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Hi Everyone,I am performing aperture photometry on Spitzer/IRAC crowded field images. The great number of point sources close to each other lead me to decide to use smaller apertures and then perform aperture correction. The instrument's handbook already provides a table with aperture and sky annuli that can be used along with the aperture correction that has to be applied on the photometry results. My concern is about the estimation of the sky background and the photometry error when using smaller apertures. The estimation of the error by the phot task is of course not correct now (the flux and stdev will be wrong). The idea of manually estimating the error through the signal-to-noise ratio (always preferred by me) is also not immediately possible because the mean sky value will be overestimated, since the sky annulus will now contain part of the star's psf, or at least that is what I think. Increasing the inner radius of the sky annuli is not much of an option since the field is very crowded. Does anyone have a suggestion on how to estimate the errors of my photometry ?Thank you Nicolas

Have you considered Point Spread Function Photometry? This should give you a good estimate of the residual background and therefore the error.

Hi AnTaR3s and thank you for your post.I haven't considered PSF photometry (yet) because I don't know how to perform it! If there's no other solution I guess I'll have to learn how to do that too. Are you refering to the psf task of the daophot package or something else ?

Yep, I'm referring to various tasks in the daophot package.Take a look here:http://www.astro.washington.edu/courses/astro480/exercises_preppoints.htmlTheir exercises helped me a lot in understanding what PSF photometry requires. (I think it was Exercise V or so...!?)hope this helpscheers

[quote:d718290b76="AnTaR3s"]Yep, I'm referring to various tasks in the daophot package.Take a look here:http://www.astro.washington.edu/courses/astro480/exercises_preppoints.htmlTheir exercises helped me a lot in understanding what PSF photometry requires. (I think it was Exercise V or so...!?)hope this helpscheers[/quote:d718290b76]Here is another really good source:http://web.ipac.caltech.edu/staff/roc/2mass/resp/appsf/appsf.comp.htmlSeeing the comparison gave me at least a better distinction of the differences between the two and helped me create a definition of PSF Photometry. Now it is time to piece together how to actually accomplish it. As far as using the software is concerned, we need to ensure that IRAF is running business continuity planning software so that the servers provide high availability, then we can go from there.

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**Telescopic Baby**