black drop, longer explanation

Peter Abrahams telscope.at.europa.dot.com wrote:

[...]

From "TRACE observations of the 15 November 1999 transit of Mercury
and the Black Drop effect: considerations for the 2004 transit
of Venus", G. Schneider, et al, Icarus 168 (2004) pp249-256, we
read:

"3. CONCLUSION

The principle cause of the Black Drop effect, which has
historically impeded ground-based planetary transit mea-
surements, is optical broadening resulting from the convo-
lution of the systemic PSF with the planetary and limb-
darkened solar disks. TRACE observations are free from
PSF instabilities caused be "seeing" in the terrestrial at-
mosphere and allow mitigation of the Black Drop effect from
the intrinsic disk images. Such stable, critically sampled,
near diffraction-limited images may be further enhanced by
PSF deconvolution, enabling very high-precision differential
astrometric position measures."