Blackdrop Effect
Inline with subject of transit, difraction, resolving
power, apperture and accuracy of timing, I would like
to know the exact reason of black drop effect & does
that effect depend upon diameter of lens/ mirror &
magnification.
Nilesh Puntambekar
It depends on seeing If eye & camera SEE things differently ?
do you mean to say that black drop effect is illusion
of eyes & not captured by camera and the effect is
independent of other things I have mentioned? Sorry, Arvind, but new evidence indicates the black drop
effect is
independent of atmospheric effects. In 1999, observations of the Mercury
transit by the TRACE satellite showed that the black drop effect was
visible from outside the Earth's atmosphere. The effect is, in the words
of the authors, "due to the interacting effects the back-light solar
limb-darkening, diffraction of photospheric light around the Mercurian
disk, and the instrument's Point Spread Function." Thus it is an
interaction between diffraction at the planet's edge and diffraction within
the telescope. Ref:
TRACE Observations of the 15 November 1999 Transit of Mercury, Schneider,
G.; Pasachoff, J. M.; Golub, L., American Astronomical Society, DPS Meeting
#33, #10.02; Bulletin of the American Astronomical Society, Vol. 33, p.1037 ADS:
Arvind Paranjapye
Nilesh
Puntambekar
Mike, I was looking at the two abstracts you have
mentioned. It is a pity
that the entire article is not available online.
The second abstract - this is what it says
-"The accuracy with which the Astronomical Unit was known has been
historically limited by the ""black drop"" effect observed
during Venus's
rare transits. It is often mistakenly attributed to Venus's atmosphere. We
report on observations taken outside the Earth's atmosphere of the 1999 and 2003 transits of
Mercury with the NASA solar satellite known as the Transition Region and
Coronal Explorer (TRACE). Though there was no contribution from the
atmosphere of Mercury or the Earth nevertheless a faint black-drop effect
was detected. We discuss and model the images showing the sources of the
black-drop effect. The techniques we discuss are applicable to ground-based
and space-based observations of the 8 June 2004 and 5-6 June 2012 transits
of Venus". I have underlined the line about Venus's atmosphere - should this have been
Earth instead of Venus ? Perhaps not, it is the atmosphere of Venus that was
being talked about. But, it is not clear to me that the Planet's atmosphere does
not play any role - and will there or will there not be a difference arising
from this,
between Mercury and Venus? After all, a bright rim was seen around the
unimmersed part of Venus, arising from its atmosphere, in many historical
transits. Blackdrop effect would have contributions from diffraction around
the planet's edge, certainly, in the absence of an atmosphere, but, how
would this compare w.r.t. Venus Atmospheric contributions? I do wish one
could obtain, the full articles :-(
Rathnasree
Rathna, I should have posted the URL where you can find a jpg of the poster that was presented on this topic in 2001. It is at: http://nicmosis.as.arizona.edu:8000/POSTERS/TOM1999.jpg. I also have a PDF of the paper (I'm not sure if it is published or not) and can send it to you, if you like.
They are referring to the atmosphere of Venus here. There had been some speculation that the black drop effect was caused -- or at least increased -- by the thin but dense atmosphere of Venus surrounding the planet. Perhaps a refraction effect. But Mercury has no atmosphere and the effect is still seen so a planetary atmosphere is removed as a possible cause.
>But, it is not clear to me that the Planet's atmosphere does not play anyContribution to the the Black drop effect comes from the
following
factors
1.A non uniform extended source that the sun is
2.The effect of the atmosphere of the planet
3.The effect of the earth's atmosphere
4.The effect of imperfect optics used in the observation
TRACE experiments on the atmosphereless Mercury and out side the
earth show that 2 and 3 have made no contribution.Pasachoffet.al have
convoluted the brightness profile of the sun near the limb and the
instrumental profile called the point spread function and have been
able to reproduce the observed intensity distribution in the
blackdrop. As for the contribution of Venus' atmosphere surely it
will add to the blur. But how much will be known after TRACE observes
the Venus transit.
Convolution is a mathematical technique that combines all the
blurring effects to reproduce a blurred observation.Deconvolution is
the reverse process.
