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We will now proceed with looking for clues about our binary that would help us set a decent starting point for automatic minimization algorithm.
UV Leonis is a detached binary system and as such we may estimate the gravity potential. PHOEBE uses modified Kopal potentials as defined in (Wilson 1979). Their WD names are PHSV for a primary star and PCSV for a secondary star. In Component-related tab (Fig. 15) you may find the calculation buttons next to initial parameter values. Clicking these brings out the calculation window (Fig. 16).
You may quickly calculate the value of the potential by supplying the instantaneous separation between both stars, normalized to the semi-major axis (thus always 1.0 for circular orbits), the mass ratio and the radius-vector to the desired point in space, again in units of semi-major axis. To calculate the modified Kopal potential, set this radius-vector to the star radius towards the companion star (if tidal forces are strong, the star is deformed into a dumbell shape, so stellar radius is direction-dependent). In case of elliptical orbit the potential will depend on synchronicity parameter and direction cosines, but this is out of scope of this tutorial. Please refer to PHOEBE Reference Manual for further assistance.
On the lowermost part of the calculation window there is a button box. The Calculate button calculates the value of potential. PHOEBE lets the user inspect it and decide whether the calculated value is acceptable or not. If not, you may close the calculation window by clicking on the OK button and there will have been no changes made. If so, clicking on the Update button will copy the calculated value to the main PHOEBE window.
UV Leonis seems to have a pretty circular orbit and in the first approximation it seems that the default values in the calculation window are as might be expected (we will be proven wrong shortly, but this in fact is the realistic presumption). Therefore click on the Calculate button. The value 11.00504 will show up in PHSV entry. Since we are happy with it, click on Update button. The value will be copied to the Advanced tab of the main PHOEBE window. Close the calculation window by clicking on the OK button.
Repeat the same procedure for the secondary potential. The value will be the same.
At this time we cannot safely presume anything about the values of gravity brightening, albedo and synchronicity parameters, so we continue without modifying anything.
