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Today: Calculating CCD gain Photometry Aperture vs. PSF Photometry Photometric Filters Effects of Atmosphere – Airmass Differential vs. Absolute Photometry Photometry of Extended Sources SNR and Exposure Time
Reading: Ch. 7 (pp. 125–132), 10
Astronomy 101 Lecture # 6
PhotometryDirect measure of integrated flux (counts per unit time
per unit area ) received from a celestial target.●
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F = specific flux from a target S = transmission function that describes
●Converting from instrumental magnitude to apparent magnitude.
Astronomy 101 Lecture # 6
m1= I1−I 2m2
mi=calibrated magnitude of star iI i=instrumental magnitude of star i
Differential Photometry
Slide Credit: Don Hoard
Astronomy 101 Lecture # 6Absolute Photometry
What if there are no calibrated stars in the same CCD frame as your target?
● Make separate observations of standard stars on the same night. Examples of star catalogs include
Landolt 1992, AJ, 104, 340 Stetson http://cadcwww.hia.nrc.ca/cadcbin/wdb/astrocat/stetson/query ● Standard star observations must span a range of airmasses (typically 1–
2.5) and a range of colors the same as that of science targets. ● Obtain observations of standard stars and targets in at least 2 filters. ● Use standard star observations to derive coefficients for the
“transformation equations” which can then be utilized to calibrate instrumental magnitudes of science targets.
Slide Credit: Don Hoard
Astronomy 101 Lecture # 6
Transformation Equations
B – V = (b–v) Tbv + Kbv X + Zbv
V – R = (v–r) Tvr + Kvr X + Zvr
R – I = (r–i) Tri + Kri X + Zri
V – I = (v–i) Tvi + Kvi X + Zvi
R = r Tr + Kr X + Zr
V = v Tv + Kv X + Zv
B, V, R, I = calibrated magnitudesb, v, r, i = instrumental magnitudesX = airmassT = color transformation coefficientsK = atmospheric extinction coefficientsZ = zero point corrections
Use IRAF package digiphot.photcal to solve for transformation coefficients and apply calibration to data.