One factor that must be taken into consideration whenever you buy a new telescope is the kind of mount it has. When it comes down to it, there are essentially 3 designs: Altazimuth, Equatorial, and Dobsonian. For the purposes of this astronomy hint we will only be talking about Alazimuth and Equatorial as Dobsonian behaves like an Altazimuth telescope mount even if it in no way resembles one.
First let us look at a telescope on an Altazimuth mount:
The mount portion consists of a simple fork that the optical tube sits in, and that in turn attaches to the tripod legs. No visible with this mount is a slow-motion rod on the other side of the fork that helps moving telescope small amounts in the altitude (up and down)
Now here is the same exact optical tube – but on an Equatorial mount:
Now you will notice this is a lot more complex. Now there are tube rings on the optical tube, and the mount is much beefier, with setting circles, counterweights and slow-motion controls attached. Its a lot of stuff, so what is it for? Let us explain.
You see, unless you are observing at one of the Earth’s poles the stars and planets you see in the sky will not be moving in a nice convenient left-to right fashion. As the Earth rotates that will actually appear to be moving in an arc pattern across the sky. This can make keeping a telescope aimed at an object tricky as it moves – when you have an altazimuth telescope mount it can be tricky to follow celestial objects. Think of it this way: if you have an etch-a-sketch, try to draw a circle with it.
This is where equatorial mounts come in. Once they are set up and aimed at a star or planet they can track it by simply turning one of the slow-motion control rods! You can even get a motor to track the objects on their own. Very handy! The equatorial mount’s setting circles, if set up properly, can be used in conjunction with a star atlas to find tricky objects!
So it might sound like the equatorial mounts are the clear winner, right? Well, not so fast. You might have noticed I said ‘Once they are set up and aimed at a star’ – That is actually a very crucial couple of steps with an equatorial mount.
To start with, you have to aim the mount properly. What that means is that to work that shaft you see running under the telescope tube has to be parallel to the Earth’s axis. This means it must be angled properly to your latitude (this sounds hard but is actually the easy part). The that shaft must be aimed so it is pointing north. At this point how much precision you want to have depends on your needs. For most purposes aligning the latitude and aiming it north is good enough, but for more precise purposes (such as astrophotography) the telescope user is going to need to do a lot of work making sure it is as accurately aligned as possible.
But for most folks the latitude adjustment and aiming north is good enough. So no more problems, right? Well again, no, because now you need to adjust your mindset of how to aim the telescope. You see, most people think in terms of up/down left/right, and that is where the Altazimuth telescope shine – they are intuitive and easy to move!
But now with an equatorial mount you need to think not in terms of up/down/left/right but in terms of Right Ascension and Declination. We’ll try to use this little drawing to illustrate the difference:
(Note that the Right Ascension line should actually be more of a curve, but we lacked that skill in Draw to make it properly).
So if you are moving from one object to another, you mind might think “just a fair amount to the right and up a little bit’ to get there, but now you have to think more like ‘down in declination and clockwise in Right Ascension.’. It can be fairly daunting! It has a learning curve, and that curve can be mean. Even more tricky us that by moving the telescope in all these odd directions will put it in strange positions. You might have noticed that counterweight on the mount? Well, while many telescope show it pointed down in product photos it rarely will be point that way (its useless when pointed down like that). In real-life use the counterweight will be countering the weight of the optical tube as it gets moved in strange-seeming positions to aim at objects.
This can be tricky enough that I have seem folks who bought telescopes with equatorial mounts end up using them as if they were altazimuth mounts. It doesn’t work very well.
There’s another factor as well – remember I mentioned how setting circles on the mount could be used to find objects? Well that is true – if you get it very well aligned. Also keep in mind on some smaller equatorial mounts that the setting circles are so small they won’t be very accurate. These things might be able to get you in the ballpark, but do not expect computer guided accuracy!
One final factor is that equatorially mounted telescopes weigh more than their altazimuth counterparts.
So let’s sum up:
Advantages: Easy to set up, lightweight, intuitive movement
Disadvantages: Poor tracking ability, cannot be motorized, no aiming aids
Advantages: Can track objects, can be motorized to track, setting circles can be used to help find objects
Disadvantages: Much trickier to set-up, unintuitive movement methods, heavy
The final choice of what kind of mount you want depends mostly on you and how you plan to view the night sky. For example, if you prefer to flit quickly from one object to another an altazimuth might work best for you as there is no need for tracking if you don’t stay on the object for very long. On the other hand if you like to take you telescope to an outdoor party and point it at the Moon you might want an equatorial mount (and a motor).
Generally speaking, with the exception of Dobsonian telescopes, as telescope’s apertures (size of their mirror or lens) get larger, the more likely they are to be on an equatorial mount. Part of this is engineering (the beefier optical tubes need beefier mounts) but the other part is that equatorial mounts are seen as more ‘advanced’ wheras the altazimuth mounts can be seen as more for beginners. How much of this is actually true is a matter for debate.