European Section * Very important: All images, reference and occultation images as well MUST be recorded with the same wavelength range,i.e. with the same color filter! International Occultation Timing Association * European Section * Example Light level Titan+star Light level deep in the atmosphere, but not necessarily without light from star. Very important measurement! Zero light level International Occultation Timing Association * European Section * What does it mean for atmospheric science? - Immersion - Emersion - Yelle's profile Variation: +/- 0.5% +/- 1.0% Nominal value for Titan's flux: 47.9% (SAAO 1m) Influence of calibration problems Inversion calculated by B. Sicardy, Paris International Occultation Timing Association * European Section * Getting the intensity out of the image sequence: Choosing the right aperture diameter: Practice has shown, in many cases a simple aperture photometry gives better results than sophisticated PSF calculations, including Gaussian ones. Schematics of measurement in the image: Stellar intensity: Background: inner circle outer circle If circles are too small, not all light is measured, if circles are to large, too much background causes noise in the lightcurves International Occultation Timing Association * European Section * How to determine the best aperture: Before and after the occultation event, we know, that the light intensity must be stable, therefore it can be used to adjust the diameter of the apertures. Looking for the standard deviations of the light curve will give us an hint, how to choose the aperture. Stdv Regions used for setting the aperture sizes. Aperture to choose Aperture size International Occultation Timing Association * European Section * Finally, from image sequence to the light curve... Correction for linearity of camera (gamma) Dark frame subtraction of data sequence Dark frame subtraction of flat field sequence Flat field correction of data sequence Choosing the aperture Running automated photometry Normalization and further data analysis of light curve International Occultation Timing Association * European Section * Using a Webcam as a Scientific Instrument Wolfgang Beisker International Occultation Timing Associatio, European Section München, [email protected] International Occultation Timing Association * European Section * Can we use a webcm for really serious photometry? It already has been shown: In many cases a webcam can replace an very expensive CCD cameasystem. But: Linearity is a problem and the 8-Bit digitization of most systems. International Occultation Timing Association * European Section * Webcams under survaillance.... Logitech 3Com HomeconnectPhillips Quickcam 4000 pro no longer manuf. ??Toucam pro 740K International Occultation Timing Association * European Section * Just an examle for imaging: 30cm instrument with B/W Toucam 740K copyright: Bernd Flach-Wilken International Occultation Timing Association * European Section * Spectral response International Occultation Timing Association * European Section * - coefficient of different Webcams, determined with a calibrated variable light source (LED, = 535nm) Quickcam 4000 Toucam 3com 225 225 200 200 175 175 150 150 125 125 35 32.5 30 27.5 25 22.5 20 17.5 100 100 75 75 12.5 50 50 7.5 25 25 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 =1.0 0.8 15 10 5 2.5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 = 1.45 0 0.1 0.2 0.3 0.4 0.5 0.6 =1.0 0.7 0.8 International Occultation Timing Association * European Section * Explanation of gamma (γ) coefficient: It represents the nonlinear relationshipbetweencode value (inan8bit systemfrom0to 255) andilluminationof the chip. Inmonitor technologyit represents the relationshipbetweencode value andluminosity. International Occultation Timing Association * European Section * But, the raw mode of Toucam has saved the problems: Etienne Bonduelle et. al. have found a way to rewrite the internal EEPROM of the TOUCAM, to cancel the compression, reset the smoothing and setting the linearity gamma value to 1.0 . Each pixel is transferred to the computer WITHOUT any manipulation, just as in a true CCD camera! After once programmed, the camera runs with all drivers and programs, from Windoofs to Linux systems. International Occultation Timing Association * European Section * What raw mode does to the data transfer: Easy control by visualizing „hot pixels“ Normal mode including smoothing Normal mode without smoothing Pure raw mode International Occultation Timing Association * European Section * How long are the exposure times? Many experiments of different authors have shown, that the exposure times marked in the drivers are not true. Therefore they had been checked using an oscilloscope by imaging the moving dot on the oscilloscope screen. International Occultation Timing Association * European Section * This allows to determine the true exposure time as demonstrated with the QASTROCAM exposure values. But keep in mind, theese values are only measured experimentally and may contain certain errors. They are only true for 5 frames per second speed of the Toucam. International Occultation Timing Association * European Section * Exposure time for 5 fps Expo time [ms] 100 10 1 55 60 65 70 75 80 85 Qastrocam exp value 90 95 100 International Occultation Timing Association * European Section * Intensity vs. Expotime 120 110 100 90 80 70 Spalte D 60 50 40 30 20 10 0 0 50 100 150 Exp. Time [msec] 200 International Occultation Timing Association Variance * European Section * Transfer curve, B/W Toucam 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0 25 50 75 100 125 Mean Intensity [rel. Units] Slope of the Transfer curve: International Occultation Timing Association * European Section * Linearity of the camera Using raw data mode, the linearity is set to 1 by software, if gamma is set to zero in the software drivers. This is not the case for the default values, if a company reset is performed. International Occultation Timing Association * European Section * The same is true, if we use an regulated light source, therefore: The camera in raw mode is linear!! International Occultation Timing Association * European Section * Finally, characteristics of the raw-mode toucam with a B/W chip and the QASTROCAM software Linear in intensity response Well determined exposure times Uncompressed data transfer using USB 1.1 Direct recording of FITS images with time stamps in it International Occultation Timing Association * European Section * What did this camera for the Venus Transit?? Using an 11 inch SCT and a 890nm filter: International Occultation Timing Association * European Section * And for occultation work... (this example even without the raw mode, made at HAKOS by K.-L. Bath)
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