Page 58 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

KNOW YOUR

SCOPE STATS

GET TO GRIPS WITH THE OFTEN MYSTERIOUS FIGURES
THAT DESCRIBE THE OPTICAL PERFORMANCE OF YOUR
TELESCOPE – FOCAL LENGTH AND FOCAL RATIO

1. FOCAL LENGTH

The focal length of a refracting telescope is lengths mean smaller fi elds of view,
the distance between its lens and the place which are not always best for observing
where light rays from that lens are fi nally wide star fi elds or star-hopping. You
brought to a focus, known as the focal point. can, however, increase a telescope’s
For a refl ecting telescope, simply swap the focal length by using an accessory
word ‘lens’ for ‘mirror’. known as a Barlow lens.
Focal length is one of the important Eyepieces also have focal
numbers if you want to fi nd out what lengths, but since they take the
magnifi cation you are viewing the night focused light and magnify
sky with. The magnifi cation is the focal the image into your eye, the
length of your telescope divided by the numbers mean the reverse. So
focal length of your eyepiece. You get the smaller an eyepiece’s focal
higher magnifi cations – good for viewing length, the higher its magnifi cation.
detail on the planets, for instance – with For example, an 8mm focal length
telescopes that have longer focal lengths. eyepiece gives you a ‘closer’
The downside to this is that longer focal view than a 20mm eyepiece.

Focal point

3. VISUAL VS PHOTO
Bearing in mind what we’ve said about fast times than their long focal ratio counterparts. have a fairly small exit pupil
and slow scopes in the ‘Focal ratio’ section, There’s also less chance of stars blurring as that can be uncomfortable
the f/number will tell you whether a scope a result of your mount’s tracking falling to look through, especially
is particularly suited to observations with behind the movement of the night sky. if you have to wear glasses.
just the eye or whether it will be good for If you’re intending to mostly use your Opting for a slower scope
astrophotography too. telescope for visual observing, then larger removes this problem. To get the
Smaller focal ratio (fast) telescopes are focal ratio (slow) instruments are ideal. To same 100x magnifi cation with a slower,
good for astrophotography – especially if get 100x magnifi cation with a small focal large focal ratio scope you’d use a longer
you want to image large star fi elds – because ratio (fast) telescope, you’d need to use a focal length eyepiece, which has longer
they can get an image with shorter exposure small focal length eyepiece. These often and more comfortable eye relief.

58 skyatnightmagazine.com 2012