Homework 8 (Aperture Photometry)

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