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Hong Wu, Xinglong Obs. NAOC PKU 2011.10.5. Scientific Goals with BFOSC CCD Observation and Strategy BFOSC Data Reduction.

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Presentation on theme: "Hong Wu, Xinglong Obs. NAOC PKU 2011.10.5. Scientific Goals with BFOSC CCD Observation and Strategy BFOSC Data Reduction."— Presentation transcript:

1 Hong Wu, Xinglong Obs. NAOC PKU

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

3 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

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

8 星系 NGC1275 二维光谱观测区域


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




14 CCD Array 2Kx2K Overscan Or Baseline 有些 CCD 没有

15  CCD Characters For example(BFOSC CCD): CCD类型 E2V 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)

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

17  BIAS Frames 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)



20  DARK Frames Dark current need to considered during a long exposure time. But, at most cases, it can be neglected


22  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。


24  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.


26 λ (Å) η CCD ( % )


28 读出速度增益选项 增 益 e - /ADU 读出噪声 e - /pixel 整幅读出时间 ( second ) Fast Median Slow

29  General principle of CCD observation  Before Observation  Observation Strategy

30 Observed - BIAS - DARK Corrected = Flat-Field - BIAS – DARK DARK is so small, and can be neglected at most cases

31 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*σ B2 ) 2 = (g* σ B1 ) 2 = r 2 (g*σ B12 ) 2 = 2*r 2 (g*σ F1 ) 2 = (F1-B1)*g (g*σ F2 ) 2 =(F2-B2)*g (g*σ F12 ) 2 = (g*σ F1 ) 2 +(g*σ F2 ) 2 + (g*σ B1 ) 2 +(g* σ B2 ) 2 =(F1-B1)*g+ (F2-B2)*g+ (g*σ B1 ) 2 +(g* σ B2 ) 2 g=( )/ (σ F σ B12 2 ) r= (g*σ B12 ) /sqrt( 2)

32  Before Observation Get to know BFOSC system and control panels Detail see 《 BFOSC Operating Manual 》 ( ) 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

33 图: BFOSC Exposure- 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)。




37 BFOSC Optics Control Panel


39  Observation Strategy Imaging : Select the filters used in the night 5-10 BIAS frames 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

40  Observation Strategy Spectra : Longslit + Grism ( G ?) Select the slit width according to resoluton and Seeing in that night Take BIAS frames 、 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

41  PSF: point spread function  Seeing FWHM of PSF  Photometric night  Airmass

42  BIAS Frames Take 5-10 frames before and after observation each night 0 sec dark frame Require Shutter Closed 、 Dome Closed 、 Light-Off 、 Mirror Cover Closed

43  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

44 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

45 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

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

47 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

48 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

49  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.


51  Standard stars If need to flux calibrate the object, must take standard stars several time at each night. Photometric standard stars select the Oke-Gunn or Landolt standrads Spectral standard stars select the white dwarfs with weak-absorption lines better to do continuum correction and high counts in blue band Generally, take standard stars several times at different zenith each night

52  Objects(Imaging) Better to take object at lower zenith First use fast CCD mode and take snapshot Check the object in the field Then use slow CCD mode to take frame To remove cosmicray, it is better to split the observaton into 3 exposures For one point sources, consider to use a small section of CCD to save readout time for some cases.

53  Objects( Spectral Observation) Better to take object at lower zenith Slit-width determined by resolution and seeing The spatial direction of slit better to along the longtitude to avoid the light leak of blue band If required, can rotate the slit direction To remove cosmicray, it is better to split the observaton into 3 exposures Suggest to take a source frame with the slit image First take the slit image, and then remove the slit during exposure.

54  General CCD redution : Object - BIAS Corrcted Frame= FF - BIAS  Imaging: General CCD reduction Photometry Flux calibration  Spectra General CCD reduction Wavelength calibration Frame distortion correction Extract spectra Flux calibration

55  Errors : Errors introduced in every step of data reduction Two types: random errors (noise) follow Gaussian Distribution systemetic errors

56  Random Noise Can be obtained from statistics Noise sources : CCD Readout noise noise from BASELINE removal noise from BIAS subtraction noise from FF correction noise from dark correction noise from sky background noise from background subtraction photon noise of source Random noise can be added as : σ 2 = σ σ

57  Systemetic Errors : difficult to be measured can not be added as random errors

58  Image Combine Improve S/N remove cosmic-ray remove bad pixels introduce larger readout noise each frame introduce readout noise once improve spatial resolution

59  Principle of image combine P1 P2 P3 P4

60  Principle of image combine For each position, we have n values a1 a2 a3 a4 … an Sum: a1+a2+…+an Mean: Sum/n Median: a(n+1)/2 3 σ clip: remove those ai mean+ 3 σ Minmax: remove the m1 largest values and m2 lowest values Then average So on..

61  Before Data Reduction Read the Log file Bad pixel table Check images FITS Header : some critical information BIAS : the difference FF Wavelength Calibration frame standard stars frames object frames

62 Extinction file Baoextinct.dat

63 Standard file # 波长 星等 带宽( A )

64 wavelength identification plot

65 Image Reduction : 1 、 Add some key word into FITS header 2 、 Remove overscan for all the images 3 、 Combine BIAS frames 4 、 Subtract BIAS from all images 5 、 Correct bad pixels 6 、 Trim images 7 、 Combine flat-field frames 8 、 Flat-field the object and standard images 9 、 Photometry the object and standards 10 、 Build up airmass-magnitude relation 11 、 Flux calibrate the object 12 、 Photometry

66 Spectral Reduction : 1. Add some key word into FITS header 2. Remove overscan for all the images 3. Combine BIAS frames 4. Subtract BIAS from all images 5. Correct bad pixels 6. Trim images 7. Combine flat-field frames 8. Normalize the combined flat-field 9. Flat-field the object/standard frames 10. Identify the wavelength calibration images 11. Wave-calibrate object/standard frames 12. Correct distortion of object/standard frames 13. Extract 1-D spectra 14. Flux-calibrate the object 15. Spectra measurement and analysis

67  Imaging: Observe a standard stars with magnitude of m Number of photons observed ε ( λ ) = number of photons accepted by telescope F ADU * gain = Δλ * A tel * F λ (0) * 10 - ( m+k( λ )*airmass ) /2.5 / ( h c / λ ) Δλ : band width of filter λ : central wavlength of filter A tel : Area of telescope F λ (0): abosulte flux for 0 mag star k( λ ): exicntion coeficient at λ airmass: airmass at observing

68 For 216+BFOSC+CCD system We observed standard stars G191-b2b (m=11.78, V) with airmass=1 at photometric night V band have central λ =5500A Δλ =890A A tel =3.14*(216/2) 2 =36644 cm 2 k(V)=0.22 (from extinction curve) F λ (0)= erg/s/cm 2 /Å From photometry, we obtained F ADU =75000 adu/sec The old BFOSC CCD have gain=1.71 e/adu From above formula, we obtain: ε (V) = 0.244

69 m inst = -2.5log F + m 0 Observe standrads m inst - m std = K * X + C K=K (UT)

70 airmass m inst - m std


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