Beginner’s Q&A – February 2009
We get many questions each week at One-Minute Astronomer from some of our nearly 10,000 subscribers. Here’s an attempt to answer some of the more frequently-asked questions. And remember, never be afraid to ask questions. Like my old chemistry professor once told me, “There are no dumb questions. Only dumb mistakes.”
Foggy Lens on Telescope
Q: “When I bring my telescope outside on a cold night, I get mist on the lens at the top of the tube. Is there a way to stop this?” (from A.J. in Great Britain)
A: Dew is forming on the lens of your telescope. This happens in warm weather and cold, and it happens even when your telescope has been outside for a long time. It’s not just a winter problem. In fact, because it’s so humid in the summer where I live, I get more dew in July and August than I do in December or January. The lens on a refractor or Schmidt-Cassegrain telescope is particularly susceptible to dewing because it radiates heat faster as it points up into the coldness of space.
One solution… reduce the rate at which your lens radiates heat by shrouding it with a dew shield that extends the length of the tube of your telescope and cuts down on the amount of sky into which the lens can radiate away its heat. Dew shields slow down the rate of dew formation, but won’t stop it altogether.
The surest way to stop dew is to use a dew heater to warm up the lens slightly, just so its above the dew point. You can buy pre-made dew heaters that fit around the circumference of the telescope tube near the lens. One size does NOT fit all, so make sure you get the right heater for your telescope. Dew heaters don’t draw much power, so you can run them off batteries for a few hours. Or, instead of a dew heater, if you have access to AC power, you can blow a little warm air on your lens for a few seconds with the help of a hand-held hair dryer. Works well.
Bright Object in the Sky
Q: “What is that bright ball in the sky just after the sun goes down. It’s high in the west, about 45 degrees above the horizon” (from many, many readers…)
A: That is Venus. It’s just reached it highest point in the sky, and is now working its way closer to the Sun each day. Through a telescope over the next few week, you’ll see Venus assume an ever more crescent-like shape.
Higher Power Eyepiece
Q: “I got a new Celestron Astromaster telescope. I can see Saturn after midnight, but it looks too small in my eyepiece. What eyepiece do I need to make it appear bigger.?” (from T. S. in the United States)
A: I’m not sure of the details of your scope. But lets say you have an Astromaster 114. That has a focal length of 1000 mm. It comes with a 10 mm eyepiece which gives you a magnification of 100x. That’s enough to see Saturn’s rings and largest moon, Titan. But as you say, the disk of the planet will look quite small. To get higher magnification, you can get an eyepiece with 5 mm; that gives you a magnification of 200x.
Or you can use your 10 mm eyepiece and get yourself a 2x Barlow lens. The Barlow will effectively double the magnification of the 10 mm eyepiece to 200x. Put the eyepiece in the Barlow, then put the Barlow into the eyepiece holder. At higher magnification, Saturn will look bigger, but it may also look a little fuzzier and fainter.
Don’t use too much magnification, or your the image will get too dim and fuzzy to enjoy. A rule of thumb is to restrict magnification to <50x the aperture of your telescope in inches. So a 4-inch telescope should not be pushed higher than 200x, approximately. It all depends on the quality of the optics and on seeing conditions.
Can I See The Southern Cross Right Now?
Q: “Where is the Southern Cross? When is it visible in the southern hemisphere?” (from K.M. in Botswana)
A: You can see the Southern Cross late at night right now from Botswana. It rises about 8 p.m. in the south-east sky, and should be high enough to see in the southeast at 10-11 p.m. It will rise about 4 minutes earlier each day, so that you can see it earlier in the night as the year progresses. Good luck… the Southern Cross is very beautiful, especially if you have dark sky.
