Hong Wu, Xinglong Obs.
NAOC
PKU 2011.10.5
•
Scientific Goals with BFOSC
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CCD
•
Observation and Strategy
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BFOSC Data Reduction
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

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Morphology and Structure
Determine Redshift
Elemental physical parameters
(Age, metal abundance, stellar
population, star-formation history,
dynamicas, etc.)
Examples:
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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)
。。。
极亮红外星系Mkn273核区[OIII]5007和Ha发射区的成像
mkn266的延展发射区的Ha成象
mkn266的[OIII]5007发射区像
星系NGC1275二维光谱观测区域
CCD: Charge-Coupled Detector
电子耦合探测器

CCD
Array
2Kx2K
Overscan
Or Baseline
有些CCD没有
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 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
Flat-Field
Bad pixel frame（table）
Overscan region （有些CCD没有）
 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）
 DARK
Frames
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)
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。
 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.
λ(Å)
ηCCD （%）
3500
4000
5000
6500
9000
18.4
49.9
86.3
88.7
38.5
读出速度
增益选项
增益
读出噪声
整幅读出时间
e-/ADU
e-/pixel
（second）
Fast
0
99.58
132.5
5
Fast
1
49.44
103.7
5
Fast
2
24.48
87.35
5
Median
0
2.33
6.80
8
Median
1
1.22
5.93
8
Median
2
0.50
6.06
8
Slow
0
2.43
3.25
28
Slow
1
1.13
2.58
28
Slow
2
0.50
2.46
28

General principle of CCD observation

Before Observation

Observation Strategy
Observed - BIAS - DARK
Corrected = ---------------------------------Flat-Field - BIAS – DARK
DARK is so small, and can be neglected at
most cases
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

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
Observation
Get to know BFOSC system and control panels
Detail see《BFOSC Operating 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
图：BFOSC ExposureS/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）。
BFOSC
Optics
Control
Panel

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

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
 PSF: point
spread function
 Seeing
FWHM of PSF
 Photometric night
 Airmass
 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

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
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
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
Super-Sky-FF
Advantages：
Uniform
Close to the observing condition
Disadvantages：
Could occupy the Observing time
Depend on weather
Lower count numbers
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
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

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.

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.
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

 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 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 ：
σ2= σ12 + σ22+...
 Systemetic
Errors：
difficult to be measured
can not be added as random errors
 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
P1
P2
P3
P4
 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..
 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
Extinction file
Baoextinct.dat
Standard file
# 波长
3200.00
3201.00
3202.00
3203.00
3204.00
3205.00
3206.00
3207.00
3208.00
3209.00
3210.00
3211.00
3212.00
3213.00
3214.00
3215.00
3216.00
3217.00
3218.00
星等
10.729
10.732
10.736
10.739
10.741
10.743
10.741
10.739
10.737
10.736
10.734
10.732
10.732
10.732
10.732
10.732
10.732
10.734
10.736
带宽（A）
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
wavelength identification plot
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
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
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
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
minst = -2.5log F + m0
Observe standrads
minst - mstd = K * X + C
K=K (UT)
minst - mstd
airmass