Aperture photometry is a widely used technique to measure the flux or brightness of a target star, by summing up the light within a defined aperture or region surrounding the star. It involves selecting a circular aperture centered on the target star and integrating the flux within that aperture while accounting for background noise and subtracting contamination from nearby stars present in the aperture.


Phoranso uses 3 concentric circles, which we refer to as a Ringset, to define an aperture, as illustrated in the image below:


  • The inner circle is called the Aperture ring. It is used to sum up the pixel values or intensities within the aperture, which represents the total light received from the target star. 

    It's important to properly select the size of the aperture ring such that it encompasses the entire target star, while minimizing contamination from nearby stars. For faint target stars, a smaller aperture will be appropriate. For brighter target stars, one should use a larger aperture. 

  • The middle circle is called the Gap ring. The area between the Aperture ring and the Gap ring is not really used for anything - it just assures that you can get far enough from the Aperture ring to start your sky background calculations.


  • The outer circle is called the Annulus ring. The area between the Gap ring and the Annulus ring is used for making sky background calculations. Background noise will eventually be subtracted in the Aperture ring to ensure that only the flux from the target star is measured within the Aperture. 

    Note the presence of a nearby star in the area between the Gap and Annulus ring (at the 5 o'clock position). While one should try to avoid such situations, in practice it will not always be possible to do so. Phoranso uses a sophisticated algorithm to neutralize the effect of nearby stars present in this area.  




Example of a Phoranso Ringset with an Aperture, Gap and Annulus ring



In addition to Aperture photometry, there are other types of photometry techniques, such as Point Spread Function (PSF) Photometry. This involves fitting a mathematical model of the PSF to the observed target star to measure its total flux. It takes into account the spread of light caused by the telescope's optics and atmospheric conditions. Phoranso does not support PSF Photometry.