Monday, August 06, 2007
Stars And Bugs
I’m all bit up.
At the end of last week we had three or four days of cloudless skies and I got the urge to do some star-gazing. So I spent an hour or two each night in my backyard under the stars with my telescope. And billions and billions of bugs.
I was looking for this thing:
The Ring Nebula, M57, in the constellation Lyra.
Of course, that’s a photograph taken by the Hubble Space Telescope. It’s beautiful, but it’s nothing at all like what you see in real life.
Through a medium-size (12”) amateur telescope the Ring Nebula looks like this:
So-called “planetary” nebula like the Ring Nebula actually are colorful. They are made of gas near a star and, typically, ultraviolet energy from the star causes the gas to glow, something like the gas in a neon sign glows. But nebula are so far away that the light is very weak by the time it gets to Earth. There’s not enough energy there to ‘activate’ the color sensors in our eyes, so even through large telescopes we—typically—see nebula as white, or white with just a tint of color.
Of course, many amateur astronomers devote lots of money and uncounted hours to taking time-exposure pictures of nebula to capture the beautiful colors.
But even through small telescopes the ‘simple’ white images are amazing.
Some people have reported seeing the Ring Nebula in good 10x50 binoculars. I’m guessing they were under perfect skies and the binoculars were either solidly mounted on a tripod or new, image-stabilized binoculars. I’ve never managed to see the Ring Nebula through my 10x50 binoculars, but I’ve never checked under good skies.
Here south of Chicago even our best nights are pretty bad. Unimaginable (to rural folks) light pollution and almost constant haze.
Last week was a good example. We had cloudless skies, but the seeing was terrible. There was some kind of haze or smog from horizon to horizon. I could see Vega and Arcturus as naked eye stars. But they are two of the brightest stars in the sky. Almost everything else required binoculars. Even the Big Dipper was almost impossible to see by eye.
Jupiter is in Scorpius, to the south, and Antares—also one of the brightest stars in the sky—was just barely visible without binoculars.
Since Vega was visible, I decided to give my small telescope a work-out and see if I could view the Ring Nebula.
My telescope is a small refractor, a 2.4 inch (60mm) Unitron on an altazimuth mount. The focal length is 900mm. That’s f/15—not fast, not a wide field of view, and with a small objective, not great at gathering light for higher powers. But it’s a well-made scope with a rock-solid mount. Unitron made good optics. If you think of a scope like this sort of as half of really great binoculars, it sets your expectations to a reasonable level. You keep the power low and you get reasonably wide fields of view (for a telescope) and dark backgrounds and bright images.
I did most of my observing last week with eyepieces that were 25mm, 20mm and 18mm, for powers of 36x, 48x and 50x. I now and then checked things with up to a 9mm eyepiece for 100x, but under poor skies high power just magnifies the glare.
Because Lyra is almost overhead this time of year, I spent most of my time in the dark kneeling in the grass, or just sitting on the sidewalk next to my lawn. A Newtonian reflector lets you stand up to observe because the eyepiece is opposite the scope’s mirror. A refractor has the lens up front and the eyepiece at the ‘bottom’ of the tube. Even with a right-angle adaptor you still have to be willing, with a refractor, to get into weird positions to accommodate the scope. And, under bad skies, it’s always a good idea to keep the light path straight, so I was doing my observing without a right-angle adaptor.
The bugs in my backyard apparently were thinking, ‘Cool, an all-you-can-eat buffet!’
Lyra is a fun constellation because Vega is always easy to find, and there really are only five other stars to know. The stars make up a simple triangle-on-top-of-a-rectangle shape most binoculars can take in with very little ‘sweeping’ necessary. My wide-angle 10x50s can show the entire top or entire bottom of the constellation at once.
The Ring Nebula is easy to find because it is between the ‘botttom’ two stars of the lower ‘rectangle’ of Lyra. In fact, there is a sequence of dim stars you can ‘hop’ along with a narrow view telescope like mine and M57, the Ring Nebula, falls just between them and off to one side. With a ‘normal’ amateur scope—a 6” or 8” scope—the Ring Nebula is easy to see and can be impressive even at high powers.
Last week I did not quite see it. The seeing was too bad, the skies were too poor.
I could just barely see the dim stars between β Lyra (Sheliak) and γ Lyra (Sulafat). Using averted vision—looking off center at the eyepiece field of view—I could make out a white ‘smudge’ where I knew the Ring Nebula was, but I couldn’t make out any structure at all.
But the summer isn’t over. We’ll have better nights.
I’ll see it. If I can survive the bugs.