Starting to use your telescope

It’s a common complaint among first-time telescope owners: ‘I’ve got this new telescope and I can’t see a thing through it’. The aim of this article is to suggest a few things that you can try to get you going.

Choose the lowest magnification

Observer using small telescope.
Using even a small telescope takes practice. Credit Robin Scagell/Galaxy

Astronomical telescopes often come with several eyepieces, of different focal lengths. The magnification they give depends on their focal length, and the focal length of the telescope – that is in effect the length of the tube. So an eyepiece will give a different magnification depending on the telescope, which is why they don’t usually have the actual magnification marked on them.

If the focal length of the telescope is 900 mm and the eyepiece has a focal length of 20 mm, its magnification on that scope will be 900 divided by 20, which is 45, often written 45x, meaning 45 times. Astronomers often refer to this as a power of 45.

The eyepiece that gives the lowest magnification of any set will therefore have the highest number on it, usually 20 mm or 25 mm. This is the one you should always begin with, and even advanced astronomers usually do this. It’s because the lower the magnification, the wider the field of view, and when you’re trying to find objects in the sky you usually need to have a wide view first of all before progressing to highers magnifications. So Rule No 1 is – start with the eyepiece with the highest number.

Align the finder

Virtually all telescopes, even Go To ones, have a finder. This is either a small telescope or a red-dot device attached to the side of the tube, the idea being that you locate the object using the finder first, then it should be in the main field of view of the telescope. Why is it needed? The field of view of the main telescope is very magnified, so you only see a tiny bit of sky at any one time. The finder shows you a wider view of the sky and acts as a peepsight to help you line up the telescope. But it needs to be accurately aligned with the main scope, and there are always adjusters for this purpose. But first you need to find an object in the main telescope, so choose the lowest magnification and to make life a great deal easier, start in daytime by finding a distant object in the main scope.

A finder telescope (top) has three adjusting screws. But if the collar doesn’t grip the tube, pack it out with tape (above)

This is for two reasons. One, it’s easier to find a distinctive object by day, such as a TV aerial or chimney pot. Two, it will stay still, unlike astronomical bodies, which move surprisingly quickly through the sky as the Earth turns. An alternative at night is to use the Pole Star, Polaris, which remains in almost the same spot for hours at a time.

But never choose the Sun or an object near the Sun. Not only does it move, but it is also a very dangerous object as it can easily blind you when seen through a telescope.

Having got your chosen object central in the main telescope, the next task is to align the finder so that its crosswires are exactly on the same object. There are always little adjusters to help you do this. In theory this is a simple task, but many of the entry-level telescopes have very poor finders which are badly mounted.

Adjusting finder focus.
To focus a finder scope, loosen the locking ring on the left then turn the dew cap to focus the lens. Credit Robin Scagell/Galaxy

If yours is a finder telescope, rather than a red-dot device, you may find that it has only one set of adjusters to move the tube, which should be held firmly by a collar around the tube. In many cases the tube is not firmly held, so you may need to wrap some sticky tape around the tube inside the collar, so as to give yourself something to adjust against.

Sometimes the finder isn’t in focus, although the finders sold with entry-level scopes such as those requiring sticky tape to make them work can’t be focused anyway.  Usually on higher-spec finders, the focus is not at the eyepiece end but at the top end. There is a locking ring that holds the dew shield in place. Slacken this, then you can turn the dew shield and objective lens together to focus the instrument. Once it’s in focus, tighten the locking ring again.

First steps with observing

Now that the finder is aligned and you have the lowest magnification, you can begin to observe. If the Moon is in the sky, that’s the thing to aim for. It’s easy to find, and if you have everything lined up and it’s in the finder, you should see it in the main scope. Now you need to focus. Try adjusting the focus in and out and as you do even if it’s blurred the image should get brighter and smaller as you approach the focus point. If it’s way out of focus and you’re not sure what’s happening, move the telescope slightly so that you can see one edge of the blurred disc rather than a general patch of light, and you should see it getting more focused.

The most difficult telescopes to focus are catadioptric scopes such as Schmidt-Cassegrains, as these have a long focus travel, and if you’re going the wrong way you could be making matters worse. So make sure you’re looking at the Moon itself and not just some internal reflection.

If the Moon isn’t around, aim for a really bright star. Its image should be a disc when it’s out of focus and it gets smaller as you approach the focus point.

What if nothing will focus, no matter what you do? It may be that if you have a refractor, you need to use the star diagonal between the telescope and the eyepiece. This device avoids the need to stoop on the ground when observing, but may also act as an essential spacer to bring the eyepiece to the right focus point. So if the image starts to get smaller when you reach the end of the travel moving out from the telescope, use the star diagonal.

Using different eyepieces

Once you’re happy with using the lowest magnification, the natural thing is to want to increase the magnification. After all, that’s what a telescope’s for. In principle all you need to do is to change the eyepiece for one of shorter focal length, and away you go. But this can be where you come unstuck.

When looking at the Moon, it usually is as easy as that. You probably need to refocus slightly, but that’s easy enough on the Moon as it’s just a matter of twiddling the focus until it’s sharp. It’s bright enough to see what you’re doing. But on stars, or deep-sky objects, it isn’t always as simple as that.

Moon at two magnifications
Part of the Moon at medium power (left) and high power (right). Notice how the field of view gets smaller and the image gets dimmer as you increase the magnification. The area shown is the Mare Tranquillitatis, where the Apollo 11 landing took place. Credit Robin Scagell/Galaxy

There’s only a certain amount of light available, and when you increase the magnification you spread that light out more, so the image gets dimmer. That doesn’t matter much on the Moon or a bright planet, but on fainter objects it can mean the difference between seeing it and not seeing it. In addition, the field of view usually decreases, and if the object is slightly offcentre and out of focus as well you might not see it at all.

Views of Crab Nebula
The left view shows the Crab Nebula as you might see it at low power. But with a higher power the object may disappear if it is not dead centre and the new eyepiece is slightly out of focus (right). Credit Robin Scagell/Galaxy

So make allowances for this when increasing the magnification. Learn how much you need to alter the focus for a particular eyepiece if necessary, and in what direction. Make sure the object you’re observing is dead centre, or, if your telescope is not driven, predict how the object will drift through the field of view while you change the eyepiece.

Can’t see anything at all?

On some occasions people with a new telescope can’t see anything at all when they look through it. If this happens, the trick is to point the telescope at a bright window or something which you can’t miss, remove the eyepiece and look into the tube. You should see a bright circle of light, but if not you should be able to establish just where the problem lies. In the case of a refractor it could be that the star diagonal is faulty, or with a reflector, the secondary mirror is seriously misaligned – known as badly collimated. But these faults are very unlikely,  and before you embark on trying to recollimate your new scope, it’s best to get someone else to look at it. Either way, knowing what the fault is should help you to sort it out.

Robin Scagell