Sat, 12/29/2018 - 09:05
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
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
“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
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
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
When I was developing a sports application, I faced similar problems in finding the right functions and libraries to solve specific problems. I’ll leave the article here, which helped me. It has been a learning journey, much like your experience with photometry.
For your question about calculating the FWHM of a star in a FITS image, you may find the photutils library useful. This library includes tools for detecting and measuring the properties of stars in astronomical images. In particular, you can use the photutils psf module, which provides functions for measuring the full width at half maximum (FWHM) of point sources.