- Email: [email protected]
A photometric study of the BAO-CCD BVRI system
Chin. Astron. Astrophys. A translation of
@ Pergamon Press Ltd Printed in Great Britain
(1992)16/2,226-253
02751062/92$10.00+.00
Acta AstrophysSin.(l992)12/1,47-53
A photometric JIANG
study of the BAO-CCD system*
Zhao-ji
LI Yong
Beijing Astronomical
CHEN
BVRI
Jian-sheng
Observatory, Chinese Academy of Sciences Beijing 100080
Abstract The BAO Schmidt telescope with a Thomson CCD of 576 x 384 pixels was used for BVRI photometry. 9 secondary photometric standard stars in 3 CCD frames were observed on Oct. 31, 1989, to establish the color equations. Some problems relevant to the capability of CCD photometry are discussed. Key words:
CCD-photometry-color
equations
1. INTRODUCTION Magnitudes and color indices are the important quantities used in many astrophysical researches. Photographic techniques can provide photometry for large field with low accuracy, while photoelectric photometry, on the other hand, can reach high precision but , in most cases, is limited to measuring one object at a time. Since the astronomical application of large format CCD, it soon became clear that as a detector, CCD combines the advantages of photographic plates and photomultiplier tubes, namely: two dimensional detection, good linear response, high quantum efficiency, and large dynamic range. In addition, with digitized CCD output from the entire frame, one can use more sophisticate statistic method to determine accurately the local sky background and can optimize the selection of the aperture for photometry to raise the S/N ratio. However, for CCD photometry, one have to take care of the readout noise as well as the noise introduced from the flat-field correction. With the improvements in performance of CCD and the developments of image processing techniques, the precision of CCD photometry is being improved and an accuracy of up to 0.0015 magnitude is obtained in the study of CCD surface photometry in M67 l
This
work
Received
was
partly
supported
19991November
by National
5; revised
version
Natural received
Science 1991
Foundation.
September
18
aCCD Photometry
227
(Gilliland & Brown[‘l), which is comparable to that of photoelectric photometry. It is undoubted that the usage of CCD for both two dimensional surface photometry as well as for large ensembles of stars is very promising. Taking advantage of fast focal ratio and large field, we have carried out CCD multi-color photometry using the 60/90 cm Schmidt telescope at Beijing Astronomical Observatory(BA0). As the first step of the study for BAO CCD photometric system, we established, in this paper, the color equations and discuss some problems relevant to the performance and capability of CCD photometry.
2. OBSERVATION The red-sensitive Thomson CCD chip of 576 x 384 pixels, attached to the BAO 60/90 cm Schmidt telescope gives a field of about 25’ x 17’. Detailed descriptions of the system can be found in Wei et r~L(~l. BVRI filters were used. Table 1 lists their parameters. Nine secondary photometric standard stars of Moffett & BarnesI (Table 2), which fall into 3 separate CCD frames, were observed on Oct. 31, 1989 in BVRI four colors. These stars are located near the equator and have magnitudes around 10. For each frame, 11 CCD exposures in each color were taken, resulting in a total number of 132 CCD exposures. In order to get as high as possible a signal-to-noise ratio, the exposures were taken out of focus to lengthen the exposure time and to avoid saturation of bright stars. The integration time is arranged as follows: 15s or 20s for B, 6s or 8s for V, 4s or 5s for R, and 2s for I, respectively. Table
1.
Filter
Specification
B
GG385/2
+ BGlt/l
V
GG495/2
+ BG18/2 + KG312
+ BGIt./l
R
RG610/2 + BG20/2 f
I
RG9/3
Table R.A. (1980)
STAR
2.
Dec.
9 Secondary
Standard
R.A. (1989.8)
I
+ KG312 KG312
Stare Dec.
