Low Power Vistas

© 1989, 2000 by Robert C. Moler

September is a good month to scan the skies with telescopes of low power and binoculars.. While the price of decent telescopes escalate out of sight, a pair of binoculars can still be had for less than $100. In September the summer Milky Way is still sprawled across the sky. Both star clusters and nebulae appear as fuzzy spats imbedded in that great sky river, except that the hazy flow itself resolves itself into a myriad of stars.

Binocular observers can pretend they're Charles Messier who cataloged many of the finest deep sky objects. That wasn't what he was locking for. Messier was a comet hunter who was frustrated by the many fuzzy comet like objects around which didn't move. So he made up a list of these things to avoid wasting your time on when hunting for comets. In the observatory telescopes we wonder how he could have thought soma of these objects were comets, but Messier didn't have large telescopes, which aren't good for comet sweeping anyway.

Most of these fuzzy objects are found in and above the constellation of Sagittarius. This area is a gold mine for binoculars or almost any telescope. A bit to the left of the top of the lid of the Teapot asterism that we recognize as Sagittarius is M22 a spectacular globular star cluster that could easily trick the novice comet hunter.

Besides the bounty in the southern Milky Way there are more things to see In the Milky Way between Cygnus and Aquila just right and obit above the back end of Sagitta the arrow is a airing of stars in a straight line with a trio of stars above. It's called the Coathanger.

One of the most impressive sights in binoculars is the Andomeda Galaxy, which is visible this time of year in the northeast. The elliptical appearance of this nearly edge-on spiral galaxy is obvious. There is much more visible of the extent of the galaxy than in a telescope. Just east of the bright star Deneb in Cygnus the swan is the North American Nebula. Though its shape may not be easily seen, the faint glow of the gasses brighten up the background of the sky. In Aquila there are many dark clouds that obscure the light of distant stars. These too can be spotted by the absence or low number of stars in various areas.

Perseus, another autumn constellation in the east hides a large star association just below unaided eye visibility. This Alpha Persei Association is seen around the brightest star on the north side of the constellation and fills the binocular field nicely. But the best binocular cluster is the Pleiades becoming easily visible in a couple of months. While visible to the unaided eye, it is beat seen in binoculars. The telescope turns out to be too powerful to see more than a few of its stars at a time.

Planetary observers aren't completely left out. When Jupiter comes into the evening sky in a few months its entourage of tiny moons can be seen moving from night to night. This December as Venus is poised to sweep between the earth end the sun, its tiny crescent will be visible in binoculars.

The perfect pair of binoculars for astronomical observation is probably 7 x 50 (pronounced 7 by 50). This or a similar pair of numbers is usually stamped on the binoculars. The first number the magnifying power, while the second is the aperture (diameter) of the front lens in millimeters. When one divides the aperture by the power the quotient becomes the diameter of the binoculars' exit pupil. The exit pupil can be seen by holding the binocular toward a bright featureless area like a wall. With your eyes well back from the eyepiece look at the spot of light in the eyepiece. As you move your head around you will notice that this disc of light is a bit back from the eyepiece itself. The exit pupil is the eyepiece's image of the front lens or objective, Since all the light travels through the objective, the light too must pass through its image. The maximum the eye's pupil can open is about 7 millimeters, but it diminishes with age. When the exit pupil matches the observer's pupil the view is the brightest it can be.

Huge 11 X 60 binoculars have been popular In astronomical circles. Division of the two numbers also gives nearly seven. However they are heavy and really need tripod support to be useful. 7 x 35 binoculars are a good compromise between the heavier astronomical binoculars and ones for day-time use. Since in bright light the eye's pupil contracts, the extra light gathering capabilities are wasted.

Join us this month at the society meeting to discuss further the wonders revealed by binoculars, and if its clear to explore the heavens at low power. Bring your binoculars if you've got them.


Questions? Send Email to me at bob@bjmoler.org

Updated: 08/16/01