Aperture Photometry

There a few techniques that astronomers could use to photometer a source, and here we going to look at aperture photometry. Aperture photometry is the measurement of the intensity of the electromagnetic radiation inside an aperture of a given size r, containing Npix pixels. We are going to make heavy use of Photutil’s aperture photometry package.

We first get the image (one with WCS) that we are attempting to photometer:

Let’s see if we have downloaded the image successfully.

Next, since we already know using Photutil, we can create an aperture object, including an annulus for local background subtraction. 

After the aperture object is created, we can photometer our astronomical object. The final_sum is the photometric measurement of our astronomical object with the local background subtracted. 

What do got so far is the ADU (analog to digital units) of the source. What you want is the magnitude of the source, expressed in the formula:

 

\(m = -2.5 log_{10} (\frac{ADUs}{t})\)

 

where \(m\) is the apparent magnitude, \(ADUs\) is the analog to digital units, and \(t\) is the exposure time for the image.

Since we already have the ADU of our source, all we need is the exposure time of our image. The nice thing about .fits image is that they come with metadata in the header that provide us with the parameters of which these images are taken. We can extract the exposure time from the header. 

Since we already have the ADU of our source, all we need is the exposure time of our image. The nice thing about .fits image is that they come with metadata in the header that provide us with the parameters of which these images are taken. We can extract the exposure time from the header. 

There is obviously a problem with the outpit: it’s too bright! What you have just calculated indeed is not the apparent magnitude of the object, but just the instrumental magnitude of the object. Unfortunately, instrumental magnitudes are quite useless in themselves for any astrophysics to be done. To derive the apparent magnitude, we have to look into adding an offset to the instrumental magnitude called a zero point.