Editing Glossary (section)

===== Focal ratio =====
----The focal ratio is how large the aperture is in relation to the focal length. A fast focal ratio (e.g. F/4) results in images taking less time to expose than a slow focal ratio (e.g. F/10). The focal ratio is worked out by dividing the focal length by the aperture - an 800mm focal length 200mm aperture newtonian will be f/4. Something which is also related to focal ratio is F/stops. This is a list of focal ratios in increments of how long they take to expose - the next F/stop takes twice as long of an exposure to produce the same level of detail.
{| class="wikitable"
!F/stop
!2.8
!4
!5.6
!8
!11
!16
!22
|-
|Exposure time
|10s
|20s
|40s
|80s
|160s
|320s
|640s
|}
In imaging deep sky objects a fast focal ratio is desirable. For planetary imaging it is not as important.