photometry on data of IRAC

hanabez wrote on Dec 05, 2007

Hello ,I have a simple question.If I photometry on fits of IRAC of spizer directly with phot task,what
is the unit of the flux?You konw the unit of the IRAC's data is Mjy.How to photometry the spizer's data and whether or not to transform the unit from MJy to ADU.

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sirmarcos wrote on Dec 05, 2007

As I understand it, the units of the IRAC mosaics are Mjy/steradian so you need to take the "flux" number that comes out of phot has to be multiplied by 34.9837 to get things into microJy, for default mosaics anyway.

http://ssc.spitzer.caltech.edu/postbcd/irac_reduction.html

there are also aperture corrections that are discussed in the IRAF data handbook. And if you want magnitudes, the "zero points" you need to use are listed for each channel here:

http://ssc.spitzer.caltech.edu/irac/iracphot.html

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hanabez wrote on Dec 05, 2007

So we could do photometry directly on data of IRAC with IRAF,isn't it?
If I get a flux from phot task,for example 2641.802,is the meaning of it 2641.802MJy?
I don't understand clearly the algorithm of the IRAF's photometry.
Thanks for your answer.

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sirmarcos wrote on Dec 05, 2007

The flux units of a mosaic from Spitzer are by default are MJy/steradian. For the default pixel scale you have to use the conversion factor I listed above.

All the aperture photometry "phot" task in IRAF does, as far as I know, is add up all the counts in the aperture and subtract a sky value from your sky annulus. So, the flux value that comes out of there needs to be massaged quite a bit - first for the pixel scale and such and then for aperture corrections. Most of the nitty-gritty is in the IRAC data handbook on the Spitzer site.

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hanabez wrote on Dec 05, 2007

Thank you.
For example,If I get a flux from phot task,2641.802,the real flux is 2641.802*34.9837mJy,right?
And is the mag -2.5*log(2641.802)+17.3(ch1)=8.745?

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Last post on Dec 05, 2007