When someone uses the term telescope they're usually referring to the whole collection of parts that it is made up of - the tripod, mount, tube and anything that is connected together to make a setup.
One of the least substantial looking elements, but which plays a vital role in the success of an observing session, is the success of an observing session, is the finderscope, or finder for short.
This bit of kit sits fairly close to the eyepiece and as the name suggests, it enables you to quickly locate night-sky objects that you can subsequently see through the main telescope. The finder is often connected to the telescope by means of a dovetail bar and bracket, which makes it easy to detach when you want to move your setup.
The reason you need a finder, and one that's aligned properly, is that most telescopes will only show you a small portion of the sky when you look through an eyepiece. Since this can make it difficult to determine exactly where you are looking, a finder's wider view helps you to get your bearings.
It's easiest to align the finder during the day by aiming the telescope at a distant object, such as a pylon, and adjusting the screws that hold the finder in its rings until the view lines up with what you see in the telescope's eyepiece. It's also good to check the alignment every time you use the finder by aiming at a bright star.
Types of Finderscopes
Finderscopes come in quite a few varieties. The original and most basic design is just like a small refracting telescope - it magnifies only a little and therefore allows you to see a bigger part of the sky. Move the telescope to point roughly at your chosen location, and then use the finders large field of view to home in on the target you're hunting. Looking through finders such as these, you'll see crosshairs the view, which makes centring an object much easier. If you're aligned the finder with the telescope correctly, then by making sure that the crosshairs of the finder are sitting over your target, you'll have the same object in the centre of the main telescope's field f view too.
The next thing to consider is the size of the finder. It's generally true that the bigger it is, the better. Some smaller telescopes come with a 5x24 finder, where 5 represents the magnification and 24 is the size of the front lens in millimetres.
These refracting finders are not your only option though. Once you are familiar with the night sky, you might prefer one that provides no magnification whatsoever - like the red-dot or reflex finders, also known as reflecting finders. These seem to project a red dot, circle or crosshair (or a mixture of all three) onto the sky by using a small glass screen.
The simple refracting finder, where you look into an eyepiece in the same direction as the telescope tube is pointing. These finders magnify the sky by varying amounts, but give you a larger field of view than the telescope, helping you to locate your target.
This type projects an illuminated red dot onto a glass screen, through which the non-magnified night sky is visible. They are mounted in the same position on the main scope as refracting finders, but unlike them, you do not need to put your eye right up to it to see anything.
The viewing position here is the same as for the red-dot finder - you don't need to put your eye up to it to see anything. Reflex finders have a non-magnified view overlaid with a variety of illuminated displays. These include differently sized circles, full crosshairs and small crosses.
So when you next come to set up your telescope for an evening's observing, make sure you take full advantage of your finder; by spending a little bit of time aligning it, you'll be enjoying the celestial sights on offer sooner and for longer.