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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.

NAOC

PKU 2011.10.5

outline

Outline

Scientific Goals with BFOSC

CCD

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)

slide4

Scientific Fields

Morphology and Structure

Determine Redshift

Elemental physical parameters

(Age, metal abundance, stellar

population, star-formation history,

dynamicas, etc.)

slide5

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)

。。。

slide6

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

slide7

mkn266的[OIII]5007发射区像

mkn266的延展发射区的Ha成象

slide10

CCD

CCD: Charge-Coupled Detector

电子耦合探测器

ccd array 2kx2k overscan or baseline ccd
CCD

Array

2Kx2K

Overscan

Or Baseline

有些CCD没有

slide15

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)
slide16

CCD Frames

Bias frame

Dark frame

Flat-Field

Bad pixel frame(table)

Overscan region (有些CCD没有)

slide17

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)

slide20

DARKFrames

Dark current need to considered during a long exposure time.

But, at most cases, it can be neglected

slide22

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。

slide24

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.

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)

slide32

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

slide33

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)。

slide39

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

slide40

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

some concepts
Some Concepts

PSF: point spread function

Seeing

FWHM of PSF

Photometric night

Airmass

slide42

BIASFrames

Take 5-10 frames before and after observationeach night

0 sec dark frame

Require

Shutter Closed 、Dome Closed、Light-Off、

Mirror Cover Closed

slide43

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

slide44

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

slide45

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

slide46

Super-Sky-FF

Advantages:

Uniform

Close to the observing condition

Disadvantages:

Could occupy the Observing time

Depend on weather

Lower count numbers

slide47

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

slide48

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

slide49

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.

slide51

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

slide52

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.

slide53

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

Photometry

Flux calibration

  • Spectra

General CCD reduction

Wavelength calibration

Frame distortion correction

Extract spectra

Flux calibration

slide55

Errors:

Errors introduced in every step of data reduction

Two types:

random errors (noise)

follow Gaussian Distribution

systemetic errors

slide56

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+...

slide57

Systemetic Errors:

difficult to be measured

can not be added as random errors

slide58

Image Combine

Improve S/N

remove cosmic-ray

remove bad pixels

introduce larger readout noise

each frame introduce readout noise once

improve spatial resolution

slide59

P1

P2

P3

P4

Principle of image combine

slide60

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

Minmax:

remove the m1 largest values and m2 lowest values

Then average

So on..

slide61

Before Data Reduction

Read the Log file

Bad pixel table

Check images

FITSHeader:some critical information

BIAS:the difference

FF

Wavelength Calibration frame

standard stars frames

object frames

slide62

Extinction file

Baoextinct.dat

slide63

# 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

slide65

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

slide66

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

Imaging:

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)

ad