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Observation and Data Reduction with BFOSC

Observation and Data Reduction with BFOSC. Hong Wu, Xinglong Obs. NAOC PKU 2011.10.5. Outline. Scientific Goals with BFOSC CCD Observation and Strategy BFOSC Data Reduction. Scientific Gaols with BFOSC. Characters of BFOSC: Image and spectra

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Observation and Data Reduction with BFOSC

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  1. Observation and Data Reduction with BFOSC Hong Wu, Xinglong Obs. NAOC PKU 2011.10.5

  2. Outline Scientific Goals with BFOSC CCD Observation and Strategy BFOSC Data Reduction

  3. Scientific Gaols with BFOSC Characters of BFOSC: Image and spectra A series of narrow band filters (to different redshift) 2D-spectra (longslit) Slitless spectra (so some survey) Multi-object spectra（Next year）

  4. Scientific Fields Morphology and Structure Determine Redshift Elemental physical parameters (Age, metal abundance, stellar population, star-formation history, dynamicas, etc.)

  5. Examples: Transit Observation:Gamma-burst、SN (Spectra , Image) Spectral identification of Infrared, X-ray, radio sources *Spectra) Stellar population of galaxies (Spectra) Gas structure and Dynamics of galaxies （narrow band image, 2D-spectra） Member identification and dynamics of galaxy cluster（Spectra, multi-object spectra） AGNs and QSOs（spectra） Star formation Regions(narrow band image,spectra) Open cluster (Multi-object spectra) 。。。

  6. 极亮红外星系Mkn273核区[OIII]5007和Ha发射区的成像极亮红外星系Mkn273核区[OIII]5007和Ha发射区的成像

  7. mkn266的[OIII]5007发射区像 mkn266的延展发射区的Ha成象

  8. 星系NGC1275二维光谱观测区域

  9. CCD CCD: Charge-Coupled Detector 电子耦合探测器

  10. CCD Array 2Kx2K Overscan Or Baseline 有些CCD没有

  11. CCD Characters For example(BFOSC CCD): • CCD类型 E2V55-30-1-348 back, AIMO • 图像大小(像元) 1242×1152 • 像元大小（微米）22.5×22.5 • 暗流（电子/像元/小时）2.4 at -100 ℃ • 满阱电荷 （电子/像元）100 K • 控制器 Lick新CCD控制器（魏名智） • Bad pixel Number(坏像元数目) • Quantum efficiency（量子效率） • Linearity（线性）

  12. CCD Frames Bias frame Dark frame Flat-Field Bad pixel frame（table） Overscan region （有些CCD没有）

  13. BIASFrames A bias current is routinely applied to CCD detectors to ensure that, as near as possible they are operating in a linear manner. But it also include some structure from readout. BIAS Frame is dark frame with exposure of 0 sec BIAS，是零秒暗场是 CCD的本底值 （含读出的附加电压值）

  14. DARKFrames Dark current need to considered during a long exposure time. But, at most cases, it can be neglected

  15. Flat-Field Flat-Field is used to correct the different quantum efficiency of different pixel and also correct the nonuniformity from some optics (such as filter, etc) 主要改正CCD的不同像素之间的差异， 以及可能成像光路中光学元件（例如滤光片）造成的的大尺度不均匀性。

  16. Overscan Region In CCD, there is a number of rows/ columns not exposed to the light. In fact, it is a constant voltage give to CCD during the readout. It is similar to bias current.

  17. 新CCD不同波长处的出厂量子效率(Quantum Efficiency)

  18. Observation and Strategy with BFOSC General principle of CCD observation Before Observation Observation Strategy

  19. General principle of CCD observation Observed- BIAS - DARK Corrected = ---------------------------------- Flat-Field- BIAS – DARK DARK is so small, and can be neglected at most cases

