Phoranso provides several options through which you can control its photometry process. To access these options select Photometry Settings from the Photometry menu in the Phoranso menu bar. You will see this dialog box:



It consists of following sections: Aperture photometry ringset, Calculate multiple ringsets, Ensemble type, Apply corrections, Other parameters, FILTER keyword mapping, Sky background calculation,and Sky brightness calculation.


  1. Aperture photometry ringset


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, called a ringset, to define an aperture, which are described here.  


Star aperture defines the inner aperture ring radius. It can be expressed in pixels or as a multiplier of the star's FWHM. When using the FWHM option, we recommend a scaling factor around 1.5. Note that "Star FWHM" photometry works best when used in conjunction with the PinPoint solver. With other solvers, we recommend to use fixed apertures.

Gap defines the gap ring radius, expressed as an increment to the Star aperture radius. Example: if the Star aperture has a radius of 5 pixels and the Gap radius is set to 7 pixels, then the Gap ring will be drawn at a radius of 5 + 7 = 12 pixels from the star's center. 


Sky annulus defines the Sky annulus radius, expressed as an increment to the Gap ring radius. 


  1. Calculate multiple ringsets


Phoranso allows to make photometric measurements for one ringset (described above) or for multiple ringsets. More precisely for a range of star aperture values. Check this option to enable working with multiple ringsets.

Apertures from defines the lowest Star aperture value to be used, and to specifies the highest Star aperture value. Phoranso will  make photometric measurements for all Star apertures in the from/to range, creating corresponding reports and light curves. 

In the example screenshot this will result in 3 reports being created for each Target star present in your FITS images: one report for the ringset (4 - 6 - 8), a second one for the ringset (5 - 6 - 8) and a third one for the ringset (6 - 6 - 8). Additionally, 3 light curve windows will be created for each Target star.

Working with multiple ringsets adds very limited calculation overhead to Phoranso, and allows a user to select the report that corresponds with the 'cleanest' light curve.


Multiple ringsets are not supported when expressing a Star aperture based on its FWHM.


  1. Ensemble type


This section applies when using Ensemble photometry. Phoranso supports 3 methods to calculate ensemble photometry, which were first described by Tim R. Crawford of Arch Cape Observatory in this paper. Note that all 3 methods provide acceptable results. Arne Henden, former Director of the AAVSO and a renowed photometrist, prefers the Mean value method.


    • Master star


This option creates a single composite "Master reference star" by summing the fluxes of all Reference stars in the ensemble. Brighter Reference stars will have a larger contribution in calculating the flux of the Master Reference star than fainther Reference stars. As the brightest star dominates completely, any error in its measure directly impacts the target star.


    • Mean


This option creates an ensemble of values for which a mean magnitude value is calculated, meaning that all Reference stars have the same weight. This option is the choice of many amateur and professional astronomers as it overcomes the main shortage of the Master star method. We recommend this option. 


    • Weighted mean


This option creates an ensemble of values for which a weighted mean magnitude value is calculated, and where the weights are the variance reciprocals of Reference star magnitudes. The strength of this method is that Reference stars with a bigger variance, will have lower contributions in the calculations. 



  1. Apply corrections

    Phoranso allows to apply First-order and Second-order Extinction corrections when calculating magnitudes. This is further explained in this section. Check Extinction Coefficients to enable these corrections.

    In a later release of Phoranso, we will also support working with Transformation Coefficients to convert instrumental magnitudes to standard magnitudes in the UBVRI photometric system.


  1. Other parameters


    • The area between the Gap ring and the Annulus ring in a ringset is used for making sky background calculations. Background noise will eventually be subtracted in the Star aperture ring to ensure that only the flux from the Target star is measured within the Star aperture. This assumes that no stars are present in the area between Gap and Annulus, which is hard to guarantee. Check the Exclude stars in sky annulus option to instruct Phoranso to ignore stars present in the area when making sky background calculations. Note that only stars which have been extracted by the used solver will be excluded.

      Background calculations furthermore are used to calculate the SNR of a reference star and its  uncertainty ('zero point error') value. Hence, excluding stars in the sky annulus will also improve both values. We recommend to turn this option on.


    • The AAVSO organization allows observers to submit fainter than observations. This can be useful for variable stars with deep minima, e.g. cataclysmic variables in quiescence or for Gamma Ray Bursts (GRBs) afterglow hunting. The AAVSO has proposed a method stating that the fainter than magnitude of a star can be considered as the limiting magnitude of the image in which it appears. The limiting magnitude itself is calculcated as the magnitude of a star with a Signal-to-Noise ratio (SNR) of 3. Check the Allow fainter-than observations option to inform Phoranso that you want to work with fainter than observations. Phoranso calculates the limiting magnitude of an image using the Reference star (or, in case of ensemble photometry, using the first Reference star).


