What type of telescope is best for me? A really rough guide based on what you want to see.
When clients come in and ask about a new telescope, the first thing we normally ask is what they want to see with it. People often look at us weirdly when I ask this. "The sky?" they tend to venture.
We're after an idea whether they want to look at planets or deep sky targets like nebulas and galaxies. What we're getting towards is what type of telescope is best for them.
Telescopes aren't the same. They're highly specific tools, each with their own area of specialisation. Get the wrong one and it's not going to give you what you want.
Ferraris and Land Cruisers
We explain by asking people what is the best type of car: a Land Cruiser or a Ferrari? They're both great for what they're intended for, but used for a different purpose... not so much.
So how do we decide what type?
Aperture and focal length Telescopes are fairly comprehensively described in two measurements, aperture and focal length.
Aperture - how much light you get
A telescope's aperture - how big the hole is out the front - determines how much light gets into the scope, and therefore into your eye (or your camera, if you're that way inclined).
Focal length- how much magnification you want
The telescope's focal length, on the other hand, determines magnification.
If you know what you want to see, you can figure out what aperture and focal length suits that target best. From there, you can begin to zero in on what type of telescope is going to suit you.
If you want to see planets, which are tiny but very bright, you're after a long focal length. This will give you high magnification. You're probably less concerned about aperture, because planets are bright and you won't need to suck in much light.
Nebulas and galaxies
If you want nebulas and galaxies, these are big and very dim. In order to get a good image, you need to get as much light into your eye as you can. This means you need a wide aperture.
I've made up a chart showing the various designs of scopes and their aperture and focal lengths. I'll post it below.
Finally, can you get a scope for both nebulas and planets? Well, yes, but you'll be paying an astronomical price - literally!
From targets to telescopes
OK, so your choice of target can suggest what focal length and aperture might suit you best.
People who want to specialise in tiny planets need long focal lengths for maximum magnification, but aperture isn't so important because planets are mostly very bright.
On the other hand, people who are interested in deep-sky objects like galaxies and nebulas need to soak up as much light as they can, so large aperture telescopes are best for them. Because deep-sky objects can be quite large (check out the Rosette Nebula), telescopes for these targets can't have have long focal lengths.
But aperture and focal length aren't all the information about your equipment, there are heaps of details.
Narrowing the field
Probably the most common question we are asked is which is better: reflector or refractor.
Another common question asked by people is about what telescope they should have in five or ten years.
I gathered a list of common telescopes and plotted their focal lengths and apertures on a scatter diagram. From this, I generalised a little to get to this scribbled diagram.
So you can tell pretty quickly what telescope is going to be better for different targets.
If you're interested in big, bright targets, such as the Moon or whale watching, a short focal length refractor will be great. Small, bright targets would be better viewed with a Cassegrain, or even better, a 180 Mak. Larger, dimmer targets are best served by a large Newtonian like a Dob: ultimately a 20" model, or that 36cm RASA for photographic work.
Small, dim objects are way, way, more debatable. A Celestron 14 Edge maybe, but we're getting into some serious technically challenging areas.
Just a rough guide
The diagram can't tell the whole story, of course. Increasing aperture for the same focal length will give better resolution on planets and other objects. Astrophotographers with a high quality mount can track for longer and that will make up for a smaller aperture, so this kinda blurs the choice a little.
There's no right answer, and maybe this is a start.
I can pretty much assure you that the scope you wanted when you began won't be the scope you want once you've got some experience. Life's full of surprises.