SP. (YPC
433
0
IS
52
+o
54
13
0
56
22.3
+0
57
23.6
G2
107
0
S5
49
+0
59
.28
0
56
19.3
i-0
02
38.6
GO
I
508
0
5s
49
-l-l
03
05.
o
56
19.3
+l
06
15.6
FS
1
510
0
56
06
-i-l
00
31
0
56
36.2
+1
03
41.5
G5
101
1
52 .16
+0
16
30
1
52
46.4
+0
19
23.3
GYP
103
1
52
19
30
17
21
1
52
49.4
+o
20
14.3
G8
241
I
54
17
+o
30
40
1
54
47.4
+0
33
32.5
G2
326
1 53
48
+0
41
I2
1
54
18.4
+O
44
04.7
F6
3
332
1
01
+o
34
20
1
53
31.4
+o
37
13.0
F8
3
53
Frame
1
1 Frame
2 2
Frame
3
JIANG
Zhao-ji et al.
3. DATA
REDUCTION
The raw CCD frames were reduced in an usual way, i.e. bias subtracted and flat-field corrected. Then the local sky background for each standard star was determined as follows. An array of 65 x 65 pixels centering at the star was extracted and a histogram built with ADU per pixel as the abscissa and frequency as the ordinate. Then the histogram is fitted by a Gaussian curve. The ADU number corresponding to the peak of the Gaussian curve was adopted as the local background for the star. The aperture size for the standard star is 15 x 15 pixels. The data array of each standard star and the local sky background are thus used to calculate the instrumental magnitude according to the formula: instrumental
= 20-2.51og(C(ADU-S#Y)/integration
magnitude
time(in
second))
C means the sum for all the pixels within the aperture. The constant 20 is artificially chosen to make the instrumental magnitudes close to the actual ones(the data not presented here due to limited space). Let the color equations be [*I(‘1: V-v=hr(B-V)+h~.X”+hs B - v =
h4(b
-
v)
+
h5
* &,”
+
h6
V - R = h7(u - r) + h8 . Xv, + hg V - I = hlo(v - i) t hll .X,;
t hlz
where BVRI and bvri denote magnitudes in the standard system(Table 3) and the instrumental system, respectively; h1,4,,,10 are color coefficients, h2,5,8,11 are extinction coefficients and h3,6,9,12 are zero points. Xs are mean air masses. Table 3. BVRI
Photoelectric
Standard Magnitudes vol. 84( 1979)
Moffett and Barnes
B-V
1
V-R
AJ
(R--l
Star
V
B-V
433
11.665
+0.684
0.029
.0.025
0.025
507
11.349
+0.961
0.031
0.020
0.018
508
11.678
+0.553
0.012
0.031
0.024
510
9.987
+1.065
0.01s
0.020
0.007
101
9.736
+0.651
0.018
0.016
0.014
103
8.829
+1.166
0.011
0.013
0.016
241
9.405
+0.8S2
0.018
0.019
0.02s
326
9.563
+0.4so
0.023
0.008
0.012
332
9.797
+0.506
0.017
0.014
0.032
(standard
deviations)
CCD Photometry
229
The equation system can be solved by iterative method. Fig. 1 Uustrates tive process. After 15 iterations, we get the solution as follows: V - v
=
O.O14(B - V) - 0.223X,
B - V
=
=
=
- 1.433 0.052
0.008
l.l84(v
- r) - 0.090X,,
+ 0.065
0.004
0.018
V-I
0.022
1.481(b - v) - 0.144& 0.031
V - R
- 0.151
0.003
0.009
0.011
1.185(~ - i) - 0.160X”; 0.013
+ 0.394
0.005
We also present the standard deviation equation. The obtained color equations magnitudes of 9 standard stars to BVRI Table 4, which gives us an estimation of
I
0.010
for each coefficient in second line of each were used to transform the instrumental standard system. The result was listed in our photometric precision.
I
Let hr=O
the itera-
h4,7,1o =
11
1 Taking hr,+r,ro aa constant, we obtain the values of hs,s,s,ii respectively by the leastsquare method. I t Taking hs.5.s.r 1 as constant, we ob&.the values of hl,r,t,lo
respectively by the leastsquare method and then get the values of hs,s,g,is No
11
Yt?S
stop
Fig. 1
JIANG Zhao-ji et al.
