Transcript Sat21_Dunham_Venus2015Oct

Venus Ashen Light Lunar Occultation
– How Dark is the Dark Side?
27 March 2016 & IOTA, 30 July
10th Trans-Tasman Symposium on Occultations
David W. & Joan B. Dunham
Observation of the reappearance of Venus from behind the dark side of
the Moon, 8-9 October 2015, north of Alice Springs
Historical Observations – Direct Visual
• The ashen light was first reported by the astronomer Giovanni Battista
Riccioli on 9 January 1643
• He named it “The Ashen Light of Venus”
• Subsequent claims have been made by various observers including Sir William
Herschel, Sir Patrick Moore, Dale P. Cruikshank, and William K. Hartmann
• A Monte Carlo simulation indicates that the lightning hypothesis as the cause of the
glow is incorrect, as not enough light could be transmitted through the atmosphere to
be seen from Earth. Observers have speculated it may be illusory, resulting from the
physiological effect of observing a bright, crescent-shaped object. Some spacecraft
looking for it have not been able to spot it — leading some astronomers to believe
that it is just an enduring myth
Venus Ashen Light during Lunar Occultations
• In the mid 20th century, some observers realized that the Venus ashen light,
if it exists, can be observed for many seconds without interference from the
glare of the sunlit side of the planet
• A visual observation of an occultation was attempted in the United Kingdom, with
an inconclusive or negative result
• A good opportunity occurred on 17 July 2001 as seen from Kwajalein, Marshall
Islands. But Peter Rejcek’s article on p. 8 of the 24 July 2001 issue of Kwajalein
Hourglass (Vol. 41, No. 58) said that “science may have to wait for another time
before the mystery of the ashen light of Venus is solved” when 67% Venus
reappeared from behind the 13% waning moon. None of the observers of this
occurrence (including some using a 61cm 'Super RADOTS' telescope) reported
seeing the ashen light. Video from the event was captured, but the camera was too
insensitive to detect even the Lunar Earthshine.
Kent DeGroff captured the 2001 Venus occultation
by the Moon using a Nikon 950 digital camera
mounted on a six-inch f/3.9 reflector with a 50mm
eyepiece, using the afocal method.
Venus Occultation 8 October 2015
• The dark side of Venus reappears from behind the dark
limb of the Moon at ~1924 UT
– The earlier disappearance will occur below the horizon for
most observers in central Australia
• Any “ashen light” should be unambiguous, if only for
up to ~37.5 seconds, until the sunlit portion of Venus
begins to reappear
• To ensure that that the spectral response of your
detector matches that of a visual observer it will be
important to use a luminance (IR/UV-blocking) filter
• From Bill Hanna’s presentation at TTSO9 in NZ almost
a year ago
Sites Along (and off of) the Stuart Highway
Occult 4.1.4.4 / C2A
Considerations
• Distribution of observers?
– Is it desirable to distribute observers widely along
the Stuart Highway to improve the odds of at least
one observer having good weather?
• Given the low elevation, it would be much better to
have good weather than to have the “ideal” geometry
• High frame rate vs. high S/N?
• Choice of Gamma?
• Use a Barlow for magnification vs. a focal
reducer for a lower f/ number?
Weather Forecast
The SkippySky weather
forecasts were quite
favorable for central
Australia, but there were
scattered clouds the day
before, and a patch of cloud
northwest of Alice Springs on
the IR weather satellite
images threatened the event.
But 3 hours before the event,
it was clear the weather
would be fine in and around
Alice Springs, and we
selected a convenient location
in the desert a few km north
of town.
Our Location, Irlpme, NT
A flat dirt parking area east of the Stuart Hwy 4 km n. of Braitling
The 10-inch ‘Suitcase’ Telescope
The camera is at prime focus, so the images are reversed (one reflection); it is normally
used for pre-pointed occultations of faint stars by asteroids (built by John Broughton).
Predicted View (from Occult 4)
The reappearance of Venus’ 29.7
40% sunlit disk would occur at a
cusp angle of 78N with the 15%
sunlit waning Moon 13 above the
eastern horizon (az. 74). The
predicted contacts were:
19:14:05 – Start dark side R
19:14:37 – Venus cusp R
19:15:25 – Venus fully reappeared
The orientation is as Joan video
recorded it with the 10-in.
