Greetings, stargazers.
I don’t know who said it first, but it bears repeating that the best telescope is the one that gets used the most. Many of you are familiar with different types of telescopes – reflectors are the ones with mirrors, refractors are the ones with lenses and a third type, catadioptric telescopes, have a combination of both mirrors and lenses. What they all have in common is that they gather light and bring it into focus.
A subject that is often missing from the discussion is how those telescopes are mounted. The specific type of mount usually has a much greater impact on the usability of a telescope than what type of optics it has.
To see the entire sky, a telescope must be able to move around two different axes (lines of rotation), and mounts are classified by how those axes are selected. Broadly speaking, mounts are either altitude-azimuth (Alt-Az), or Equatorial.
The simplest way to point a telescope is to have one axis move the telescope up and down (the altitude axis), and the other axis move left and right (the azimuth axis). For daytime viewing, this is always desirable. For nighttime viewing with low magnification, such as what you might get with binoculars, this is by far the easiest and most intuitive way to navigate around the sky.
My favorite type of alt-az mount is a Dobsonian. It is a simplified, low-cost mount developed by and named after John Dobson and makes the telescope look like a canon. This configuration allows relatively large Newtonian reflector telescopes to be pointed easily by hand. The scope rests on Teflon furniture sliders and, if balanced properly, will stay in place no matter which direction it is pointed. Dobsonian mounts easily provide the most bang for the buck of any telescope mount.
The disadvantage of these mounts is that as the Earth rotates, objects in the sky don’t move in straight lines so both axes must be moved to follow a target. At high magnification, both axes must be moved every few seconds to keep an object in view.
Smaller telescopes can be mounted in an alt-az configuration on a tripod. These mounts often come with a motorized, computer control that allows the scope to find and track arbitrary targets. However, the alignment process (telling the computer which way to point) is often difficult, even for experienced users. And there is no standard method for alignment among different manufacturers. I must admit that I am behind the times with using many of these mounts because computer control and automatic GPS syncing are changing and improving from year to year.
German Equatorial Mounts, or GEMs, have one axis pointing toward the celestial pole, making it parallel with Earth’s rotational axis. The other axis is at a 90 degree angle. The advantage of this configuration, especially with high magnification, is that objects can be followed through the sky by moving a single axis, and a simple motorized clock drive can keep the telescope pointed at the same object all night without any computer assistance.
The disadvantage is an increased mechanical complexity. Different latitudes require a different tilt of the pole-pointing axis, and large counterweights are required to keep the scope balanced on both axes. But when done successfully, very small motors can smoothly control very large telescopes. A high-quality GEM will typically cost more than the optics attached to it, but these are the mounts of choice for astrophotographers and many serious observers.
The nights are still short because we are so close to the solstice, and it doesn’t get truly dark until after 10 p.m. But if you can find a dark, moonless location, the summer Milky Way is one of the most spectacular sights you can see.
Venus is the prominent evening star visible in the western sky at sunset. Saturn and Jupiter are rising at 10 p.m. and 11 p.m. respectively. Next month they will both reach opposition, their closest approach to Earth, and will make great telescope targets.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.
Useful links
DOBSONIAN TELESCOPE: https://en.wikipedia.org/wiki/Dobsonian_telescope.
ASTRONOMY PICTURE OF THE DAY: http://apod.nasa.gov/apod.
OLD FORT LEWIS OBSERVATORY: www.fortlewis.edu/observatory.
AN ASTRONOMER’S FORECAST FOR DURANGO: https://bit.ly/2eXWa64.
FOUR CORNERS STARGAZERS: https://bit.ly/2pKeKKa.
