# Automatically calculating aperture / annulus sizes

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stratis
Automatically calculating aperture / annulus sizes

I’m building a batch differential photometry app in Python using astropy and affiliated packages such as photutils. This is mainly a learning exercise - I know there are apps already out there that do batch photometry.

I’m stuck on how to automatically calculate the aperture / annulus sizes. I’m intending to set these as multiples of the max FWHM of the comp star ensemble but can’t find a Python function to calculate FWHM (in pixels) of a star, given a FITS image and the centroid (x,y in pixels) of the star concerned.

Can anyone point me to a function that does that.

Many thanks

Nigel

Tonisee
To find stars, use

To find stars, use DAOStarFinder with 0 order approximation for FWHM. To estimate actual FWHM,  you can fit found sources that have good SNR by e.g. 2-D Gaussians.

One of my students did that in this spring, however I did conclude to myself that pyraf and IRAF would work much better for stellar photometry... ;-)

Best wishes,

Tõnis

stratis
“you can fit found sources

“you can fit found sources that have good SNR by e.g. 2-D Gaussians.”

Thanks but how do I do that? Sorry my math is limited so I was hoping for a function that would take as input the FITS image (or a numpy array corresponding to the image ADU values), together with the x,y pixel location of the star and would return the FWHM.

Identifying the stars and getting their centroids isn’t a problem - I have functions that do that.

Nigel

stratis
After a bit more research I

After a bit more research I’ve found Numpy and Scipy functions that will do a 2D Gaussian fit or an spline interpolation to give me a FWHM estimate but I really need a 3D solution so am still hoping there is a library function that will do this as I don’t think my math skills are up to rolling my own.

Any advice welcome. And Happy New Year all!!

Nigel

Tonisee
2-D (or actually 3-D) Gaussian

Hello Nigel,

I'm sorry I kept answer too brief, I answered first using mobile phone...

This 2-D Gaussian is more like "two-axis Gaussian", a 3-D function that's widths along e.g. x and y axes are different.

You can actually use function from Astropy itself, too:

from astropy.modeling.models import Gaussian2D

However, a lot of examples about photutils usage are given on their documentation page: https://photutils.readthedocs.io/en/stable/index.html . I'd suggest to look at PSF-related topics. I also noticed that probably I have to revise my former opinion about the usefulness and quality of photutils - progress till ver 0.6 has been impressive.

Best wishes,
Tõnis