230
Table 4. BVRI Star
V
CCD Photometric
V-R
B-V
Magnitudes
v-1
N
by color equations(this V
(
B-V
1
(standard 433
11.683
507
11.352
508
11.698
V-R
paper) (
V-l
deviations)
0.564
0.922
11
0.019
0.067
0.018
0.035
0.924
0.745
1.231
11
0.010
0.077
0.025
0.027
0.546
0.501
0.817
11
0.018
0.072
0.032
0.027
0.631
510
9.964
1.059
0.814
1.327
11
0.022
0.030
0.025
0.022
101
9.734
0.668
0.538
0.883
11
0.018
0.022
0.009
0.019 0.019
103
8.834
1.173
0.847
1.397
11
0.014
0.023
0.Ol.J
241
9.415
0.850
0.718
1.191
11
0.012
0.036
0.015
0.016
326
9.549
0.494
0.395
0.664
11
0.015
0.026
0.012
0.013
332
9.780
0.543
0.468
0.764
11
0.019
0.024
0.012
0.020
B-V ccd
0.4
0.6 V-R
V
ted
0.8 ccd
‘-’ ccd Fig. 2
CCD Photometry
231
4. DISCUSSION 1. We plot in Fig.2, for the 9 standard stars, the magnitudes given by Moffett & Barnes[31 against the magnitudes calculated by our color equations derived above to show the goodness of the fitting. We have also studied the problem in another way. The instrumental magnitudes of the 9 standards for each color were corrected for the atmosphere extinction. The extinction coefficients were obtained with the CCD
I
K-0.30&.01
X (AIR
lIEiS)
Fig. 3 Table L. CCD Photomath rtar
Magnitudes (MO-CCD
(1989,[email protected]) I
R
v
B
watt)
433
13.2482
0.0424
11.8249
0.0152
Jt.4225
0.0109
11.3602
0.0217
507
13.1143
0.0474
11.4963
0.0083
10.9012
0.0142
10.7911
0.0202
SO8
13.1878
0.0495
11.8253
0.0134
11.5i69
0.0116
11.4873
0.0130
II0
11.8029
0.0232
to. 1249
0.0161
9.4639
0.0107
9.3069
0.0149
tot
33.3t3e
0.0286
9.8943
0.0207
9.4842
0.0170
9.4326
0.0170
to3
10.7476
0.0226
e.9921
0.0153
8.3082
0.0157
a.1120
0.0190
241
11.0643
0.0363
9.s354
0.0161
a.9at3
0.0198
8.8474
0.0211
326
10.9477
0.0289
9.668s
0.0196
9.4113
0.0124
9.4434
0.0174
332
Il.2415
0.0320
9.9242
0.0260
9.1770
0.0229
9.177s
0.02te
232
JIANG Zhao-ji et al.
K=O.995i.O06 I
1
10
12
’ (baoccdsyrtcm)
-
X=1.014*
, 8
1 a
12
10 R (boo,ccd ryslcm)
0.4 -
Fig.
‘QE”
4
Curve of BAO Tbomsm
Fig. 5
10
’
CCD -
(bawccd
syrtem)
016
233
CCD Photometry
photometric data of the 9 standards taken at different zenith distances on the same night. Fig.3 shows the results of the star No. 510 for 4 colors. The extinction corrected instrumental magnitudes are listed in Table 5. We again plot the magnitudes given by Moffett & Barnes131 against the magnitudes in Table 5 in Fig. 4. The data can well be fitted by a straight line indicating the reliability of the photometric system. 2. It is notable that the color coefficient of the B - V relation in color equations is 1.481, far greater than 1. This means that the sensitivity of our CCD-filter combination bias to longer wa.velength in B. We believe that such a bias is not due to the filters themselves but due to the fast drop of CCD quantum efficency toward shorter wavelengths within the B ba.nd where most of the ADU numbers detected are coming from the longer wavelength part. In fact, the integration time for B color is about 10 times as long as that for I color. We thank Dr. SUN Yi-li of Beijing Astronomical Observatory and Dr. YAO Bao-an of Shanghai Astronomical Observatory for their valuable discussions and help. We also thank BAO CCD group for providing the quantum efficiency curve of the Thomson chip. Acknowledgement
References
c31 r41
Gillilond, R.L.. Brown, T.M., PASP, lOO(l988). Wei, M., Chen, J., Jiang, Z., PASP, 102(1990), Moffett, T.J., Barnes. T.G.. AJ, S4(1979),628 Schild, R.E., PASP. 95(1983), 1021
[51
Hiltner,
111 [21
W.A..
