Greetings, stargazers.
A regular occurrence for me is being asked what kind of telescope someone should buy. My answer is always the same – the best one is the one that will get used the most. For most stargazers that means a pair of binoculars. I have written several columns about binoculars and binocular targets, but for those committed to seeing even more, here is a quick rundown on why I think an 8-inch Dob gives you the best bang for the buck and is the way to get started.
Useful links
Dobsonian Telescopes
https://en.wikipedia.org/wiki/Dobsonian_telescope
Astronomy picture of the day
An Astronomer’s forecast for Durango
http://www.cleardarksky.com/c/DrngoCOkey.html?1
Old Fort Lewis Observatory
http://www.fortlewis.edu/observatory
hakes_c@fortlewis.edu
A Dobsonian-mounted Newtonian reflector (much easier to just say Dob) looks like a canon. It is a hollow tube, often made out of cardboard, that holds the mirrors and eyepieces in the simplest configuration for a reflector telescope. The tube is balanced on a rotating base with furniture sliders so it can be pointed up, down, left and right very easily. When it is well-balanced, it will stay pointing wherever it is aimed.
For observing dim, deep-sky objects, the primary consideration is the light-gathering ability of the telescope. Light-gathering of a telescope increases with the square of its diameter, so an 8-inch scope can detect things four times dimmer then a 4-inch scope. Bigger is always better as far as what you can see. The biggest Dob I have looked through was a 32 inch monster of a telescope that gave me the best view I have ever seen. However, it was so big it took several people to move and quite a while to assemble, because it was too big to move as one piece.
The reason I like the 8-inch version is that is about the biggest one that can be picked up and moved in one step. Setup simply involves setting it on the ground and making sure it doesn’t wobble. It is the telescope we use in the introductory astronomy labs at Fort Lewis College. The targets that are barely visible with the naked eye, or are easy to spot with binoculars, gain a new level of detail with the extra light available through an 8-inch scope. To get the same factor of 16 increase in light gathering going from 2-inch binoculars to an 8-inch Dob would require going to that 32-inch giant.
Almost all the cost of manufacturing a Dob goes into the optics, and very little is required of the mount. Many readers could probably build a great Dob mount in an afternoon using hand tools. A similar-sized reflector on a tripod would need a very sturdy tripod with some sort of mount and multiple counterweights. The supporting hardware would likely cost more than any Newtonian reflector they could hold.
To reduce their size, most tripod-mounted scopes are one of several types of Cassegrain designs that have a folded optical path. Although I am a huge fan of the Cassegrain design, they have much longer focal lengths and thus a much narrower field of view. That makes targets much harder to get into the eyepiece. To “help” the user, many have computers for targeting. But unless you have a permanent mount, the tracking and go-to features require a careful setup and synchronization every time the telescope is taken out. That might be great for star parties or extended observations, but is not the most conducive to spur of the moment quick looks at the sky.
The summer Milky Way as seen from Southwest Colorado is a spectacular sight. Just scanning it with low magnification reveals numerous clusters and nebulae. Near the brightest part of the Milky Way, Sagittarius, the asterism that looks like a teapot, is a good anchor for finding several of my favorite nebulae.
M8, the Lagoon Nebula is about 5 degrees to the right of the topmost star in the teapot. After the Orion Nebula, the Lagoon Nebula is the second brightest star-forming region we can see. You can see the Lagoon with your naked eye. With binoculars, numerous stars are visible as part of the open cluster associated with the nebula. The glowing hydrogen gas that makes M8 so interesting will show up red in photographs, but because of the limitations of our eye sensitivity, we will only see this as a pale blue-gray fuzzy patch.
Slightly to the north of M8 is M20, the Trifid Nebula. Dark bands divide this nebula into three, thus its name.
Less than 10 degrees north of the teapot is M17, called the Omega Nebula, the Swan Nebula or the Checkmark Nebula, depending on what your imagination says it looks like.
Venus is still dominating the western sky after sunset. Although Mars is in the same direction, it is getting quite dim because it is on the far side of the sun. Jupiter is bright in the morning sky, and we can look forward to nice evening views of it in the fall.
As a final disclaimer this month, I need to confess that among my numerous telescopes, I don’t own an 8-inch Dob. But I have watched hundreds of students learn to use them in lab and get excited when they find their targets.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory.
