Author: Glenn Gombert Date: 970308 Revision: #1 970308 Key Words: CCD, astrometry, photometry, observation, techniques
In this TASS Technical Note I will expound upon the photometric calibration techniques that have been developed here in Dayton for reducing "raw" TASS images in FITS file format to calibrated star lists. This Technical Note details the techniques that have been developed here in Dayton over the last three months or so for TASS Data analysis. I have made use of several piece's of software that have been developed by several different individuals and combined them together to make positional and brightness measurements from TASS images.
The techniques that are described in this Technical Note are not limited to TASS images but should be applicable to just about any type of CCD image that has been saved in FITS format. These techniques that are described here should be general enough to be used on just about any ccd image or scanned photograph.
It is desired to measure precisely the RA and DEC for each object that is detected in an image as well as an estimate of the Stellar magnitude in a particular photometric band. These are usually the Johnson system (UVBRI Photometric) bands.
In part we will demonstrate how ccd images (with accurate known stars in the field) can be photometrically reduced using the "Sextractor" program. The demonstration image is a deep field image of the well known (studied) area of M67 taken with a 16 inch Newtonian telescope and an SBIG ST-6 ccd camera equipped with a "V" Band photometric filter.
The image is particularly suitable for calibration purposes since many of the stars in the field have been accurately measured as a part of several journal article(s):
Publications of the Astronomical Society of the Pacific 96, 1021, 1983
Publications of the Astronomical Society of the Pacific 102, 1004, 1990
The information that is used as a reference is part of the ccd observation package developed by the AAVSO as a part of their ccd variable star observing program. It was furnished for use in the TASS project by Dr. Janet Mattei the Director at the AAVSO.
The following stars in the M67 "finder chart" are used as calibration and "check" points in the M67 image:
Star Num "V" Band Mag. "I" Band Mag 170 18.644 16.625 149 21.543 20.191 111 21.734 20.423 I-228 21.387 19.861 I-242 21.085 18.753 81 19.971 18.420 108 18.703 16.651 130 21.893 20.684 134 21.262 19.919 135 20.435 18.995 127 21.776 20.447 124 21.136 19.904 117 21.625 20.030
Click on the image below to download a FITS version of the image.
The image shown above (M67V.FTS) was reduced using E. Burton's Sextractor program by setting the "zero magnitude" point as accurately as possible using one of the reference stars in the list shown above. The approximate center of the field is RA 08:50:32 and DEC 11:52:24, it is about 9.5 arcminutes per side.
"Sextractor" is able to compute four different types of photometric instrumental magnitudes:
Several of these methods were tried while using the M67V.fts image, they gave similar results on the reference stars in the image and simple aperture photometry was finally used in the test catalog that was generated from the image.
Show below is the "parameter" file that was used to generate a reference catalog for the M67 "V" band image:
=============================================================================== # Default configuration file for SExtractor # Glenn Gombert 03/08/97 #-------------------------------- Catalog ------------------------------------ CATALOG_NAME m67.cat # name of the output catalog CATALOG_TYPE ASCII # may be ASCII or FITS PARAMETERS_NAME default.par # name of the file containing catalog contents #------------------------------- Extraction ---------------------------------- THRESHOLD_TYPE SIGMA # may be SIGMA or MAGNITUDE THRESHOLD 1.5 # threshold above above background DETECTION_TYPE CCD # may be CCD or PHOTO CONVOLVE Y # convolve image before thresholding (Y or N)? CONVOLVE_NAME gauss233.con # name of the file containing the mask CONVOLVE_NORM Y # is the mask to be normalized. EXTRACT_MINAREA 5 # minimum number of pixels above threshold DEBLEND_NTHRESH 32 # number of deblending sub-thresholds DEBLEND_MINCONT 0.005 # minimum contrast parameter for deblending CLEAN Y # Clean spurious detections? (Y or N)? CLEAN_PARAM 1.5 # Cleaning efficiency CLEAN_OBJSTACK 5000 # Size of buffer containing objects #------------------------------ Photometry ----------------------------------- PHOTOM_APERTURE 10 # FIXED aperture diameter in pixels PHOTOM_KPAR 3.5 # Kron parameter for AUTO PHOTOM_KSIG 6.0 # radius in sigmas for AUTO PHOTOM_KMINSIG 6.0 # min radius in sigmas for AUTO SATUR_LEVEL 65556.0 # level (in ADUs) at which arises saturation MAG_ZEROPOINT 31.740 # magnitude zero-point MAG_GAMMA 4.0 # gamma of emulsion (for photographic scans) GAIN 6.7 # detector gain in e-/ADU. PIXEL_SCALE 1.0 # size of pixel in arcsec. #------------------------- Star/Galaxy Separation ---------------------------- SEEING_FWHM 2.5 # stellar FWHM in arcsec STARNNW_NAME default.nnw # Neural-Network_Weight table filename #------------------------------ Background ----------------------------------- BACK_XSIZE 64 # width of the background mesh-size BACK_YSIZE 64 # height of the background mesh-size BACK_FLTRXSIZE 3 # width of the background filtering mask BACK_FLTRYSIZE 3 # height of the background filtering mask BACKPHOTO_TYPE LOCAL # may be GLOBAL or LOCAL BACKPHOTO_THICK 24 # thickness of the background LOCAL annulus #------------------------------ Check Image ---------------------------------- CHECKIMAGE_TYPE NONE # may be one of NONE, BACKGROUND, -BACKGROUND, # OBJECTS, SEGMENTATION, APERTURES or CONVOLVED CHECKIMAGE_NAME M67CK.fts # Filename for the check-image #--------------------- Memory (change with caution!) ------------------------- MEMORY_PIXSTACK 50000 # number of pixels in stack MEMORY_BUFSIZE 512 # number of lines in buffer #---------------- Scanning parameters (change with caution!) ----------------- SCAN_ISOAPRATIO 0.6 # maximum isoph. to apert ratio allowed #----------------------------- Miscellaneous --------------------------------- VERBOSE_TYPE NORMAL # may be QUIET, NORMAL or FULL
The catalog shown below was generated with the parameter file that is shown above. There are 10-11 reference stars that were detected in the image that were referenced back to the AAVSO chart shown the field in M67.