in Astronomical
Techniques,
754 698
(1962),
Chapter
8, 178
@ Pergamon Press Ltd Printed in Great Britain
(1992)16/2,226-253
02751062/92$10.00+.00
Acta AstrophysSin.(l992)12/1,47-53
A photometric JIANG
study of the BAO-CCD system*
Zhao-ji
LI Yong
Beijing Astronomical
CHEN
BVRI
Jian-sheng
Observatory, Chinese Academy of Sciences Beijing 100080
Abstract The BAO Schmidt telescope with a Thomson CCD of 576 x 384 pixels was used for BVRI photometry. 9 secondary photometric standard stars in 3 CCD frames were observed on Oct. 31, 1989, to establish the color equations. Some problems relevant to the capability of CCD photometry are discussed. Key words:
CCD-photometry-color
equations
1. INTRODUCTION Magnitudes and color indices are the important quantities used in many astrophysical researches. Photographic techniques can provide photometry for large field with low accuracy, while photoelectric photometry, on the other hand, can reach high precision but , in most cases, is limited to measuring one object at a time. Since the astronomical application of large format CCD, it soon became clear that as a detector, CCD combines the advantages of photographic plates and photomultiplier tubes, namely: two dimensional detection, good linear response, high quantum efficiency, and large dynamic range. In addition, with digitized CCD output from the entire frame, one can use more sophisticate statistic method to determine accurately the local sky background and can optimize the selection of the aperture for photometry to raise the S/N ratio. However, for CCD photometry, one have to take care of the readout noise as well as the noise introduced from the flat-field correction. With the improvements in performance of CCD and the developments of image processing techniques, the precision of CCD photometry is being improved and an accuracy of up to 0.0015 magnitude is obtained in the study of CCD surface photometry in M67 l
This
work
Received
was
partly
supported
19991November
by National
5; revised
version
Natural received
Science 1991
Foundation.
September
18
aCCD Photometry
227
(Gilliland & Brown[‘l), which is comparable to that of photoelectric photometry. It is undoubted that the usage of CCD for both two dimensional surface photometry as well as for large ensembles of stars is very promising. Taking advantage of fast focal ratio and large field, we have carried out CCD multi-color photometry using the 60/90 cm Schmidt telescope at Beijing Astronomical Observatory(BA0). As the first step of the study for BAO CCD photometric system, we established, in this paper, the color equations and discuss some problems relevant to the performance and capability of CCD photometry.
2. OBSERVATION The red-sensitive Thomson CCD chip of 576 x 384 pixels, attached to the BAO 60/90 cm Schmidt telescope gives a field of about 25’ x 17’. Detailed descriptions of the system can be found in Wei et r~L(~l. BVRI filters were used. Table 1 lists their parameters. Nine secondary photometric standard stars of Moffett & BarnesI (Table 2), which fall into 3 separate CCD frames, were observed on Oct. 31, 1989 in BVRI four colors. These stars are located near the equator and have magnitudes around 10. For each frame, 11 CCD exposures in each color were taken, resulting in a total number of 132 CCD exposures. In order to get as high as possible a signal-to-noise ratio, the exposures were taken out of focus to lengthen the exposure time and to avoid saturation of bright stars. The integration time is arranged as follows: 15s or 20s for B, 6s or 8s for V, 4s or 5s for R, and 2s for I, respectively. Table
1.
Filter
Specification
B
GG385/2
+ BGlt/l
V
GG495/2
+ BG18/2 + KG312
+ BGIt./l
R
RG610/2 + BG20/2 f
I
RG9/3
Table R.A. (1980)
STAR
2.
Dec.
9 Secondary
Standard
R.A. (1989.8)
I
+ KG312 KG312
Stare Dec.