  20. Calculate the Gain and Readout Noise Take two Flat-Field frames (F1,F2)with same exp. Take two BIAS frames(B1,B2) B12 = B1 – B2 F12 = F1 - F2 σ: stdev of frame (adu) r: Readout noise (e) g: Gain (e/adu) (g*σB2)2 = (g*σB1)2 = r2 (g*σB12)2 = 2*r2 (g*σF1)2 = (F1-B1)*g (g*σF2)2 =(F2-B2)*g (g*σF12)2= (g*σF1)2+(g*σF2)2+ (g*σB1)2 +(g*σB2)2 =(F1-B1)*g+(F2-B2)*g+ (g*σB1)2 +(g*σB2)2 g=(<F1>-<B1>+<F2>-<B2>)/ (σF122 - σB122) r= (g*σB12) /sqrt(2)

  21. Before Observation • Get to know BFOSCsystem and control panels Detail see《BFOSCOperating Manual》 （http://www.xinglong-naoc.org/doc/216/BFOSCmanualv2_chinese.pdf） • Confirm the filters/grism used in the night • Prepare the sources list and standard list identification map, observing sequence, exposure time , etc. • Prepare the candidate source list, if weather is not good enough

  22. 图：BFOSCExposure-S/N estimation curve CCD V波段20等星在V波段测光的信噪比随时间的变化（上）； G6光栅加1.8角秒狭缝拍摄的15（蓝色）、17.5（红色）、20（黑色）等星光谱在4500埃的信噪比随时间的变化（下）。

  23. BFOSC CCDControl Panel

  24. BFOSCOptics Control Panel

  25. Observation Strategy Imaging： Select the filters used in the night 5-10 BIASframes 5-10 Flat-field frames for each filter Images of standard stars Images of observed object The general observing sequence as : BIAS—FF—standards—objects—standards—objects--…--—FF—BIAS

  26. Observation Strategy Spectra：Longslit + Grism（G？） Select the slit width according to resoluton and Seeing in that night Take BIASframes、Flat-Field for each Grism should used wavelength calibration spectra Standard star spectra Object spectra General observing sequence as: BIAS—FF—Wave-Cali--standards—object—standards—objects--…--Wave-Cali—FF—BIAS

  27. Some Concepts PSF: point spread function Seeing FWHM of PSF Photometric night Airmass

  28. BIASFrames Take 5-10 frames before and after observationeach night 0 sec dark frame Require Shutter Closed 、Dome Closed、Light-Off、 Mirror Cover Closed

  29. Flat-Field Better to take before and after observation at each night Three type of Flat-Fields： Dome-Flat Twilight-Flat Blank-Sky-Flat (Super-sky-Flat) Select the type of Flat-Field taken according to Imaging/spectra Scientific goals

  30. Flat-Field for Imaging Dome-FF Advantages： not depend on weather not occupy the observing time high count numbers Disadvantage： illumination difficult to be uniform Spectra quite different from that of night sky

  31. Twilight-FF: Advantages： be uniform for small FOV Not occupy the observing time high count numbers Disadvantages： not uniform for large FOV (>0.5deg) The time used to take FF is short Pollution from bright stars The spectra is quite differennt from that of night sky

  32. Super-Sky-FF Advantages： Uniform Close to the observing condition Disadvantages： Could occupy the Observing time Depend on weather Lower count numbers

  33. Combined-FF FF: include pixel-to-pixel variation large scale variation Dome-FF( pixel-to-pixel variation ) +Twilight(Super-Sky)-FF( large scale variation ) Take Advantages of above FF： Uniform Close to the observing condition high count numbers

  34. Flat-Field for Spectra Observation Correct the different QE of CCD and the nonuinform from Optics (such as grism and slit etc.) Two types of FF：Dome-FF、Twilight-FF Dome-FF： Adv.：high counts、continumm spectra Disadv：could be not uniform in spatial direction, lower counts at blue wavelength Twilight-FF： Adv.：uniform in spatial direction Disadv.：possible emission line Combined-FF：Dome-FF+Twilight-FF

  35. Wavelength Calibration (For Spectra) Lamp：Fe/Ar、Ne Ne-Lamp: strong isolated emission lines easy to be identified better for red range Scare emission line in blue band Fe/Ar-Lamp： Many emission lines in either red and blue band, some are blended and weak easy to mis-identified. Generally ，Take once before or after the observation. To high accuracy, can take one before and after object.

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