X-POS Y-POS Magnitude FWHM
270.60 1.50 26.1207 3.63
133.75 3.15 24.6282 2.35
167.53 3.70 25.2204 2.24
296.66 5.36 23.1184 3.88
264.34 9.34 23.6828 2.50
32.82 11.81 21.7928 2.30
189.07 17.63 23.6475 2.48
256.72 21.04 25.6973 1.00
139.22 15.13 21.3028 2.12
147.33 17.53 21.5191 2.28
94.57 23.90 22.2671 2.29
52.14 17.79 22.4633 2.21
58.87 27.04 22.2127 2.23
88.17 29.81 22.3082 2.24
311.34 5.98 22.5201 2.85
171.91 34.03 26.3778 2.45
110.50 32.71 23.9912 2.31
304.92 7.69 22.4060 2.37
237.41 35.51 24.8665 2.30
70.75 33.75 22.2326 2.31
64.20 38.42 19.2993 3.97
3.82 23.16 21.6797 2.31
4.77 36.05 20.7479 3.90
189.38 40.11 24.1183 2.41
193.01 42.03 24.5331 2.22
191.38 34.43 24.3506 2.07
94.99 43.15 21.6689 2.16
228.52 42.35 22.7350 2.32
302.61 48.38 27.1983 4.34
269.98 49.13 25.1509 2.68
131.12 52.06 25.0987 2.97
261.05 52.95 22.9631 2.17
120.50 56.75 25.6339 3.42
302.22 60.17 99.0000 3.90
48.20 50.06 19.4682 2.30
104.03 59.70 24.7169 2.39
138.79 60.96 25.4943 2.94
308.10 61.20 24.8055 5.08
221.47 55.06 22.6023 2.24
223.75 61.60 22.4950 2.27
90.48 61.78 23.2218 2.26
80.85 65.30 21.7809 2.28
256.85 66.80 22.3413 2.25
42.78 75.01 25.2285 3.13
109.52 78.79 25.7995 1.71
215.97 78.16 25.4169 2.08
180.75 70.30 23.0551 2.30
164.18 69.72 19.3130 2.24
66.66 72.03 23.1564 2.29
71.68 79.89 23.5105 2.22
273.91 76.12 20.5327 2.31
25.71 80.98 22.8009 2.32
308.08 84.82 25.9451 3.89
314.40 25.08 99.0000 44.33
42.75 89.04 26.5180 3.10
60.02 88.95 27.6936 0.00
139.19 86.95 22.6132 2.16
2.64 92.82 25.0025 2.28
319.28 91.55 99.0000 8.15
96.01 81.93 19.4705 2.18
22.29 96.63 24.7727 2.38
42.36 103.84 23.7160 2.24
65.21 103.00 21.5172 2.24 # V-Band Mag = 21.543 (Star #149)
290.43 107.73 25.8228 1.96
291.89 113.65 26.4008 4.62
183.63 106.14 21.6155 2.26
191.39 112.24 21.9698 2.04
221.00 119.89 24.0427 2.09
231.96 123.36 26.8474 2.69
83.83 117.84 21.0468 2.30
80.43 122.48 21.0290 3.67
78.01 108.01 23.4252 2.41
218.36 126.99 24.2288 3.96
267.72 122.49 21.3987 2.32
11.03 116.72 18.7301 2.65 # V-Band Mag = 18.665 (Star #170)
144.76 129.21 21.5863 2.22 # V-Band Mag = 21.625 (Star #117)
130.23 130.91 21.0670 2.22 # V-Band Mag = 21.136 (Star #124)
160.74 131.00 21.9730 2.18
156.07 135.80 20.9919 2.31
29.44 139.10 23.8486 2.35
104.48 146.51 22.1653 2.26
127.50 146.11 21.7482 2.28 # V-Band Mag = 21.667 (Star #127)
9.95 159.51 27.2567 2.39
279.68 158.46 24.3957 2.24
220.77 163.41 22.1960 2.30
17.01 167.31 25.7114 0.76
249.10 167.19 23.5497 2.08
264.35 171.51 22.4046 2.28
318.26 182.97 29.6096 6.76
229.31 186.06 24.0778 2.19
160.21 177.66 22.8448 2.28
152.66 184.91 21.7067 2.28
308.73 183.96 22.4707 2.28
122.00 166.63 21.8052 2.30 # V-Band Mag = 21.893 (Star #130)
