How to Use a Telescope Finderscope to Navigate the Nights Sky

How to use the telescope finderscopes that came with your big telescope to help locate celestial objects in the sky and aid star hopping.

When somebody makes use of the term telescope they are in most cases referring to the whole collection of parts that it is made up of - the tripod, mount, tube and anything else that is connected to make a setup.

One of the least substantial looking components, but which plays an important role in the success of an observing session is the finder scope, telescope finder scope or just "finder" for short.

This bit of equipment sits quite close to the eyepiece and as the name suggests, it lets you quickly find night-sky objects that you can subsequently see through the primary telescope. The telescope finder scope is often attached to the telescope by the use of a dovetail bar and bracket, which makes it easy to detach when you want to move your setup. Some also attach through the use of velcro or sticky pads.

The reason why you need a finder, and one that's aligned correctly, is that the majority of telescopes will only show you a very small portion of the sky when you look through an eyepiece. Since this will make it tricky to determine precisely where you're looking, a finder's wider view lets you get your bearings.

Aligning a Telescope Finderscope

Finderscope needs to be accurately aligned, otherwise, they are useless. Aligning the finderscope means you can quickly locate objects in the wide field of view, or star hop. When you have the object in the centre of the finder, it should be the centre of the telescope eyepiece, or close enough that you can see it and tweak it.

It's easiest to align the finder during the day by aiming the telescope at a faraway object, such as a pylon, and adjusting the screws that hold the finder in its rings until the view lines up with what you notice within the telescope's eyepiece. It's additionally good to check the alignment every time you use the finder by aiming at a bright star.

Methods for aligning vary by type of finderscope, but in most cases, there are three screws which form a triangle. By tightening or loosening screws you can adjust the aim by small amounts.

Types of Telescope Finderscope

Finderscopes come in quite a few types. The original and most basic design is just like a small refracting telescope - it magnifies only a little and therefore permits you to see a larger part of the sky. Move the telescope to point roughly at your preferred location, and then use the finder's large field of view to home in on the target you're searching. Looking through finders such as these, you'll see crosshairs in 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 primary telescope's field of view too.

The next thing to consider is the dimensions of the finder. It's typically 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 may 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 display.

Straight-Through Finders

The simple refracting finder, where you look into an eyepiece in the same direction as the telescope tube is pointing. These finders enlarge the sky by various amounts but give you a larger field of view than the telescope, helping you to locate your target.

Red-Dot Finders

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 red dot finder scope is typically included when you buy Skywatcher bundles, it was part of the package when I bought my StarTravel 102 refractor. It's much smaller than my Telrad reflex finder (see below) and I find it much easier to use as well. It mounts directly onto the finder scope bracket, unlike the Telrad which has its base. The advantage of the separate base is that you can use a 9x50 finder scope as well.

It features adjustable brightness control as well as two alignment screws for calibration. Quick and easy to use and once calibrated it rarely needs doing again.

Reflex

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.

One of the first gadgets I acquired was my Telrad Reflex red dot finder. I was experiencing difficulty locating objects in the sky, mainly because I haven't learned every star visible yet. I know the general position of the objects and can star hop between bright stars, but looking through the finder scope dramatically increases the number of stars visible and the finder scope projects an inverted view to your eye - up is down and left is right. I rapidly got confused, lost my way and lost my patience.

The reflex finder works by superimposing an electronic cross-hair over the sky (which is not magnified, you just look through a small piece of glass) and it makes star hopping that much easier by limiting to eye-visible stars only. The cross-hair has the added feature of being able to measure the distance between stars. The inner circle has a diameter of 1/2 degree, the middle of 2 degrees and the outer circle has a diameter of 4 degrees. A regular finder scope can still be used to pinpoint accuracy even further with its magnification.

The device works brilliantly and the cross-hair has a variable brightness control, however, I feel the unit is a bit bulky and could have been made a lot smaller, given that there is a lot of unused space inside (as shown in the final picture).

So when you next come to set up your telescope for an evening's observing, make sure you take full benefit of your finder; by spending a little bit of time aligning it, you'll be enjoying the celestial sights on offer quicker and for longer.