“suitcase” f/4 reflector using a
Watec 120N+ camera with 2-frame
integration, while I observed
visually using a 25mm eyepiece
with a 20cm SCT. Show the video.
Analysis of the Video
Observations
•
•
•
•
Scotty Degenhardt
Tony George
Roger Venable
Important Critiques by Dave Herald
Venus Lunar Occultation Reappearance
Detection of the Unlit Side of Venus
2015-Oct-08
Observation by David and Joan Dunham
https://youtu.be/kFq7BE4SYHA
Analysis initiated by Richard Wilds
Analysis by Scott Degenhardt
Event and video details:
•Dunham’s video captures the reappearance of Venus on the earth lit limb of the moon, with
the unlit side of Venus emerging for ~20 seconds prior to the sunlit crescent of Venus.
•The video is taken with a Watec 120N+ running in 2 frame average mode at about 30 fps
reaching at least 12th or more limiting magnitude.
•The video has tracking issues making reduction with LiMovie or Tangra unreliable.
•Intensity measurements of Venus past 19:14:45.5 UT are not useable because the sunlit
portion of Venus reaches saturation.
•Venus is ~30” of arc in diameter.
•The image scale is ~1.89” per pixel making Venus ~16 pixels wide.
•Video is not in complete focus. Relative photometry could not be calculated, but intensity
profiles still demonstrate differences between earth lit moon, unlit Venus, and 2 field stars.
•ImageJ was used to align each frame individually to hold the star field constant.
•A target aperture from ImageJ was placed on the lunar limb where Venus emerges.
•Background apertures above and below Venus were measured to normalize intensity.
•The final lightcurve is 30 frames of data binned for 1 second data points boosting S/N.
Relative intensity measurements show the unlit disc of Venus to be
fainter than the Earth lit moon and a 9.6 magnitude field star, but
brighter than an 11.4 magnitude field star.
Unlit side of Venus emerged prior to sunlit cr
Zoomed and contrast enhanced
The Y pixel row scan yielded 5 good chords through the unlit disk of
Venus.
This provided two sets of parameters:
1. Chords representing the size of Venus blocking the background sky. (i.e. we can map its shape)
2. Peak intensities at that chord’s position on the disk of Venus. (we
can map the intensity gradient of the disk of Venus)
A side by side comparison of the image derived chord profile of the
disc of Venus (left arrow) next to the high precision Guide 8
simulation of the same UT time (right arrow) we find the visible disc
of Venus matches the calculated visible disc of Venus very well.
The intensity gradient of Venus demonstrates a drop of intensity
towards the limb typical of a spherical object. We also estimate the
peak intensity coincides with an object radius of 8 pixels matching
the image radius of Venus of a 16 pixel diameter.
Relative intensity of Earth lit moon, Venus’ disc, and 2 field stars
Moon LC
Venus
TYC 830 638
Johnson V magnitude: 11.372 ± 0.123
TYC 830 188
Johnson V magnitude: 9.612 ± 0.038
Unlit Venus
Sunlit Venus
Unlit Venus
Analysis by Tony George
Apertures for
the analysis
Light curves for the three apertures
Tony George’s Conclusions
• Because of the substantial amount of movement present in the registered
images, there is quite a bit of ‘noise’ and variation in the light
curves. However, all three light curves tend to vary in the same degree, both
temporally and in the measured image intensity. I do not see any indication of a
pre-emergence brightening. The degree that all three light curves overlay and
track each other up to the emergence of the Venus bright cusp is to me totally
amazing.
• It is interesting to note that the deviation of the light curves between
images 360-400 is the visible [sunlit] cusp appearing.
• Unfortunately, this work does not corroborate Scottie’s findings. It was
also discouraging to hear from Scottie that the program and processing
he used is too complicated to be reviewed or reproduced by another
independent analysis. For now, I think we are again left with a
conundrum. I do agree with Scottie that we have learned a lot from this
exercise and how future observations may be improved.
