The Optical Tube Assembly

A telescope has to be as portable as possible if you want a large telescope to be easy to get from star party to star party with the least amount of effort. That's where the Telescope Dobsonian comes in. The refractor telescope is a tube with a lens at each end and has no mirrors. These OTA's are rarely over a 6 inch diameter tube. There are several reasons for this, mostly it's the cost of the lens. Because of the size and the cost of the OTA you don't see them set up as a telescope Dobsonian style very often if ever.

Now the Newtonian Reflector is a different story. The Newtonian was invented by Sir Isaac Newton way back in 1672. It was actually quite the research instrument and typically hung on a German equatorial mount.

This style OTA is capable of huge diameter scopes! The primary mirror is supported from underneath therefore it dose not suffer from the same limitations as the refractor does because of the thin lens edge or the sagging glass. There are telescope Dobsonian styles that have ten meter mirrors! That is 30 feet across. Huge aperture that collects a lot of light and allows us to see WAY out there! See the Keck Telescope on Mauna Kea in Hawaii

How does the newtonian reflector work

Before the Newtonian telescope, the scopes used were mostly refractors and very small ones at that. The Newtonian reflector has a series of mirrors that are supported by some sort of structure. Be it a cardboard tube or a set of poles connected to make a truss type support or even two mirrors connected to a pipe!

Here is a link to some science techie stuff for building a Newtonian reflector

The most common Newtonian OTA is a tube type like this.

The Idea was to make telescopes that were inexpensive and easy to cart around..The Dobsonian style telescope base was usually made of plywood or any thing you had to use actually. The tubes typically were a piece of sonnotube. That is a trade name for these concrete forming tubes used in construction. They are tough as nails and already made water proof for the intended job of holding in wet concrete. A rolled Aluminum tube is great for a Newtonian tube.

The real beauty of a newtonian reflector is you don't really need a tube at all!

You can make a Newtonian with just about anything! If you look at the telescope dobsonian to the right. You'll notice the bars that hold the secondary mirror up in the air. Now you might think there would be a lot of light leaking in this telescope ...Thankfully most astronomy is DONE IN THE DARK LOL. I know it seems obvious but SO many people ask..Dont the light ruin the view?

These are all newtonian reflectors on a rocker style base. The base made famous by John Dobson and the San Francisco Sidewalk Astronomers! What a cool name.

This is a telescope dobsonian style that is made with two poles that run from the rocker box (or base) and support the secondary mirror. This scope does not have a tube to speak of, But it does have some what of a optical tube ASSEMBLY. The assembly is not a tube but it does assemble the parts needed to hold the secondary and eyepiece exactly where they need to be. You can accomplish the same trick with a single pole

For the longest time telescopes were mostly equatorial mounts. They are more precise for tracking and astrophotography. And still are today. Fortunately John Dobson didn't’ give a hoot about research or astrophotography at the time. He just wanted to get more people to enjoy the night sky. So some buddies and he started setting up scopes on the streets of San Francisco and they invited everybody that walked by to look through the telescopes...The original astronomy out reach program!

A good site for the beginning astronomer is My First Scope for more information

Here is a shameless advert from Astronomy Magazine on the truss tube dobsonian. A good video for a the beginner!

Next lets look at the TELESCOPE DOBSONIAN BASE

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