SP. (YPC
433
0
IS
52
+o
54
13
0
56
22.3
+0
57
23.6
G2
107
0
S5
49
+0
59
.28
0
56
19.3
i-0
02
38.6
GO
I
508
0
5s
49
-l-l
03
05.
o
56
19.3
+l
06
15.6
FS
1
510
0
56
06
-i-l
00
31
0
56
36.2
+1
03
41.5
G5
101
1
52 .16
+0
16
30
1
52
46.4
+0
19
23.3
GYP
103
1
52
19
30
17
21
1
52
49.4
+o
20
14.3
G8
241
I
54
17
+o
30
40
1
54
47.4
+0
33
32.5
G2
326
1 53
48
+0
41
I2
1
54
18.4
+O
44
04.7
F6
3
332
1
01
+o
34
20
1
53
31.4
+o
37
13.0
F8
3
53
Frame
1
1 Frame
2 2
Frame
3
JIANG
Zhao-ji et al.
3. DATA
REDUCTION
The raw CCD frames were reduced in an usual way, i.e. bias subtracted and flat-field corrected. Then the local sky background for each standard star was determined as follows. An array of 65 x 65 pixels centering at the star was extracted and a histogram built with ADU per pixel as the abscissa and frequency as the ordinate. Then the histogram is fitted by a Gaussian curve. The ADU number corresponding to the peak of the Gaussian curve was adopted as the local background for the star. The aperture size for the standard star is 15 x 15 pixels. The data array of each standard star and the local sky background are thus used to calculate the instrumental magnitude according to the formula: instrumental
= 20-2.51og(C(ADU-S#Y)/integration
magnitude
time(in
second))
C means the sum for all the pixels within the aperture. The constant 20 is artificially chosen to make the instrumental magnitudes close to the actual ones(the data not presented here due to limited space). Let the color equations be [*I(‘1: V-v=hr(B-V)+h~.X”+hs B - v =
h4(b
-
v)
+
h5
* &,”
+
h6
V - R = h7(u - r) + h8 . Xv, + hg V - I = hlo(v - i) t hll .X,;
t hlz
where BVRI and bvri denote magnitudes in the standard system(Table 3) and the instrumental system, respectively; h1,4,,,10 are color coefficients, h2,5,8,11 are extinction coefficients and h3,6,9,12 are zero points. Xs are mean air masses. Table 3. BVRI
Photoelectric
Standard Magnitudes vol. 84( 1979)
Moffett and Barnes
B-V
1
V-R
AJ
(R--l
Star
V
B-V
433
11.665
+0.684
0.029
.0.025
0.025
507
11.349
+0.961
0.031
0.020
0.018
508
11.678
+0.553
0.012
0.031
0.024
510
9.987
+1.065
0.01s
0.020
0.007
101
9.736
+0.651
0.018
0.016
0.014
103
8.829
+1.166
0.011
0.013
0.016
241
9.405
+0.8S2
0.018
0.019
0.02s
326
9.563
+0.4so
0.023
0.008
0.012
332
9.797
+0.506
0.017
0.014
0.032
(standard
deviations)
CCD Photometry
229
The equation system can be solved by iterative method. Fig. 1 Uustrates tive process. After 15 iterations, we get the solution as follows: V - v
=
O.O14(B - V) - 0.223X,
B - V
=
=
=
- 1.433 0.052
0.008
l.l84(v
- r) - 0.090X,,
+ 0.065
0.004
0.018
V-I
0.022
1.481(b - v) - 0.144& 0.031
V - R
- 0.151
0.003
0.009
0.011
1.185(~ - i) - 0.160X”; 0.013
+ 0.394
0.005
We also present the standard deviation equation. The obtained color equations magnitudes of 9 standard stars to BVRI Table 4, which gives us an estimation of
I
0.010
for each coefficient in second line of each were used to transform the instrumental standard system. The result was listed in our photometric precision.
I
Let hr=O
the itera-
h4,7,1o =
11
1 Taking hr,+r,ro aa constant, we obtain the values of hs,s,s,ii respectively by the leastsquare method. I t Taking hs.5.s.r 1 as constant, we ob&.the values of hl,r,t,lo
respectively by the leastsquare method and then get the values of hs,s,g,is No
11
Yt?S
stop
Fig. 1
JIANG Zhao-ji et al.
