Page 59 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO USE

2. FOCAL RATIO
The focal ratio of any telescope is its focal length
divided by the diameter of the front lens or mirror.
This leads to another way of describing it – the
f/number. Let’s say you end up with a ‘6’ once
you’ve done the calculation. The resulting focal
ratio would be written as ‘f/6’. A scope
with an f/number lower than six is said to have
Light path a small focal ratio. F/numbers of nine or above
are considered large. Knowing your focal ratio
is important for astro imaging.
There’s yet another way of describing a scope’s
focal ratio, as fast or slow, a throwback to the
days when cameras used fi lm. Small focal ratios
meant the aperture of a camera’s lens was open
wide, which let in a lot of light and caused a
‘fast’ reaction between the chemicals on the
fi lm and the light. The opposite happened with
large focal ratios: the narrower apertures in the
lens let in less light, causing a ‘slow’ reaction
with the light in the chemicals on the fi lm.

F/NUMBER PROS & CONS
This general guide compares fast and slow focal ratios, and applies
to most (but not all) telescopes
FAST SLOW
Smaller focal ratio: f/4 and below Larger focal ratio: f/9 and above
Shorter focal lengths: shorter Longer focal lengths: longer
telescope telescope
Wide fi eld of view: good for Narrow fi eld of view: good for
observing large swathes of the zooming in on planets or viewing
night sky double stars
Smaller eyepiece eye relief: have Larger eyepiece eye relief: can
to use lower magnifi cations or use higher magnifi cations
viewing can be uncomfortable
Smaller depth of focus: precise Greater depth of focus: more
focusing required for a crisp image tolerance in focusing
Telescopes can be smaller and Telescopes may be larger and
easier to transport heavier and so not as portable

BEST OF BOTH WORLDS

Choosing a telescope is not or awkward to move. However,
simply about deciding on the knowing the limitations of
best focal ratio. It may be fast and slow telescopes is a
that portability overrides useful addition to the buying
everything; many scopes process. If you want to hedge
languish in sheds and your bets, then it’s best to go
garages because for the area between fast
they are too heavy and slow scopes.

skyatnightmagazine.com 2012 59