    • To determine the brightness or flux of a star, Phoranso centers the Aperture ring on the centroid pixel of that star, based on the star's position retrieved through astrometry. When using very faint stars, astrometric solvers will not succeed in retrieving their coordinates and the photometry fails. You can overrule this by checking the Allow very faint target star option. In that case, Phoranso will use the astrometric solution of the image to calculate the pixel coordinates of the target star and will center the Aperture ring on that pixel. 


  1. FILTER keyword mapping


FITS image acquisition software, such as MaxIm DL, AstroImageJ, PixInsight, and IRAF, write out FITS images with meta data stored in FITS keywords. Common keywords include those for the date and time of observation, the exposure time, etc. While many FITS keywords and their values are standardized across FITS image acquisition software, some critical keywords are not. One such example is the FILTER keyword. 

The FILTER keyword is essential for Phoranso to perform accurate photometric reduction. However, different FITS image acquisition packages may use different values for the FILTER keyword. For instance, when no filter or a Clear filter is used in your CCD/CMOS camera, some packages might set the FILTER keyword value to “Clear”, others to “C” or “None”, etc.

Mapping FILTER keywords to Standard Color bands


To ensure Phoranso correctly interprets your FILTER values, you need to map the FILTER keyword values used by your acquisition software to the standard Color bands. The standard Color bands encompass photometric filters in either the BVRI photometric system or the Sloan g', r', i', z' (SG, SR, SI, SZ) photometric system. 

Follow these steps to map your FILTER keyword values

    1. Click the FILTER keyword mapping button to open the FILTER mapping table. 




    1. Enter the FILTER keyword values used by your FITS image acquisition software in the left column cells, corresponding to each Color band in the right column.


      • If your camera setup does not utilize filters, you only need to modify the first row in the FILTER mapping table. In the left cell (labeled Clear, C, None in the example above), enter the FILTER keyword value used by your system, e.g. Clear. 

        If you use multiple image acquisition software programs, you might encounter different FILTER keyword values for the same filter. For example, software X might label your Clear filter as Clear, while software Y might label it as C or None. You can enter all these values, separated by commas, in the FILTER keyword value field, as shown in the example. Whenever Phoranso processes a FITS image with a FILTER value of Clear, C or None, it will map it to CV Color band.

        Similarly, you can specify one or more comma-separated FILTER keyword values in any of the other cells in the left column. 

      • If your camera setup uses multiple filters, ensure the correct FILTER keyword values are set for each filter in the table. Retain the default values suggested by Phoranso for filters not in use. For example, if you do not use a Sloan g' filter, keep "SG" as the default value in the corresponding cell.

      • The Clear filter (first row in the table) should be mapped to either the CV or CR Color band. During photometry, Phoranso will retrieve either the V or R magnitude of the Reference star(s), based on your choice. In Phoranso reports, these observations will be denoted as CV or CR band observations, indicating the use of V-band or R-band comparison magnitudes, respectively. Note that CV is more commonly used than CR.

 

Finalizing the mapping

 

Press the OK button to confirm your selections. This action saves your mappings and applies them during the photometric processing of FITS images in Phoranso. In general, you won't need to revisit the FILTER mapping table as long as your filter system and FITS file acquisition software remain unchanged. 


Note that Phoranso does NOT modify the FILTER keyword value of your FITS image. The FILTER mapping table serves solely as a reference for Phoranso's internal processing and does not affect the original headers of your FITS files.


  1. Sky background calculation


As mentioned above, calculating the sky background noise involves measuring the background signal in the area between the Gap and Annulus rings. It helps in isolating the flux from the Target star by subtracting the background noise.

Two methods can be used  to calculate background noise: mean or median:


    • The mean method involves averaging the background flux values across the area. It is sensitive to extreme outliers, making it slightly more susceptible to the presence of anomalies (e.g. cosmic rays) in the background. 

    • The median method involves finding the middle value in the background flux values. It is more robust against outliers, and hence recommended.


  1. Sky brightness calculation

    The value of the FITS header keyword PEDESTAL typically is added to each pixel value in a FITS image to get a zero-based ADU, forcing the image to contain positive pixel values. 


Check the option to overwrite the value of the FITS header keyword PEDESTAL. Phoranso uses this value only when calculating the sky brightness of an image. 


The sky brightness of an image is estimated by dividing the mean background flux in the area between gap and annulus of the Reference star(s) by the area. This value is then converted to magnitude per arcsec2. It assumes that the background within the area between gap and annulus of the Reference star(s) is representative of the overall sky brightness.