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Log, Jan 9, 2009

32 people. Sorry for the late posting of this week's logbook but I had to leave early Saturday for a five day trip to New York City. We had a nearly full Moon this week, and since the Moon was so bright, we chose to insert the polarizing filter. Since we were pointed at the bright planet Venus at the time, I decided to use it to refocus after we inserted the filter. And I was dumbfounded. The polarizing filter had a spectacular effect on Venus - it allowed me to see for the very first time, the limb darkening on this planet. Limb darkening is no big surprise because as the surface moves from the sub-solar point (the point on a planet where the Sun stands exactly at the zenith) to the terminator (the ring where the planet is either undergoing sunrise or sunset), the amount of light trails off. However, for bright objects, the human eye usually cannot see any difference in shading. For Venus, the whole surface looks extremely bright.

With the filter on, it was quite a different story. When sunlight hits a surface and bounces off it, it becomes somewhat polarized. The angle at which it hits determines the orientation and degree of polarization. You can actually try this at the beach in the morning or afternoon. Wear polarized sunglasses and look at the water where the Sun reflects. Twist the glasses back and forth and you will notice a great deal of light changes. This is what happened on Venus, but on a planet wide scale. The top of the Venus image (sub-solar point) was bright white, barely dimmed. The image grew more gray blue towards the terminator. I even saw some faint indications of clouds (sort of semicircles of slightly darker shades). I cannot confirm that this wasn't a trick of my eye, but it is true that towering cloud masses would present slightly different angles to the Sun and hence different polarization.

I should explain that besides the polarizing filter, I had also installed a hexagonal mask on the telescope. The purpose of the hex-mask is similar to the purpose of the apodizing mask we tried last year. It causes images of bright stars to present not a circle but a six sided star burst. This star burst causes light to be pulled from the star into the points leaving the star a little dimmer. We planned to use it to try to see the Pup (Sirius B) which is 10,000 dimmer than its partner Sirius A (the Dog star). Perhaps the hex mask affected the Venus image - possibly even creating the hints of cloud definition.

The Moon was exceedingly bright. In fact, it was brighter this Friday than it will be in any other Friday during 2009. This limited what we could see but we certainly looked into Messier 42/43 [The Great Nebula in Orion]. We used a fairly high power magnification initially with reasonable success. Later in the evening we used a lower power eyepiece and got a much nicer image.

-Les Coleman

Leslie Coleman
Leslie Coleman
Entry Date:
Jan 9, 2009
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Leslie Coleman's Log
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