transit of venus: images of black drop in T>200?

Hi,

To complete an article about the transit of Venus and "side effects" for my
educational website, I am looking for any quality hi-res picture showing the
black drop, arc of light or halo in Venus (with all technical data).

These documents will be published at this URL :
http://www.astrosurf.com/lombry/imag...nsit-venus.htm
and also in French
(http://www.astrosurf.com/lombry/imag...t-venus-fr.htm )

Up to know only two amateurs at my knowledge reported have observed the
black drop in scope of 10 and 12.5". Other osbervations are welcome in scope
larger than 200 mm.

Thanks in advance

Thierry
LUXORION

Here is a picture takern by Dodi Dierick of the black drop effect:
http://users.pandora.be/vcsb/dodi/drop.jpg

I believe it was taken with a 105mm Traveler in white light.

Roland Christen

Thanks Roland. I haven't see that one.
One more time that confirms well that this effect is mainly visible in small
scopes
I know Dominique as he published his H-alpha image on my web
I will contact him.

To all : nobody else recorded a black drop using a large scope ?

Thierry

"Chris1011" wrote in message
...
Here is a picture takern by Dodi Dierick of the black drop effect:
http://users.pandora.be/vcsb/dodi/drop.jpg

I believe it was taken with a 105mm Traveler in white light.

Roland Christen

These images show smearing between the contact points, which appears as
a stretch in the circumference of Venus, slightly reaching out to the
edge of the Sun. I think there have been other images showing this (see
www.spaceweather.com for a large selection).

This, to my understanding, is from diffraction, similar to the blurring
you see when you separate your thumb & forefinger by about 1mm & hold
the gap right up to your eye.

18C & 19C observations of 'black drop' were sometimes of a different
nature. A black thread was seen stretching between the edge of Venus &
the edge of the Sun, of varying length but up to nearly a diameter of
Venus. If Venus is an O, and the edge of the sun is a shallow curve, it
would look like this: O-|

Only a minority of the illustrations from 18C & 19C transits show this
effect. If all illustrations were assembled, there would be a continuum
of increased effect, from no black drop to a long thread.

This effect is nicely illustrated in:
Observations of the Transit of Venus 9th December 1874, Henry
Chamberlain Russell; Sydney 1892
See especially the front cover, and plates 13 & 14 (1.4 meg files),
posted at:
http://www.astro.univie.ac.at/
~wuchterl/Kuffner/2004/Venustransit/russell_vt.html
===
Here is a post to another list, about what this means regarding historic
telescopes:


Looking through the images of the transit that have been posted on the
internet, it is pretty clear that almost no one saw or imaged the 'black
drop'.

18th and 19th century observers made drawings showing a distinct thread
connecting the black disc of Venus with the black outline of extra-solar
space. In some cases, the thread was as long as a diameter of Venus.
There are also illustrations in Philosophical Transactions of the 18C
transit, showing a glowing ring around the black disc of Venus, as it
transits the middle of the sun. Nothing like this has been imaged in
the recent transit. (This is distinct from the very narrow 'halo' on
entrance or exit.)

I believe it is safe to assume that most of these observers were not
hallucinating, or deluded, or wrong. A few might have tried to see what
their predecessors had seen, or their directors suggested they should
see.

It seems likely that this 'black drop' was an artifact of the
instruments, which should give us very useful clue about the telescopes
used at that time.
This could also be a very useful clue for historians of observational
astronomy, who need to sift observer's reports for what 'really
happened' in the sky, and eliminate instrument artifacts & observer's
errors.

Were these observations of the black drop accomplished with refractors,
silvered glass reflectors, speculum reflectors?
Gregorians are notoriously difficult to baffle, were they more likely to
result in such an observation?
What characteristic of the instrument could introduce an artifact
similar to the black drop or the illuminated annulus?
It isn't very useful to simply say, 'these old telescopes can cause an
annulus around black discs against a bright background'; we need to be
able to extrapolate this to observations of the sun, moon, & possibly
dimmer objects like double stars, etc.

There are many questions related to instruments that are raised by this
issue.
--Peter

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

Hi,

Interesting to note about this image recorded in H-alpha (overexposed):
Dodi told me that he doesn't think it's real but a kind of photographic
artefact (bleeding).
This drop effect of Venus came out as well. He has it on all images, so he
think that it's definitely not a seeing effect as visually, nothing of this
kind was seen...

So take care in 2012 ;-)

Thierry
..
"Chris1011" wrote in message
...
Here is a picture takern by Dodi Dierick of the black drop effect:
http://users.pandora.be/vcsb/dodi/drop.jpg

I believe it was taken with a 105mm Traveler in white light.

Roland Christen