## Direct Distance Estimation

Direct distance estimation means that the distance, D, is derived in a least-squares analysis without the a posteriori step discussed in Section 5.1.3. If D is determined in this way, it is consistent with all local physical surface quantities, including proximity effects and any other phenomena supported by the model, avoiding the need to resort to mean radii. The only model requirement is the ability to compute distance-dependent flux, Fjh, in standard physical units, e.g., cgs units. In the analysis, these fluxes are then compared to the observed fluxes, Fobs, by exploiting the relation

with

Thus, observed light needs to be available also in these standard units, or should be properly converted from standard magnitudes or relative flux to these units. As discussed already in Sect. 5.1.2.3 for the Johnson UBV system, Fobs has been calibrated to standard physical units.

In the case of the WD program, there has always been an explicit absolute coupling between 4n sr passband luminosity, L, and calculated flux, F. Use of the normal emergent intensity, /pole, at a reference point (usually, a pole) as a scaling factor, made it easy and required only minor changes to compute light curves in any standard system of physical units by introducing a second scaling step from user-defined flux units to physical units by the relation

where superscript "mod" refers to WD intrinsic model quantities. Thus, absolute Ipde yields absolute Lph and Fjh, given that the geometry is correctly followed.

The distance-dependent flux, Fjh, in physical standard units, in the WD program, is computed as

## Telescopes Mastery

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