Nirupama Raghavan
I am not aware whether this link was posted in an earlier, however, if not, members would like to check out this link on a paper on the phenomenon of the black drop and astigmatism.
http://www.bo.astro.it/~biblio/Horn/Blackdrop.htm
Thanks Manoj, but can u explain in ur own words what
exactly is this
Black drop effect. Regards
Kumar Gaurav
The precise timing of the contacts, in particular the most important
contacts 2 and 3, gets limited by the appearance of an undefined meniscus
that is usually seen between the two disks - Sun and Venus - between the
inner edge of the Sun and the outer edge of Venus, just as they touch each
other, at contacts 2 and 3.
Some have seen this as the ultimate cause for the failure of historical
transit observations to determine the Astronomical Unit accurately - as this
gives rise to uncertainties of a few seconds in contact timings.
The contributions to this tiresome blackdrop could come from -
(1)
Atmosphere of the Earth
(2) Atmosphere of the transiting planet, in this
case, Venus
(3) diffraction effects in the optics of the observing
instrument and
(4) Diffraction effects around the limb of the planet.
The
most recent observations of the transits of Mercury, through spacecraft,
seem to indicate that the effect survives, even in the absence of
contributions from the atmospheres of Earth and the obscuring planet (I need
to add to this, after I can download Paschaoff's paper and add the
reference. There are also some multiwavelength reports of observations of
Blackdrop that I need to study, and add notes from).
Jeremiah Horrocks, who observed the 1639 transit, for a very brief while,
might not have noticed this effect, since he did not observe close to the
contact timings. The earliest of the observations of this effect came from
Pingre, Lalande, Cook, and many others during the 1761 and 1769 transits. It
also plagued the 19th century observations, although Gould, who observed
from Chile in 1882 did not seem to detect the black drop.
A bright rim of emission around Venus, due to Sunlight refracted through its
atmosphere is also seen, when Venus would be about half inside the disk of
the Sun, the entire disk of Venus would be discernable due to the rim of
emission appearing as a bright half ring around the unimmersed part of the
disk of Venus. This is described in the observations at Hudson Bay, by Wales
and Dummond, in Philosophical Transactions, 1769, as follows -
"Soon after Venus was half immerged, a bright crescent or rim of light,
encompassed all that part of her circumference which was off the Sun,
thereby rendering her whole periphery visible. This continued very bright
until within a few minutes of the internal contact, and then vanished away
gradually."
There would be a connection, I suppose, between the Venus atmospheric
contribution to the Black Drop and this bright rim that is usually seen.
Rathnasree
What is blackdrop effect
Nisha Vincent
Black drop is usually observed during the transit
of Mercury or Venus. More prominently during the Venus
transit. The extreme edge of any circular celestial
object be the it sun, moon or a planet is called the
limb. As the planet passes across the limbs of the
sun, it appears to be connected to the sun, with a
small black portion between the planet and the sun.
Actually it an illusory effect when the limbs of the
sun and planet are not actually in contact, a black
drop like object appears to join them. This hampers
the timings of the transit. To a ground based
observer, when venus crosses into the sun it continues
to be connected with the small black drop, while in
reality the transit was already in progress and
thereby giving the incorrect time. This had happened
during the observation of venus transit of 1761 and
again during the 1769 transit. During those years it
was extremely important to get the exact timings of
the transit to determine the distance of the Sun.
However like all other celestial illusory
effects its
of little importance now. Clear skies
Manoj Pai
This particular
"illusion" is created by the limitations of the
eye. It is technically called the resolving power of your eye.You can
actually be calculate and find out the closest separation between the
two fingers held at a certain distance that will not seem to touch
each other.
Nirupama Raghavan
Is black drop effect
really an illusion of eye?
i had raised this question previously also and we had
discussed related factors such as apperture, resolving
power, magnification, diffraction, solar limb
darkening, atmosphere of pnanet etc. even someone
informed that cameras on spacecraft had recorded
blackdrop effect. can anyone summerise exact physics behind the
phenomena for all members.
Nilesh Puntambekar
When
one has a continous video sequence between contacts I and II and
later - would some kind of image processing not be feasible in terms of
completing the sphere of Venus - use the limb from the regions unaffected by
the Black Drop and complete the sphere and thus obtain more accurate contact
II (or III) timings?
Rathnasree
This
is what is attempted in Pasacoff's paper in their modeling
Nirupama Raghavan
Compiled from the Venus Transit Group
