Aperture Photometry
November
6, 2015
Ryan
Hall
Abstract
This
lab took images of Vega from Baker Observatory and applied an aperture
photometry code to them. This particular code was provided Dr. Plavchan and the
code gave us information on the x and y coordinates of the stars on our image,
instrumental magnitude, error of magnitude, full width at half maximum,
aperture size, and correction applied. This information is then used to
calculate other information such as the apparent magnitude and seeing. However,
calculating apparent magnitude of a science target requires a comparison
target, with a known apparent magnitude, to be tested. Once all the appropriate
data has generated then conclusions are made from it.
Introduction
Aperture
photometry is a process of measuring all of the counts in an aperture, or a circle
with a fixed size, of a star or celestial object. The size of this circle is
dependent on the brightness, or number of pixels with sufficient light. By then
detecting the average counts of the sky around the object, a baseline of counts
can be created for your science target. This lab does aperture photometry with
the use of an IDL code, apphot_STI.pro. This particular code finds stars and
bright objects in an image and outputs their x and y coordinates, instrumental
magnitude, error of data, full width at half maximum (fwhm), aperture size, and
correction applied. Most of this information is fairly intuitive; however, fwhm
is a portion of the light curve of an object containing the points from the
maximum value of counts to half of the maximum.
All
of the data outputted from the code can be used to calculate values such as the
apparent magnitude and seeing. Apparent magnitude is generated from the
instrumental magnitudes of the science and comparison and the apparent
magnitude of the comparison target. The relationship is shown in the following
equation.
apparent magnitude = inst magsci - inst magcom +app magcom + 2.5*log(tsci/tcom)
The end of the equation
containing the logarithmic function is required if the exposure times of the
science and comparison targets are different. The error of the apparent
magnitude can be calculated using error propagation. This is done by taking the
square root of the sum of the squares of the errors for each magnitude used in
the previous calculation. The equation is shown by the following.
apparent magnitude error = (error1^2 + error2^2 + error3^2)^1/2
Seeing can be
calculated multiplying the fwhm and the plate scale of the particular CCD.
Seeing = plate scale * fwhm
Procedure
The
lab begins by logging into EXO and creating a directory for this assignment.
The zip file provided by Dr. Plavchan, containing the aperture photometry
codes, must then be moved into the new directory. There are multiple ways of
accomplishing. The method used in my particular case was typing in the command
“firefox” and opening a Mozilla Firefox window through EXO. The file can then
be found online and downloaded. This puts the file in a directory titled
Downloads. The zip file along with all of the reduced science and comparison
images must be moved into the directory created for this assignment. The zip
file can then be opened by using the command “unzip filename.zip”. After this
all of the IDL procedures for the assignment should be able to be seen in the
directory.
Next
the command “idl” is typed into EXO to access IDL from EXO. The aperture
photometry code must next be compiled by typing “.compile apphot_STI.pro”. At
this point images must be run through the aperture photometry code by typing
“apphot_STI,’imagename.fit’”. If one would want to, all of the data that the
code outputs can be saved to a text file by typing “,outfile=’textname.txt’” at
the end of the previous code. Once all of the data is generated from the IDL
code, it can then be used to calculate the apparent magnitude of the science
target, the value of its error, and the seeing of both targets.
Results
and Conclusion
This lab used Vega as its science target. Since there
were no other stars found in the science images of Vega, this required that I
use one of my classmates images from the same night and use them as a
comparison image. These were images of a double star system so I choose to pick
Epsilon Lyra 91919B. After a couple of corrections with the IDL code, done by
Dr. Plavchan, all of the images ran through it and the output data was found to
be reasonable. All of the data, images, and text files can be found in EXO at
/home/ryan/Homework8. The comparison images are found further in at /home/ryan/Homework8/calcdata/reduced.
