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Quick Point Source Photometric Measurement |
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This is a brief guide on how to obtain a quick flux
measurement for a point source from Spitzer IRAC and MIPS data.
The best final photometry for your particular data probably
will come from a more detailed analysis, where you
go back to individual BCDs or apply post-BCD tools first
to clean up instrument artifacts, etc. However,
the description below will let you get quick answers that
are reasonably accurate.
The quick method is to measure integrated flux through a circular aperture using the SSC pipeline-created mosaic images, then apply appropriate aperture corrections. There are a variaty of tools which can perform aperture photometry, for example, IDL, IRAF apphot package, etc. If you start with the mosaiced image from the SSC pipeline, the image is already in physical unit of surface brightness, including the effect of exposure time. The total integration time map can be computed by multiplying the coverage map with the exposure time of a single BCD image. The physical unit of the mosaicked images is surface brightness unit, i.e., MJy/sr for IRAC and MIPS data. Therefore, the final aperture flux density should equal to (area of 1 pixel) multiplied by (summed surface brightness through your choice of aperture). To find out the exact pixel size of the pipeline mosaiced images, we recommend checking the FITS header keywords, CDELT1 and CDELT2 (in degrees). Specifically, for IRAC, the pixel size of the pipeline mosaicked images is 1.2 arcsec. To convert MJy/sr to uJy/pixel, you can convert steradian into arcseconds squared, then multiply by the area of the pixel. Thus, the conversion factor between MJy/sr and uJy/pixel for IRAC is: 1 MJy/sr = (1E12 micro-Jy)/(4.254517E10 arcsec**2) x 1.2 arcsec x 1.2 arcsec = 33.846uJy/pixel. Similarly, for MIPS data, the pixel scale is 2.45" per pixel (pipeline mosaicked image) for 24 um data, and 4.0" per pixel for 70 um pipeline produced mosaicked images. The convertion factor is: 1 MJy/sr = (1E12 micro-Jy)/(4.254517E10 arcsec**2) x (2.45)**2 arcsec^2 = 141.08uJy/pixel (MIPS 24um) and 376.07 uJy/pixel (MIPS 70um). See the MIPS Data Handbook, Table 3.7, for more details on pixel sizes.
IRAC dataFor bright point sources, set the aperture size to 10 pixels and the sky annulus to between 10 and 20 pixels. The IRAC calibration is based on an aperture this size, so for this aperture there is no aperture correction necessary. For fainter stars, it is better to use a smaller aperture and then apply an aperture correction. For example, for 5 pixel radius with sky annulus between 5 to 10 pixels, the aperture corrections are 1.061, 1.064, 1.067 and 1.089 for the four channels, respectively. Again, you should multiply your measured fluxes by these corrections. The full aperture correction table for various sizes are listed in the IRAC Data Handbook, Table 5.7.So, if you use 10-pixel radius aperture, and the photometry measurement is X, the final flux density should be f=33.846X uJy. If you wish to compute a magnitude for IRAC bands, then mag = 2.5*log10(f (in Jy)/f(0)), where f(0) is the zero point, which is listed in http://ssc.spitzer.caltech.edu/irac/calib/. The zero point, f(0), is in Jy; for example, 280.9 Jy for IRAC channel 1. Equivalently, for example, if one is using "phot" or "qphot" in IRAF/DAOPHOT, for the pipeline-generated mosaic images (where the pixel size is 1.20 x 1.20 arcsec), set the "zmag" keyword in photpars to 17.30 (ch1), 16.82 (ch2), 16.33 (ch3) and 15.69 (ch4).
MIPS data24 um data: Let us assume we start with the pipeline mosaicked images with pixel scale of 2.45"/pixel. The MIPS 24 um calibration stars were measured through aperture radius of 35'' and background annulus of 40-50 arcseconds, see http://ssc.spitzer.caltech.edu/mips/apercorr/. For this large aperture, the aperture correction is small, 1.082 (again, multiplicative). However, for fainter sources, you can use smaller aperture radius, for example, 13". For this aperture, with sky annulus of 20"-32", the aperture correction is 1.167. See http://ssc.spitzer.caltech.edu/mips/apercorr/ for a complete aperture correction table. In this example, if the measurement through the aperture is X, the final flux density f = X*141.08*apcorr uJy = 141.08*1.167*X uJy = 164.64*X uJy = 0.165*X mJyTo compute magnitudes in the Vega system, magnitude = 2.5*log10(f(0)/f), where f(0) for a 0 magnitude Vega star is listed at http://ssc/spitzer.caltech.edu/mips/calib/, i.e., f(0) = 7.14 Jy, 0.775 Jy and 0.159 Jy for 24 um, 70 um and 160 um, respectively. 70 um data (coarse scale data): Start with the pipeline *filtered* and mosaicked images. The pixel scale is 4"/pixel. We recommend using 30" (7.50 pixel) radius aperture with sky annulus of 40"-60" (10-15 pixel). The aperture correction is 1.295. Therefore, the final flux density f = X*376.07*apcorr uJy = 487.01*X uJy = 0.487*X mJy. Finally, we note that the above crude estimates of aperture flux density for both IRAC and MIPS data do not include any color corrections. For details of this correction, see Spitzer Observer's Manual at http://ssc.spitzer.caltech.edu/documents/SOM/. |
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