A Simple Dobsonian Telescope

In the late months of 2008, I bought the telescope I should have bought first. I bought it for the right reason. I wanted to know about the sky.

All my telescopes have names. Maybe it comes from the habit of all my sailboats having names, but it was influenced by David Levy’s writing about his scopes having names. That first Meade SCT is Virginia. She is a refined package and was purchased when my family lived in that state. The Celestron 4se is “GT” which is short for GoTo and seems like the sort of name that a computer with a telescope strapped onto its frame should have. The Orion XT10, a ten inch commercial version of John Dobson’s volkscope, is Texas. It is big and it was bought in San Antonio.

As the summer and fall of 2008 marched along, I learned about the deep sky from little GT. I reacquainted myself with celestial mechanics when I refurbished Virginia. With Texas, I fell headfirst into the inky black sky of southwest Texas. During the winter months of 2009, I began to prepare for the Messier Marathon to be held in the last weekend of March. Texas was just what I needed for that task.

A Dobsonian scope is a very simple version of a Newtonian reflecting telescope. It is characterized by a large primary mirror near the ground and an angled secondary near the top. The mounting system is super simple. There is a central pivot in the base which allows the scope to be rotated in azimuth and there are large bearings on each side of the optical tube assembly which allow the scope to be moved in altitude.

I started with a simple, unmagnified finder, the Telrad. That meant that I needed to learn where something was in the sky and point the scope by hand at that area. For almost all the objects I was interested in observing, I could not see the object with my unaided eyes. That meant that I needed to learn about the constellations. Then I needed to find some relationship between the stars that I could see and the object I could not see.

I use two major techniques for finding objects in the sky with a Telrad.

- The first is to use the concentric circles in the Telrad. They have diameters of 1, 2, and 4 degrees. If an object is near an easily observed star, perhaps within a degree or two, I can place the correct Telrad circle on the star and have the central circle oriented toward another star. Then looking through a low power eyepiece (I use 32 mm for 38X magnification) I find the object of interest.

- The second is to use two stars and to imagine the object of interest as part of a triangle of a certain shape (right, isosceles, equilateral) or if the object is directly between two stars, I estimate the position as half-way, 1/3, ¼ etc of the way between the stars. An example of this last technique is in finding the ring nebula, M57, between two stars in Lyra. A second example is M13 in Hercules.

Those techniques are sufficient for double stars and Messier objects. For the Hershell list that I have just begun working on, I have added a finder scope to refine the area of interest after initial pointing with the Telrad. The spotter allows some initial star-hopping across dim stars.

The medium aperture “dob” is a great scope for many purposes. There is no scope which can so quickly teach an observer about the sky – to learn where objects are and how they are related to the stars around them. A dob with a primary mirror between 8 and 12 inches in diameter is reasonably easy to transport, not prone to going out of adjustment, and requires no on-site calibration or alignment. (When I pull my truck up at a public star party, it takes me about 2 minutes to pull the base out and place it on the ground and then the scope goes right on the base. I have a stool which is good for sitting on for prolonged careful observing or for kids to kneel on if they are not tall enough to see in the eyepiece.) I am often showing folks the wonders of the sky well before some of my friends are able to see stars to begin their alignment.

A dob, especially at a power between 40X and 50X. is intuitive for beginners or children to use. When they point it somewhere, that’s what they look at. No mystery, no gears, no computers, no alignment. A dob teaches folks that the stars have a constant and incessant travel from east to west. A dob is perfect for hunting for pretty objects, scanning for comets, finding Messier objects, and with investment in a good 8-10mm eyepiece, for looking at planets and the moon. It is short enough for many children to use on their own. It is easy to transport in the back seat of almost any car. It’s not perfect, but it is pretty close.

I have begun to recommend a scope like this as the best family telescope. It is heads above the scope I bought for my family in the 1980s and a lot cheaper. I will probably begin to look for a dob with a larger mirror, that’s called aperture fever. But it will take some looking to find a scope that I like better than Texas.