Dr. Carol Stoker is a planetary scientist in the Space Sciences Division at NASA Ames Research Center, Moffett Field, CA. She received her Ph.D. in AstroGeophysics from the University of Colorado in 1983. At NASA since 1985, she has done theoretical and experimental research on a variety of problems related to the origin, evolution, and search for life in the solar system. She is actively involved in planning for robotic and human exploration of Mars. Since 1990, Carol has led a NASA Ames project to develop telepresence and virtual reality technology to enhance control of mobile rovers on other planets. This work has focused on field studies of space-analog environments on the Earth using robots. She has been the lead scientist in a number of field experiments simulating robotic rover missions to Mars. She has also been involved in robotic exploration of underwater environments relevant to searching for life on other planets. She was a participating scientist on the Mars Pathfinder mission where she provided a three-dimensional interactive virtual reality model of the Pathfinder landing site as tool for operating the rover mission.
The March 30, 2001
Venus passes between the Earth and sun every 14-to-24 months with most intervals at about 19 months. During these inferior conjunctions, Venus becomes increasingly more slender, larger in size and a very bright crescent and then disappears in the sun's glare as it goes from the evening to the morning sky. Only rarely does Venus actually transit the sun since the respective orbits (Earth's and Venus") are not coplanar. The next transits will occur in 2004 and 2012. Conjunctions are very difficult to see because the planet is seldom more than five degrees from the sun, the glare of which becomes both overwhelming and dangerous.
During the March 30th 2001 inferior conjunction, Venus passed 8º
6" north of the sun and was well placed for northern observers. That
is good news; the bad news is that the instant of passage occurred at
0400 UT, or 9:00 p.m. local time. Consequently, the best we could do was
to follow its
I enjoy following planetary motion and decided to follow this conjunction
with my CCD, even though several things are stacked against the imager.
Once Venus gets really close to the sun, the daylight sky overwhelms the
sensitive CCD chip even with a built in infrared blocking filter. Since
Venus was at magnitude 4.02, the daylight sky could be managed with neutral
density filters and the planet still came through. The second challenge,
sunlight entering the telescope is twofold; heat causes tube expansion
which translates to focus changes, and the CCD chip to warm up. Working
quickly and then rotating the dome to block the sun meets these challenges.
The third challenge, atmospheric turbulence from working so close to the
sun, is best countered by taking lots of short exposures, easily done
by programming the CCD camera in sequence mode.
I used a Barlow lens yielding F-28 and two ND-1 (neutral density) filters
that pass 0.1 percent total light. At this high focal ratio and attenuated
light, Venus produced about 10 percent saturation on my Santa Barbara
Instrument Group ST8-E CCD with the shortest possible exposure of 0.11
sec and the smallest pixel size of 9 microns.
The night before imaging Venus I focused on a bright star in approximately
the same RA and Decwhere I would image the next day using the exact optical
configuration. Focus is arguably the most critical criterion in CCD imaging
and it is practically impossible to obtain precise focus on an extended
object, such as Venus, in turbulent conditions, let alone in daylight.
Indeed, turbulence generally causes the image to move out of focus about
15 percent on a reasonable day, and 25 percent is not uncommon. To achieve
an exact focus under those conditions reduces to a mediocre guess. In
contrast, focusing on a star the night before is a walk in the park. Similarly,
I prepared and median-combined my dark, flat, and bias frames the night
At imaging time the next day, I would set the scope on Venus' RA and
Dec then make sure the CCD reached its operating temperature of 0º
C; full image calibration was set, before opening the dome. Venus was
easily brought in the center of the field as a rapidly bouncing crescent
in the eyepiece. I would then take images in sequences of ten each with
a 3-second interval, so I could compensate for any drive errors. As the
images downloaded, I would note, "toss," "possible,"
and "keep." On most clear days, about 5 percent were keepers
on the first pass and about 2 percent overall; on somewhat hazy days,
the keeper rate is higher as haze stabilizes image movement.
I selected an array of images to show that two changes occur at the same
time: the crescent becomes ever more slender, and it "tilts"
more as it passes above (north) the sun. I oriented the crescents as they
would appear in an erect imaging telescope. The only image processing
was to adjust the "input and output" in Photoshop, making the
sky appear black.
I wish I could have been able to capture the actual instant of conjunction. However, the exercise proved CCD imaging in daylight is possible. The next two challenges are the coming conjunction of Mercury on June 16th. and catching a very slender moon, say 24 hours, in daylight. Both are much more difficult because neither is as bright as Venus and the moon is an extended object with low surface brightness. However, I've got other filter options I haven't yet used, and exposure times can be extended.