Astronomer Book
your interests to be equally divided between Solar System and deep-sky
objects, then the f/5–f/9 instruments are probably the ones that will likely
interest you most.
One further factor when considering the speed and focal length of a
potential purchase is the use of an optical amplifier to alter these figures. An
amplifier, such as a Barlow lens, can be used between the telescope and
eyepiece to effectively make it seem that the focal length of the telescope
has been multiplied by the power of the Barlow. For example, consider a
200mm-aperture telescope with a 2000mm focal length (f/10). Using a
2x Barlow increases the effective focal length of the scope to 4000mm but
reduces its speed to f/20. The caveat here is that amplifiers with powers
greater than 1 can make viewing harder, the high focal ratios produced giving
rise to dimmer images.
Optical amplifiers also come with powers less than unity, called focal
reducers. These reduce the effective focal length and increase the speed
of the instrument, delivering a wider, brighter view. Careful and consideredA large reflector fits this bill well, and there are plenty of examples of this
"light-bucket" type of telescope available on the market. One very popular
and relatively cheap version of a deep-sky light bucket is the Dobsonian
reflector. This is typically a large-aperture telescope on a simple left-right/
up-down (alt-az) mounting platform. The ethos behind this telescope,
originally designed by John Dobson, was that this should be a telescope
that was simple to make from readily available components. If you are
good with your hands, there are many Dobsonian plans available
online which would allow you to create a large-diameter scope for very
little money.
The distinction between a planetary scope and the deep-sky scope is
often made by referring to the speed of the instrument. This is a measure of
the instrument’s focal length divided by its diameter using the same units.
So a 200mm (8-inch) reflector with a 2000mm focal length has what’s
called a focal ratio of 2000/200 = 10. This is normally written as f/10 and
is an indication of the speed of the telescope, in this instance f/10 is being
considered slow.
A telescope with a similar aperture of 200mm but a shorter focal
length of 600mm would have a focal ratio of 600/200 = f/3, which is
considered fast. If you’re wondering where the terms "speed", "slow" and
"fast" come from, they echo the days of film photography. The term
"speed" is a measure of how quickly a lens or telescope can deliver a set
amount of light to photographic film. A "fast" lens would achieve delivery
quicker than a "slow" lens. Basically, a slow lens requires a longer exposure
to achieve the same depth of image that a fast lens can deliver in a relatively
short exposure.
The basic rule of thumb is that slow instruments are good for the Moon
and planets while fast ones are better suited to deep-sky objects and
the stars. What are the definitions of slow and fast? Well, this is open to
interpretation but a reasonable assumption would be that any scope with
a focal ratio of f/5 or lower is considered to be a fast instrument, while any
scope with a focal ratio of f/9 or higher is considered to be slow. Of course
this leaves those with focal ratios in the region of f/5–f/9 uncategorized but
these are the instruments which are suited to both camps. If you regard[4] If you’re just starting out, a pair of 7x50 or 10x50 binoculars are ideal. [6] A basic alt-az mount provides side-to-side and up-and-down motion.[5] A big-aperture and low-focal-ratio (fast) scope is ideal for deep-sky viewing.4AltitudeAzimut
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