black drop, longer explanation

I should have read the earlier posts; as Tom Van Flandern wrote, "the
black drop effect is caused by variable refraction from moving air cells in
Earth's atmosphere."

Bradley Schaefer has written on the subject, The Transit of Venus and the
Notorious Black Drop, B.A.A.S. 32 (2000) 1383-1384; giving the cause as
terrestrial atmospheric smearing, which blurs the image.

Also a long article by Schaefer, 'The Black Drop Effect', Journal for the
History of Astronomy 32:4 (Nov. 2001) 325-336.

p334: "the ideal image...will suffer smearing...that will produce a
somewhat fuzzy image with contour lines (i.e., what is perceived as the
edge) that are shaped like the Black Drop. The primary causes of smearing
are the usual astronomical seeing (associated with small angle scattering
in our Earth's atmosphere) and the usual diffraction in the telescope (the
Airy pattern). Other contributing smearing mechanisms that generally do
not dominate are impoerfections in the telescope's optics, imperfections in
the observer's eyes, the finite angular resolution of the detector, and
even the physical size of the telescope's aperture."


Peter Abrahams
The history of the telescope and the binocular:
http://home.europa.com/~telscope/binotele.htm

Many thanks Pete for these additional information.

Thierry

"Peter Abrahams" telscope.at.europa.dot.com wrote in message
...
I should have read the earlier posts; as Tom Van Flandern wrote, "the
black drop effect is caused by variable refraction from moving air cells
in
Earth's atmosphere."

Bradley Schaefer has written on the subject, The Transit of Venus and the
Notorious Black Drop, B.A.A.S. 32 (2000) 1383-1384; giving the cause as
terrestrial atmospheric smearing, which blurs the image.

Also a long article by Schaefer, 'The Black Drop Effect', Journal for the
History of Astronomy 32:4 (Nov. 2001) 325-336.

p334: "the ideal image...will suffer smearing...that will produce a
somewhat fuzzy image with contour lines (i.e., what is perceived as the
edge) that are shaped like the Black Drop. The primary causes of smearing
are the usual astronomical seeing (associated with small angle scattering
in our Earth's atmosphere) and the usual diffraction in the telescope (the
Airy pattern). Other contributing smearing mechanisms that generally do
not dominate are impoerfections in the telescope's optics, imperfections
in
the observer's eyes, the finite angular resolution of the detector, and
even the physical size of the telescope's aperture."


Peter Abrahams
The history of the telescope and the binocular:
http://home.europa.com/~telscope/binotele.htm

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."