230
Table 4. BVRI Star
V
CCD Photometric
V-R
B-V
Magnitudes
v-1
N
by color equations(this V
(
B-V
1
(standard 433
11.683
507
11.352
508
11.698
V-R
paper) (
V-l
deviations)
0.564
0.922
11
0.019
0.067
0.018
0.035
0.924
0.745
1.231
11
0.010
0.077
0.025
0.027
0.546
0.501
0.817
11
0.018
0.072
0.032
0.027
0.631
510
9.964
1.059
0.814
1.327
11
0.022
0.030
0.025
0.022
101
9.734
0.668
0.538
0.883
11
0.018
0.022
0.009
0.019 0.019
103
8.834
1.173
0.847
1.397
11
0.014
0.023
0.Ol.J
241
9.415
0.850
0.718
1.191
11
0.012
0.036
0.015
0.016
326
9.549
0.494
0.395
0.664
11
0.015
0.026
0.012
0.013
332
9.780
0.543
0.468
0.764
11
0.019
0.024
0.012
0.020
B-V ccd
0.4
0.6 V-R
V
ted
0.8 ccd
‘-’ ccd Fig. 2
CCD Photometry
231
4. DISCUSSION 1. We plot in Fig.2, for the 9 standard stars, the magnitudes given by Moffett & Barnes[31 against the magnitudes calculated by our color equations derived above to show the goodness of the fitting. We have also studied the problem in another way. The instrumental magnitudes of the 9 standards for each color were corrected for the atmosphere extinction. The extinction coefficients were obtained with the CCD
I
K-0.30&.01
X (AIR
lIEiS)
Fig. 3 Table L. CCD Photomath rtar
Magnitudes (MO-CCD
(1989,[email protected]) I
R
v
B
watt)
433
13.2482
0.0424
11.8249
0.0152
Jt.4225
0.0109
11.3602
0.0217
507
13.1143
0.0474
11.4963
0.0083
10.9012
0.0142
10.7911
0.0202
SO8
13.1878
0.0495
11.8253
0.0134
11.5i69
0.0116
11.4873
0.0130
II0
11.8029
0.0232
to. 1249
0.0161
9.4639
0.0107
9.3069
0.0149
tot
33.3t3e
0.0286
9.8943
0.0207
9.4842
0.0170
9.4326
0.0170
to3
10.7476
0.0226
e.9921
0.0153
8.3082
0.0157
a.1120
0.0190
241
11.0643
0.0363
9.s354
0.0161
a.9at3
0.0198
8.8474
0.0211
326
10.9477
0.0289
9.668s
0.0196
9.4113
0.0124
9.4434
0.0174
332
Il.2415
0.0320
9.9242
0.0260
9.1770
0.0229
9.177s
0.02te
232
JIANG Zhao-ji et al.
K=O.995i.O06 I
1
10
12
’ (baoccdsyrtcm)
-
X=1.014*
, 8
1 a
12
10 R (boo,ccd ryslcm)
0.4 -
Fig.
‘QE”
4
Curve of BAO Tbomsm
Fig. 5
10
’
CCD -
(bawccd
syrtem)
016
233
CCD Photometry
photometric data of the 9 standards taken at different zenith distances on the same night. Fig.3 shows the results of the star No. 510 for 4 colors. The extinction corrected instrumental magnitudes are listed in Table 5. We again plot the magnitudes given by Moffett & Barnes131 against the magnitudes in Table 5 in Fig. 4. The data can well be fitted by a straight line indicating the reliability of the photometric system. 2. It is notable that the color coefficient of the B - V relation in color equations is 1.481, far greater than 1. This means that the sensitivity of our CCD-filter combination bias to longer wa.velength in B. We believe that such a bias is not due to the filters themselves but due to the fast drop of CCD quantum efficency toward shorter wavelengths within the B ba.nd where most of the ADU numbers detected are coming from the longer wavelength part. In fact, the integration time for B color is about 10 times as long as that for I color. We thank Dr. SUN Yi-li of Beijing Astronomical Observatory and Dr. YAO Bao-an of Shanghai Astronomical Observatory for their valuable discussions and help. We also thank BAO CCD group for providing the quantum efficiency curve of the Thomson chip. Acknowledgement
References
c31 r41
Gillilond, R.L.. Brown, T.M., PASP, lOO(l988). Wei, M., Chen, J., Jiang, Z., PASP, 102(1990), Moffett, T.J., Barnes. T.G.. AJ, S4(1979),628 Schild, R.E., PASP. 95(1983), 1021
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