Table 1: Science
Target, Vega
Science Target: Vega
(exposure time= .001s)
|
||||||||
Apparent Magnitude
|
Apparent Error (±)
|
Instrumental Magnitude
|
Instrumental Magnitude Error (±)
|
Error
|
Full Width Half Max
|
Seeing
|
Aperture
|
|
1
|
-0.4854
|
0.04628
|
10.57
|
0.0381
|
0.0036
|
2.76
|
2.042
|
4
|
2
|
0.2706
|
0.12696
|
11.36
|
0.1238
|
0.0109
|
2.26
|
1.672
|
4
|
3
|
-0.5844
|
0.04774
|
10.54
|
0.0379
|
0.0036
|
3.57
|
2.642
|
4
|
4
|
-0.5944
|
0.04400
|
10.48
|
0.0346
|
0.0033
|
3.39
|
2.509
|
4
|
5
|
-0.8564
|
0.04184
|
10.41
|
0.0260
|
0.0025
|
2.99
|
2.213
|
4
|
6
|
-0.6094
|
0.03833
|
10.49
|
0.0252
|
0.0024
|
4.51
|
3.337
|
4
|
7
|
-0.7234
|
0.03617
|
10.37
|
0.0228
|
0.0022
|
2.52
|
1.865
|
4
|
8
|
-1.9404
|
0.03210
|
9.14
|
0.0101
|
0.0011
|
3.57
|
2.642
|
4
|
9
|
-0.9164
|
0.03334
|
10.17
|
0.0203
|
0.002
|
1.95
|
1.443
|
4
|
10
|
-0.6834
|
0.03980
|
10.43
|
0.0282
|
0.0027
|
2.99
|
2.213
|
4
|
avg
|
-0.7123
|
0.04566
|
10.396
|
0.0357
|
0.00343
|
3.051
|
2.258
|
4
|
Comparison Target: Epsilon
Lyra 91919 B (exposure time= .6s)
|
||||||||
App Mag
|
App Error (±)
|
Inst Mag
|
Inst Mag Error (±)
|
Error
|
FWHM
|
Seeing
|
Aperture
|
|
1
|
4.593
|
0.01
|
8.703
|
0.0244
|
0.0028
|
7.82
|
5.787
|
7
|
2
|
4.593
|
0.01
|
8.737
|
0.0262
|
0.003
|
7.57
|
5.602
|
7
|
3
|
4.593
|
0.01
|
8.772
|
0.0272
|
0.0031
|
8.67
|
6.416
|
7
|
4
|
4.593
|
0.01
|
8.722
|
0.0253
|
0.0029
|
9.17
|
6.786
|
7
|
5
|
4.593
|
0.01
|
8.914
|
0.0312
|
0.0035
|
7.82
|
5.787
|
7
|
6
|
4.593
|
0.01
|
8.747
|
0.0271
|
0.0031
|
8.74
|
6.468
|
7
|
7
|
4.593
|
0.01
|
8.741
|
0.0262
|
0.003
|
7.4
|
5.476
|
7
|
8
|
4.593
|
0.01
|
8.728
|
0.0288
|
0.0033
|
10.88
|
8.051
|
7
|
9
|
4.593
|
0.01
|
8.734
|
0.0245
|
0.0028
|
7.48
|
5.535
|
7
|
10
|
4.593
|
0.01
|
8.761
|
0.0263
|
0.003
|
7.98
|
5.905
|
7
|
avg
|
4.593
|
0.01
|
8.756
|
0.0267
|
0.00305
|
8.353
|
6.181
|
7
|
Given that we know that the apparent magnitude of
Vega should be 0, my average value of -.71 ± .0456 appears to be off by a
little. This variation is somewhat understandable due to a combination of
factors such as imperfect instruments, the fact that the comparison image was
not from the same telescope and CCD, and not great seeing with an average of
2.26 for Vega and 6.18 for Epsilon Lyra 91919B.
Figure 1: Vega, Science
Image 1
Figure 2: Epsilon Lyra
91919B, Comparison Image 1