Roger Venable’s Analysis
To make a long story short: I agree with Scotty's analysis.
After stacking the video to eliminate the effects of
image motion -- no small task in this case -- I used
the measurement frame shown to the left. The
Venus dark side reappearance is shown below.
Roger Venable’s Light Curve and Comments
The detected light from Venus brightens rapidly, even saturating pixels, before the actual cusp is
geometrically visible. This could all be due to light from the dark side and the twilight zone, but
some of it may be due to light diffracted by the limb of the Moon from the bright part of Venus
that is geometrically hidden from view. The graph in the file "Venus's Reappearance" shows this
effect. In the graph, A marks the first visibility of the twilight zone. B marks the geometrically
determined, first detection of light from the northern cusp of the crescent Venus. C marks the first
frame that contains a saturated pixel, and D marks the last frame in which there was no saturated
pixel.
Roger Venable’s Conclusions
• The regression lines intersect at frame 373, which is thus the point of
first detectability of the light of the darkside. This frame has a timestamp
at 19:14:24.0, which is 8 seconds after the geometric first sliver of
darkside reappearance. In summary, light from the dark side of Venus is
definitely detected on this video, 19 seconds before the first detection of
light from the twilight zone of Venus and 20 seconds before
the geometrically determined first reappearance of the northern cusp.
This is a remarkable agreement with Scotty's analysis.
• Roger speculates about an origin from cytherean twilight, or refracted
light from the sunlit side, rather than light from the dark side.
• The detection of the dark side of Venus in this video can be presumed to
be in the 1010 nm window that is known to reveal the surface of Venus.
The CCD chip can be expected to have a quantum efficiency of about
2% at this wavelength.
• I think that this certainly should be written up for publication. However,
there is no significant new science in it. It may be of interest to
professional astronomers, but it will be intensely interesting to A.L.P.O.
members who study Venus.
Overall Conclusions
• Scotty and Roger claim that the dark side of Venus was detected in the
video, but Tony George disagrees with that.
• If Scotty’s analysis is used, the dark side of Venus was comparable to a
10th-mag. star, or about 1/3rd the brightness of the lunar Earthshine.
David’s inability to see the dark side visually (and the similar 2001
Kwajalein visual observation with an even larger telescope) show that
the dark side of Venus, at best, is very hard to see when the sunlit part is
not visible, so it seems impossible to see the dark side directly (visually)
with the overwhelming glare of the sunlit part of Venus (unless there
might be times when the dark side is much brighter visually).
• Certainly, attempts might be made with better equipment at future lunar
occultations of Venus, so I will conclude with some of the next chances.
But any “ashen light” of Venus is quite faint, according to the 2001 and
2015 observations, so although new observations might better quantify
whatever “ashen light” is there (some is likely to exist from thermal
emission), I think the conclusion that it can’t be detected by direct visual
observation will remain unchanged.
2015 December 7
This occultation occurred under rather favorable conditions two months
after the October event, but was only visible from part of Alaska
(Anchorage and Fairbanks). As I remember, weather reports said it was
clear, or only light clouds, but as far as I know, it was not observed.
Some in the “lower 48” states saw the daytime occultation.
Next chances, 2019 & 2022
Kwajalein may have another
chance less than 3 years from now
Not shown are events that occur
too close to the Sun, or waxing
phase (sensor might be blinded by
sunlit side before the dark side D),
and a few are only in inhospitable
polar areas.
In 2022, southern Madagascar,
Mauritius, La Reunion, and
Kerguelen may have a chance.
2028 September in New Zealand
Venus will be 67% sunlit, so not quite as favorable as the 2015 Oct.
occultation. As you can see, good opportunities are about as rare as total
solar eclipses.
2029, 2031, and 2032
2029 Oct. has WAXING chance
for California & Pacific N.W.
Below, good waning chances for
eastern North America, Azores,
Ireland, & maybe UK, Iberia, and
other parts of western Europe.
Next Australian Chance
The next chance in Australia, but
it’s WAXING and low altitude on
the Western Australian coast
New Zealand has another good
waning event a year later.