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.

Robots such as this will enable scientists on Earth to explore Mars with the use of virtual reality visors and gloves.

The March 30, 2001
Inferior Conjunction of Venus
by Jim Scala

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
diminishing disk up to and after the actual transit.

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 before.

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.
Image Galley (see top of page)

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.
Upcoming Challenges


From Left to right: March 2nd, March 17th, March 29th, March 30th, April 3rd - each image is 70 by 70 arc seconds and shows the crescent Venus several days before and after conjunction. The slender crescent is truly a beautiful sight and is larger at over 58" than any other planet can ever become.

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.