Astronomy FAQ 002

Firstly, the pun in the second question was intentional, and the answer is not much scope at all. For that money you should consider a decent pair of binoculars. A really good pair can be even more than that, but you should be able to get a reasonable pair from £60 upwards. The scopes in the catalogues and chain stores, (generally), are not worth considering, even as a starter scope. They can be so frustrating to use that they can put people off for good. More on that later but first let´s deal with binoculars. These are very much underrated by the new would-be astronomer so I will spend a bit of time on them first.

The strength of the telescope is magnification and resolving power. Astronomical objects are so far away from us that bodies as large as the planets look like points of light to the naked eye. To see any details therefore, magnification is required. Even then, the stars are so far away that they still look like points of light in the telescope but the magnification can separate stars that the naked eye sees as one. Some astronomical objects are quite large and faint, however, so that too much magnification makes them impossible to see in a telescope. The field of view in the eyepiece, even on the lowest power, might only cover a small fraction of the object. Secondly, spreading out the light by magnification to increase separation, decreases the brightness. It is a trade-off, so compromise is required. This is where the binoculars come in. With their lower power and object lenses much bigger than our eyes, they are just what is needed for diffuse, faint objects. A good example of this is the Plieades. These are a wonderful sight in most binoculars but a bit disappointing in many telescopes. The serious astronomer needs both binoculars and a telescope, so a decent pair of bins is the place for the newcomer to start.

What sort of binoculars should I get? Can they be used for bird watching too?

Usually, advice on buying binoculars applies to both bird watching and astronomy. You want as much magnification and light gathering power as you can manage but since you need to hold them steady, they need to be light. (Here we go, compromise again - and puns).

Binoculars are rated as 8x40, 7x50, 10x50, etc. The first of these numbers is the magnification and the second is the diameter of the object lens in millimetres. The smallest useful magnification is about 6. Normal are 7 or 8, above that it starts to become difficult to keep them steady and they start to get heavy due to the weight of the glass. A magnification of 10x will need something to steady the binoculars; ideally use a sturdy tripod, or rest on your elbows in the bird hide. A tripod bush is therefore useful, but you can get adaptors for binoculars that don't have tripod mountings. With a firm tripod, a magnification of 12 or even higher could be considered but remember, we shall probably get a telescope later which will provide much better magnification. However, if someone offers you a pair of 16x80 ex-naval ones as a gift, don't turn them down.

What about the second number? Well as you increase the magnification the second number needs to increase by the same amount to get the same brightness. So 7x35, 8x40, 10x50 will all give the same brightness of object but with increasing size. However, the weight does go up quite quickly as the objective size increases.

What ratio should I choose then? If you divide the second number by the first you get what is known as the exit pupil size. If you hold the bins up, some distance from your eyes, you will see a little circle of light in the eyepiece. The exit pupil is the size of this circle in millimetres. It is actually the image of the object lens in the eyepiece lens. If you imagine the eyepiece next to your eye now, you should realise that this bundle of light, (collected by the objective and focused by the eyepiece) is what enters your eyes. If you are out bird watching on a bright sunny day your pupil will be quite small, 2 or 3 millimetres. That is why those very compact 8x20 pocket binoculars can be surprisingly good in those conditions. The extra light gathered by a pair of 8x40s doesn't get in to your stopped-down iris anyway, so they don't perform any better. But in the woods or late in the evening, when your iris opens up, it is a different story. What you need depends on how you are going to use your binoculars. As a rough guide you could consider the following.

• 11cm spherical mirror
• 900mm focal length.
• interchangeable lenses 4mm, 20mm.
• 450x magnification.
• £229 (Autumn 2000).

So what am I likely to see through it? Well, let us find out. Let us do an imaginary field trial. Suppose there is a very large field which has a track across it and a house at the end of this track. On the roof of this house is a TV aerial and a bird is sitting on the aerial. Well this is the field and this is the naked eye view of it. I said it was a large field, it is difficult to see the house at the end, but you can just make it out. Certainly I can't even see the roof, let alone the TV aerial and don't bother to mention the bird. So what happens if we look at the house through our magnification 10x binoculars? Let's find out.

Ah yes. A house with a red roof and what might be a TV aerial. In fact through these binoculars, although I can see the house, I can't see any bird. Of course one normally looks at birds at lot nearer than that. But this an exercise to do with telescopes, not bird watching. OK then, so I can see the TV aerial, what would I be able to I see through a bird watching scope. 25x or 30x is a bit more normal for bird watching but we'll pretend we have a 50x one. It had better be mounted on a firm tripod (with a tilt/pan head) or else all we will see is the chimney flashing past our eyes occasionally as we try to point the scope in the right direction.

OK, now we can see a TV aerial with a small dark blob on it. I guess that must be the bird. I can see the brickwork of the chimney and the cross elements on the TV aerial. That is pretty good detail, seeing as I could only just see the house as a small blob of colour with the naked eye. OK then, so what are we waiting for? What about the big one, the 500x astronomical scope. What can we see through that?

Uh? Everything is upside down. Yes that's normal. It is possible to get a prism device to turn the image the right way up. However most astronomers will put up with the inconvenience to avoid losing a bit of light in the extra glass. When it comes to star diagonals to get a more comfortable view however, and avoiding a cricked neck, that is a different matter (but I digress). I hope you can all see that the bird is clearly a starling. This is what you can expect from 500 magnification. Will you see detail like this through one of these catalogue scopes? I'll come back to this in a minute, but first let's just be sure about this exercise. There is an important lesson (see below) contained in it which needs to be grasped.