My Astro Blog

Quest for 100

Wed, 29 Jun 2011 16:51:09 -0500

The AAVSO has come up with some “observer challenges” as part of its Centennial Celebration. One challenge is to observe 100 different variable stars in 100 days. I’m trying to meet that challenge by the 3rd of July. Challenge completed!

Binoviewers

Sat, 6 Mar 2010 12:14:52 -0600

Years ago when I was taught how to shoot a rifle, I was told to aim with both eyes open. Apparently that helps you relax and hold steady. When I started looking through telescopes, I tried to do the same thing. I kept both eyes open but concentrated only on what I saw through my dominant (right) eye. But wouldn't it be better if you kept both eyes open and actually used them both at the same time? That's the idea behind binoculars...and binoviewers.
Some thoughts about binoviewers:
1. For many, observing is more "comfortable" with binoviewers, and the view is more 3-D like. Other people, however, have difficulty merging the images entering each eye into one. Sometimes I do; sometimes I don’t.
1. Although the total light is obviously split in half, the overall brightness of the image doesn't actually decrease that much. Maybe a half magnitude is lost? I don’t notice it much.

1. Because the required light path is lengthened, some telescopes won't focus. A barlow lens must be inserted to re-lengthen the light path, thus increasing the magnification.
2. With most Cassegrain scopes, no barlow is needed, but because the distance between the primary and secondary mirror changes when you focus, the magnification is still increased.
3. Because of (3), (4), and vignetting, it’s difficult to use a binoviewer for low power, wide field views. The diameter of the cone of light entering a binoviewer is necessarily larger than the diameter of the cone of light normally entering an eyepiece. Binoviewers without a large clear aperture (the less expensive ones) reduce the amount of light reaching your eyes, especially near the edge of the field.
4. Using a binoviewer improves the quality of the image, especially for us old guys who have floaters in our eyes. What one eye misses, the other can pick up.
Conclusion: For low power views, buy binoculars! For high power views, especially of the moon and planets, consider binoviewers. Two eyes are better than one!

Jupiter, the Police, and the Speed of Light

Sat, 6 Mar 2010 10:42:34 -0600

Francois was an exchange student at a school where I taught. He was originally from France, but moved to Montreal. He had already taken every math and science course we had to offer, but he had never done any lab work! So we decided to do lots of labs together.
Since I was interested in astronomy, I suggested trying to determine the speed of light by the method first used by Ole Roemer in the 17th century. Eclipses of Jupiter’s moon Io occur at very regular time intervals, but they seem to occur sooner than expected when the Earth is traveling towards Jupiter, and later than expected when the Earth is traveling away from Jupiter.
The motion of the Earth (and to a smaller extent, the motion of Jupiter) changes the distance between the two planets, thus affecting the amount of time it takes for light to travel from one to the other. For more info, click HERE.

The Best Telescope

Wed, 13 Jan 2010 10:18:18 -0600

How important is the quality of a telescope? To what extent does optical quality affect how much you can see and how well you can see it? Provided that you don’t own one of those 2 inch in diameter, 600x refractors, advertised by some discount department stores, telescope “quality” is vastly overrated. Other factors are much more important.

1. The transparency of the sky. This one is obvious. If it’s cloudy, you can’t see anything. And even a slight amount of haze makes a huge difference when you are trying to observe faint objects.

1. The amount of light pollution at your observing site. To see many faint objects well (or at all), there must be sufficient contrast between them and the background sky. That’s why many catalogs of deep-sky objects list the “surface brightness” together with the overall magnitude. Even bright objects don’t show up very well when that brightness is spread out over a large area. Light polluted skies (or moonlit skies) drastically decrease the amount of contrast between object and sky.

1. The “seeing” conditions. Even when the sky is very dark and clear, the atmosphere is often “unsteady.” The stars twinkle beautifully, but planetary images look like quivering mush. Increasing the magnification only increases the fuzziness.

1. The experience of the observer (you). You have to learn how to “see.” It takes patience and practice. Some people have difficulty seeing anything through a telescope. Your eye has to be positioned correctly, you need to relax, and sometimes you have to wait for a brief moment of excellent seeing.

1. The quality of your eyesight. Near-sightedness and far-sightedness are not a problem (just adjust the focus), but astigmatism, the size of your dilated pupil, and the presence of “floaters” are all a factor. At my age (63), objects no longer look as bright or as sharp as they once did.

1. How precisely are the optics of your telescope collimated? Usually not a problem for refractors, but definitely important for Newtonians and Cassegrains.

1. Ok, now we finally get to the quality of your telescope. How precisely have the mirrors and lenses been figured? Are the eyepiece lenses well coated to prevent internal reflections? How steady is the mount?

90% of the time you’re not going to be limited by the quality of your telescope. Other factors almost always play a much larger part. Most importantly, does your telescope get used? Quality doesn’t matter at all if the telescope stays in a closet.

Perfect Beauty of Venus

Mon, 27 Jul 2009 09:20:25 -0500

Venus revolves around the sun 13 times in almost exactly the same time it takes the Earth to revolve around the sun 8 times. In other words, 13 Venus years equals 8 Earth years.

If you connect the positions of the Earth to the positions of Venus every two weeks or so, you get the “string art” pattern shown here. In his book A Little Book of Coincidence, John Martineau calls it “the perfect beauty of Venus.”

If, instead, you put the Earth at the center of the solar system and plot the path of Venus relative to Earth for 8 years, then you get the second pattern shown to the right.

To download Planet Chase , a shareware program that allows you to explore this and other planetary relationships, go to:

http://www.tombretl.com/Software/Science.html

Observing Variable Stars

Thu, 16 Jul 2009 14:51:36 -0500

"The American Association of Variable Star Observers (AAVSO) is a worldwide, nonprofit, scientific and educational organization of amateur and professional astronomers who are interested in variable stars. Its purpose is to coordinate, collect, evaluate, analyze, publish, and archive variable star observations made largely by amateur astronomers, and to make these observations available to professional astronomers, researchers, and educators." http://www.aavso.org/aavso/