Software

To convert instrumental magnitudes into the standard photometric system, transformation coefficients are used. This process corrects for color-dependent differences between the instrumental and catalog magnitudes. The transformation is implemented using a set of custom Python scripts that process AAVSO Extended File Format output files. The scripts apply transformation coefficients created with AAVSO Transform Generator. They match observations in different filters based on timestamps and iteratively refine the standard magnitudes by applying the following equations. 

Let us, bs, vs, and rs be the instrumental magnitudes of the variable star, and uc, bc, vc, and rc those of the comparison star, with catalog values Uc, Bc, Vc, Rc.

The transformation equations are:

Bs = Vs + (Bc − Vc) + Tbv · [(bs − vs) − (bc − vc)]

Vs = vs + (Vc − vc) + Tv_bv · [(Bs − Vs) − (Bc − Vc)]

Rs = Vs − (Vc − Rc) − Tvr · [(vs − rs) − (vc − rc)]

Us = Bs + (Uc − Bc) + Tub · [(us − bs) − (uc − bc)]

Since the transformed magnitude in one band depends on the result in another, the equations become interdependent and cannot be solved directly. To overcome this, an iterative approach is applied. After a few iterations, the values converge to stable, internally consistent solutions. This method is simple, reliable, and recommended in the AAVSO CCD Photometry Guide and Transform Applier documentation.

The Python scripts created with the help of ChatGPT can be downloaded here. Last update: 11 Sep 2025