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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

Observation and Data Reduction with BFOSC

Hong Wu, Xinglong Obs.


PKU 2011.10.5



Scientific Goals with BFOSC


Observation and Strategy

BFOSC Data Reduction

Scientific gaols with bfosc

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)

Scientific Fields

Morphology and Structure

Determine Redshift

Elemental physical parameters

(Age, metal abundance, stellar

population, star-formation history,

dynamicas, etc.)


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)







CCD: Charge-Coupled Detector


Ccd array 2kx2k overscan or baseline ccd





Or Baseline


  • 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)

CCD Frames

Bias frame

Dark frame


Bad pixel frame(table)

Overscan region (有些CCD没有)

  • 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


  • DARKFrames

    Dark current need to considered during a long exposure time.

    But, at most cases, it can be neglected

  • 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)



  • 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.

Ccd quantum efficiency
CCD不同波长处的出厂量子效率(Quantum Efficiency)

Observation and strategy with bfosc
Observation and Strategy with BFOSC

General principle of CCD observation

Before Observation

Observation Strategy

General principle of ccd observation
General principle of CCD observation

Observed- BIAS - DARK

Corrected = ----------------------------------

Flat-Field- BIAS – DARK

DARK is so small, and can be neglected at most cases

Calculate the gain and readout n oise
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)

  • Before Observation

  • Get to know BFOSCsystem and control panels

    Detail see《BFOSCOperating 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

图:BFOSCExposure-S/N estimation curve CCD



Bfosc ccd control panel
BFOSC CCDControl Panel

BFOSCOptics Control Panel

  • Observation Strategy


    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 :


  • Observation Strategy

    Spectra:Longslit + Grism(G?)

    Select the slit width according to resoluton and

    Seeing in that night


    BIASframes、Flat-Field for each Grism should used

    wavelength calibration spectra

    Standard star spectra

    Object spectra

    General observing sequence as:


Some concepts
Some Concepts

PSF: point spread function



Photometric night


  • BIASFrames

    Take 5-10 frames before and after observationeach night

    0 sec dark frame


    Shutter Closed 、Dome Closed、Light-Off、

    Mirror Cover Closed

  • Flat-Field

    Better to take before and after observation at each night

    Three type of Flat-Fields:



    Blank-Sky-Flat (Super-sky-Flat)

    Select the type of Flat-Field taken according to


    Scientific goals

Flat-Field for Imaging



not depend on weather

not occupy the observing time

high count numbers


illumination difficult to be uniform

Spectra quite different from that of night sky



be uniform for small FOV

Not occupy the observing time

high count numbers


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




Close to the observing condition


Could occupy the Observing time

Depend on weather

Lower count numbers


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:


Close to the observing condition

high count numbers

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


Adv.:high counts、continumm spectra

Disadv:could be not uniform in spatial direction,

lower counts at blue wavelength


Adv.:uniform in spatial direction

Disadv.:possible emission line


  • Wavelength Calibration (For Spectra)



    strong isolated emission lines

    easy to be identified

    better for red range

    Scare emission line in blue band


    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.

  • 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

  • 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.

  • 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.

Data reduction
Data Reduction

  • General CCD redution:

    Object- BIAS

    Corrcted Frame= -----------------------------

    FF- BIAS

  • Imaging:

    General CCD reduction


    Flux calibration

  • Spectra

    General CCD reduction

    Wavelength calibration

    Frame distortion correction

    Extract spectra

    Flux calibration

  • Errors

    Errors introduced in every step of data reduction

    Two types:

    random errors (noise)

    follow Gaussian Distribution

    systemetic errors

  • Random Noise

    Can be obtained from statistics

    Noise sources:

    CCD Readout noise

    noise from BASELINEremoval

    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 :

    σ2= σ12 + σ22+...

  • Image Combine

    Improve S/N

    remove cosmic-ray

    remove bad pixels

    introduce larger readout noise

    each frame introduce readout noise once

    improve spatial resolution





Principle of image combine

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

3σ clip:

remove those

ai< mean- 3σ and ai> mean+ 3σ


remove the m1 largest values and m2 lowest values

Then average

So on..

  • Before Data Reduction

    Read the Log file

    Bad pixel table

    Check images

    FITSHeader:some critical information

    BIAS:the difference


    Wavelength Calibration frame

    standard stars frames

    object frames

Extinction file


# 波长 星等 带宽(A)

3200.00 10.729 7.0 3201.00 10.732 7.0 3202.00 10.736 7.0 3203.00 10.739 7.0 3204.00 10.741 7.0 3205.00 10.743 7.0 3206.00 10.741 7.0 3207.00 10.739 7.0 3208.00 10.737 7.0 3209.00 10.736 7.0 3210.00 10.734 7.0 3211.00 10.732 7.0 3212.00 10.732 7.0 3213.00 10.732 7.0 3214.00 10.732 7.0 3215.00 10.732 7.0 3216.00 10.732 7.0 3217.00 10.734 7.0 3218.00 10.736 7.0

Standard file

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


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

The efficiency of 216 bfosc
The Efficiency of 216+BFOSC


Observe a standard stars with magnitude of m

Number of photons observed

ε(λ) = ---------------------------------------------------------

number of photons accepted by telescope

FADU * gain

= -----------------------------------------------------------------

Δλ * Atel * Fλ(0) * 10-(m+k(λ)*airmass)/2.5 / ( h c /λ )

Δλ: band width of filter λ: central wavlength of filter

Atel : Area of telescope Fλ(0): abosulte flux for 0 mag star

k(λ): exicntion coeficient at λ airmass: airmass at observing

The efficiency of 216 bfosc1
The Efficiency of 216+BFOSC

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

Atel =3.14*(216/2)2=36644 cm2

k(V)=0.22 (from extinction curve)

Fλ(0)=3.67 10-9erg/s/cm2/Å

From photometry, we obtained FADU =75000 adu/sec

The old BFOSC CCD have gain=1.71 e/adu

From above formula, we obtain:

ε(V) = 0.244

Flux calibration of magnitudes
Flux Calibration of magnitudes

minst = -2.5log F + m0

Observe standrads

minst - mstd = K * X + C

K=K (UT)

Flux calibration of magnitudes1
Flux Calibration of magnitudes

minst - mstd