Telescopes are not the tube: they're the glass!
Think of a telescope and you think of a long tube with stuff on the eyeball end.
That long tube is actually the least important part of the telescope: it's just there to keep the other equipment in place, hopefully without vibrating too much.
The most important part of the telescope is the lenses and/or the mirrors. The tube is just there connect them together. Solid tube connectors are cheaper and they block out stray light, but some scopes just have struts between the optical ends of the scope.
The two ends are known as the objective (the object/target end) and the ocular (the eye end).
You could use a paper towel roll if the size were right. In fact, some vintage inexpensive dobsonian telescopes used essentially larger paper towel rolls as their structure.
Two ends: Objective (object) and Ocular (eyeball)
So, the important part is the glass, and both ends have a relationship that controls all the optical characteristics of the scope.
Generally, you want to have some flexibilty with the images you see, so you can change some of the glass to get a different view.
The Objective and the Tube are generally a unit
The objective lens or primary mirror actually drives the diameter and length of the tube, so the tube (or other support system) is generally a permanent component of the objective lens cell or the primary mirror cell.
The Ocular end is the Variety end
The other end, the ocular side - the user end - is where all the customization (and a most of the fun) occurs. It ends with interchangeble eyepieces.
Focuser
Because you are likely putting a lot of different equipment on the ocular end, there is almost always a focuser. The objective/primary has a fixed focal plane that the eyepiece needs to be positioned at, the focuser allows you to move the eyepiece precisely to that plane. That's how focusing works moving the eyepiece to the focal plane. Some scopes (like Schmitt-Cassegrains (and cameras) have fixed "receiver"; focusing moves the glass to move the focal plane to the sensor/eyepiece).
Eyepieces: A Labyrnth of Fun (and Money?)
Magnification is the focal length at the objective side of the eyepiece divided by the focal length of the eyepiece. A 1000mm focal length hitting a 25mm focal length eyepiece gives you 40x. Want higher power, put in a shorter focal length eyepiece. 10mm gives you 100x.
That number on the side of the eyepiece is the focal length.
If you buy a new entry level telescope package, it generally will contain two or three eyepieces.
One of the most common questions asked is "Are my included eyepieces any good?" The short answer is (and I seldom give short answers): NO!
Are they sufficent for getting started? Yes.
Are you going to get the best experience with these cheap eyepieces? No.
If you're enjoying the hobby, will you want to buy better eyepieces? Absolutely.
Here's a little secret: you can spend a lot more money on eyepieces than on telescopes. But you don't have to and you shouldn't.
I've rotated through dozens of telescopes, but I have a set of good eyepieces and I hang on to them.
If money were no object, here's the recommendation: buy a Televue Nagler 31, and a bunch of Televue Ethos eyepieces. They $700 or more, each. I own the Nagler, but have cheaper alternatives for everything else.
The eyepieces that came with your telescope kit, may be worth $30 collectively (beginner scopes have a lot of competition for price, the eyepieces and the mount/tripod get a lot of "cost reduction").
The good news is that there is a lot high-value, high-performance eyepieces between your "cheap" eyepieces and the bank breakers.
Note: there are two standard diameters for "real" telescope eyepieces: 2" and 1.25". Some cheap telescopes only accept 1.25" eyepieces. Check yours.
- The old standard was 0.96", these are mainly used by collectors who enjoy the vintage experience.
So what's the difference between a $40 Celestron 32mm eyepiece and the $700 Televue Nagler 31mm?
- Diameter - the 2" barrel of the Nagler can show a wider swath of sky than the 1.25" barrel of the Celestron. The 1.25" barrel essentially clips the image delivered to it by the telescope. For "point objects" (Jupiter, Saturn, Mars, planetary nebula - anything small), this isn't as big a deal. For big objects and wide fields - it is a big deal. Check your focuser!
- Quality - better glass, better coatings, better correction for fast focal ratios
- AFOV - Apparent Field of View. This is the big one and where most of the pricepoint jumps. It's how wide the view looks to your eye. It deserves explanation (Apparent Field of View)
Barlows
Barlows are magnification multipliers that allow you to use an eyepiece for multiple magnifications. A 2x Barlow with a 24mm eyepiece is provides the magnification of a 12mm eyepiece and maintains the properties of the eyepiece. Even a premium barlow is cheaper than most premium eyepieces and they can be used with many eyepieces. They're not only a magnification multiplier, they're an "eyepiece" multiplier.