Transit of Mercury, November 8, 2006
by Matt Allard, Diane Murray and Philip Steffey
Passages of the innermost planet Mercury in front of the Sun as seen from Earth, occur about 11 times per century. Actual intervals range from three years to almost two decades, and an event may or may not be visible from a given observing site depending mainly on whether the Sun is above or below the horizon. Also, not every "transit" is equal. In some the planet's path barely grazes the solar disc for an hour or so, and in the best events, lasting six hours, the path bisects the disc or nearly so.
Daytona Beach was fortunate to have two recent visible transits seven years apart. Matt and Phil observed the November, 1999 event, a grazer, from a site in southern Flagler County. We just watched in small telescopes, but luckily a colleague, Tony Viviano, had the materials, patience and skill to make a fine composite drawing, including numerous sunspots, which is reproduced in an article on Phil's Webpage (please click here to see that article).
When Matt and Phil contemplated the 2006 event, about six weeks before it was due, our top priority was to obtain a hard record, with images that could be monitored in real time or at least quickly. Videography was an available and tried method and we had a second: digital imagery with Diane's Sony CyberShot DSC-P73 camera, which recently had produced excellent pictures of the Moon. (We soon confirmed that Diane would join us for the transit.)
Our second consideration was where to observe the transit, necessarily involving what telescopes to use. This event was to be five hours long but with a drawback for us, beginning after 2 P.M. with sunset 3 1/2 hours later. The Sun's altitude at Mercury's ingress would be 35°, so in order to get two to three hours observation we needed a site with a west-southwest horizon clear to about 5.° Due to a misconception that the large air conditioning/heating structure adjacent to the B-CC Observatory dome would block the 14-inch telescope's view of the low Sun (see first picture following the text), thought was given to observing from the ground with small telescopes. For reasons including probable lack of adequate electric power, remote sites were ruled out, leaving the B-CC campus as one of very few prospects and hardly an ideal one. But in mid-October, Matt checked the capability of the 14-inch to go where we wanted and found, as he suspected, that the rooftop structure would not be an obstacle! Henceforth we planned to use the big 'scope. By the month's end our concern had shifted to the weather.
After five months of Arizona-like weather that refused to produce repeatedly forcasted hurricanes or even local rains, Daytona Beach finally got some clouds and a little rain in late October. And this extended into the first week of November, to our chagrin. On the 6th we had overcast but the forecast was for clearing beginning late the next day. Would it bring us a clear-enough sky by the early afternoon of the 8th? We each made tentative preparations for observing and went to bed on the 7th under mostly cloudy skies with fingers crossed.
Amazingly, the sky was mostly clear by early morning and rapidly getting clearer! Matt and Phil met at B-CC just after noon and we soon opened the observatory dome, which had been in strong sunlight for three or four hours. Outside the air was cool and breezy. Near 1 P.M. we put the full-aperture filter on the telescope and acquired the Sun for a first look. The "seeing" was, not surprisingly, lousy, but we expected it to improve. Two small sunspots were seen near what proved to be the preceding ('off') limb. Next a crucial fitting of the Celestron f/7 focal reducer to the 'scope was made, and we could view the entire Sun with a low-power (70X) eyepiece. A fairly large, single sunspot was seen just inside the following ('on') limb. Diane joined us and we were almost ready to begin taking pictures.
Largely because we had no experience using it for solar observing, Phil had chosen not to use the Polaris video camera but instead his Orion camera with TV monitoring and tape recording. (B-CC's somewhat newer Orion camera was available as a backup.) This choice probably sacrificed spatial resolution, but with at most 2 1/2 hours to observe Mercury on the Sun, reliability and efficiency were paramount. By 1:30 the camera, with its f/6 focal reducer, was installed on the telescope and connected to the TV-VCR used many times before. The small sunspots were taped for a minute, and considering the still poor seeing the streaming image quality (spatial resolution) was satisfactory; see second picture. But two features were worrisome. The area of the solar disc shown was less than expected and the disc's color was a sickly pink. With the transit looming, these problems had to be temporarily put aside. The low-power eyepiece was inserted in place of the camera.
Phil believed Mercury's ingress would occur on the limb near the two
small sunspots, but when nothing appeared there by ~2:13 a quick glance at
the opposite limb revealed a black notch about 23° in position angle south of
the large, single spot. (The confusion about the ('on') limb remains a
mystery.) Diane was ready with her digital camera and began shooting
pictures a couple minutes after second contact. The two best, taken at
f/2.8, are reproduced in Figures , processed with Photoshop mainly to
smooth a jagged solar limb presumably due to atmospheric turbulence. The
raw disc color was desaturated somewhat to resemble the visual appearance,
and the contrast was enhanced. The shading in the upper left of both images
was produced by the telescope's defocused secondary mirror, which Diane
handled well but could not be effectively removed by Photoshop processing.
The times of these pictures are based on measurements of the planet image
diameter and the known motion rate of one diameter per 1.7 minutes.
Accuracy is +/- 1 minute.
Videography of the event required a few minutes for equipment changes, and the first images were recorded at 2:20. A single still frame from the tape is shown in , converted to black-and-white because the Orion camera's color adjustment control refused to work properly! The larger image scale than the digital camera's yielded somewhat more detail in the sunspot and clearly showed some faculae near it, but the frame's coverage of the solar disc area was disappointing--only ~10 x 8 arcminutes. Evidently the camera's focal reducer had little or no effect and we were stuck with an effective telescope focal length of near 98 inches. Another surprise!
The progress of the transit was videotaped for 24 minutes, then following a break for 10 minutes from 3:09. A mosaic of stills from these segments, all 2-frame averages, is shown in . Two four-frame-average stills at original scale, , are more contrasty and a little sharper than the single frames, but Mercury's image is imperfect even after considerable processing. Diane made good use of this period to take pictures of the telescope and eventually us three observers . Around 3:30 we had visitors including our good friend, retired Professor Richard Copeland. With the wide-field eyepiece in the telescope, Diane obtained a few more transit pictures . None of these or later digital pictures had adequate timings when taken, so the times had to be calculated from measurements on the images and on a diagram in the November 2006 Sky and Telescope, p.57. Accuracy is only +/- 5 minutes. Videotaping resumed at ~4:05, by which time Mercury and the big sunspot could not be simultaneously imaged. Luckily, enough of the Sun's limb area was captured in images of the separate features that "joints" could be made . Better still, Diane's camera could produce pictures with planet, spot and room to spare. Two of her late shots are reproduced in .
By 4:30 the telescope was still getting sunlight without obstruction outside the dome. But a corner of the six-feet high equipment cabinet was casting a growing shadow on the corrector lens, so we stopped observing about 15 minutes later. We therefore obtained images of half the transit and some 80% of what we could have covered at an ideal site before sunset. Diane's digital images--only half of the good ones are reproduced here-- are remarkable for hand-held camera shots and nicely complemented the video. The realistic color, except for excessive hue which was easy for Photoshop to handle, was welcome. The 50 minutes of video recording was adequate for images with mostly bare solar photosphere. As with most of our previous astro-videography, the tape has segments with more details in streaming playbacks than the stills shown here reveal, including photospheric granulation. But automatic frame-stacking capability, which we still lack, would be needed to create stills with matching definition. The excessively red color of the raw video images and the malfunction of the Orion camer's focal reducer could have been avoided if we had been able to observe the Sun a few days before the transit, but the bad weather then prevented this.
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