122.19 176.02 22.0348 2.27
110.11 174.85 20.4286 2.24 # V-Band Mag = 20.435 (Star #109)
114.08 185.77 21.1847 2.31 # V-Band Mag = 21.262 (Star #113)
124.73 186.10 22.0507 2.23
49.32 192.83 26.2069 2.62
232.52 193.37 26.2234 2.22
270.96 192.22 28.1693 2.47
319.28 191.45 99.0000 5.29
281.76 194.63 25.0397 2.38
69.10 193.40 23.1021 2.29
26.89 200.47 24.7575 2.30
317.86 200.12 99.0000 8.61
293.33 204.55 28.6850 2.53
288.80 204.95 28.7945 4.87
302.07 205.68 99.0000 4.23
312.83 206.55 27.4023 2.58
315.88 208.40 99.0000 1.89
16.91 205.45 21.9251 2.35
120.64 208.13 24.6600 2.33
299.70 212.67 99.0000 3.28
300.00 215.50 99.0000 2.11
293.48 216.42 99.0000 7.45
134.05 218.58 25.5643 2.38
11.97 222.58 26.9625 2.31
158.51 210.61 18.7512 2.45 V-Band Mag = 18.703 (Star #159)
166.35 195.37 22.0776 2.51
226.65 211.56 18.9675 2.31 V-Band Mag = 19.036 (Star #226)
282.31 225.02 29.3702 1.22
297.36 224.77 99.0000 4.91
276.47 224.07 28.2092 8.97
290.55 220.15 30.3795 -0.10
290.89 225.00 28.9568 7.45
301.84 227.53 99.0000 3.35
305.60 222.63 99.0000 12.61
118.79 226.64 22.0675 2.30
296.30 232.64 99.0000 9.03
195.35 228.41 19.8608 3.17 V-Band Mag = 19.971 (Star #I-242)
275.64 237.86 99.0000 4.34
86.86 239.51 27.7560 3.70
288.84 239.04 99.0000 0.00
296.56 240.68 99.0000 3.15
277.49 243.73 99.0000 1.19
102.34 245.63 26.6066 3.34
286.04 245.24 99.0000 4.58
301.28 240.35 99.0000 5.17
303.50 245.82 99.0000 4.64
298.03 247.34 99.0000 5.85
290.17 245.72 99.0000 7.49
194.22 246.61 23.4905 7.40
277.78 251.70 99.0000 4.43
281.61 250.53 99.0000 8.04
172.56 251.09 22.4485 2.29
155.37 253.68 24.4733 2.17
263.38 254.62 99.0000 4.11
267.25 255.11 99.0000 3.40
273.33 257.83 99.0000 3.95
289.48 258.64 99.0000 11.27
317.33 234.09 99.0000 36.03
311.75 214.75 27.1462 3.30
316.99 217.14 99.0000 10.10
308.71 232.92 99.0000 14.37
308.94 255.60 99.0000 16.47
49.60 259.22 24.3620 2.38
274.31 263.95 99.0000 5.67
267.00 265.19 99.0000 3.49
285.40 265.60 99.0000 3.28
319.63 265.33 99.0000 5.55
315.97 266.58 99.0000 5.73
282.59 272.14 99.0000 3.38
291.65 266.49 99.0000 5.63
297.80 265.34 21.2208 11.12
294.68 271.29 32.2415 2.29
305.18 267.23 25.4388 6.94
308.54 269.55 99.0000 4.77
308.51 261.87 99.0000 6.60
270.02 273.24 29.0356 3.34
274.54 275.87 29.6646 2.95
100.82 269.96 22.9642 2.27
107.15 273.36 22.9621 2.60
295.95 277.88 21.3388 7.96
263.79 278.58 21.6592 7.64
276.79 279.92 26.9087 2.51
284.39 279.25 21.6594 5.28
281.06 280.01 21.7492 6.35
316.05 278.93 20.4708 9.95
306.55 279.23 21.3144 9.39
274.30 284.08 25.8467 2.89
51.19 284.34 20.0763 2.35
163.30 290.93 18.8082 31.27
7.08 284.72 21.1285 2.29
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