What do the specifications mean?
Optic
al tube f
o
c
al l
ength is simpl
y a
measur
ement of the l
ength of the
optic
al tube. In other wor
ds, this is
the dis
tanc
e light tr
av
els in the t
e
le
-
sc
ope bef
or
e being br
ought t
o
f
ocus in y
o
u
e
yepiec
e. Y
o
ur tube is 700mm l
ong.
Objectiv
e l
ens diamet
er is how big the l
ens
is on y
our sc
ope. T
e
lesc
opes ar
e al
ways
described by how lar
ge their objectiv
e l
ens
is. Y
our t
e
lesc
ope is 60mm or 2.4 inches.
Other t
e
lesc
opes ar
e 90mm, 8 inches, 16
inches, or e
ven 3 f
eet in diamet
er
. The
Hubbl
e T
e
lesc
ope’
s objectiv
e l
ens has a
diamet
er of 2.4 met
er
s (that’
s 7.8 f
eet
acr
os
s!).
The f
o
c
al r
atio helps det
ermine how f
a
s
t the
phot
ogr
aphic speed of a t
e
lesc
ope is. The
lo
wer the f
o
c
al r
atio number, the f
a
s
ter the
e
xposur
e. f/5 is f
a
s
ter than f/10. The f
a
s
ter
the r
atio, the f
a
s
ter e
xposur
e time is needed
when a c
amer
a is hook
ed up t
o
the t
e
le
-
sc
ope. Y
our t
e
lesc
ope has sl
o
w
er f
o
c
al r
atio
at f/11.7. Sometimes, as
tr
onomer
s use f
o
c
a
l
reduc
er
s t
o
mak
e sl
o
w
e
xposur
e t
e
lesc
opes
hav
e f
a
s
ter f
o
c
al r
atios.
Alt
a
z mounting simpl
y means y
our t
e
lesc
ope
mo
ves up and down (altitude or “alt”), and
side t
o
side, (azimuth or “az”). Other
mounting c
onfigur
ations ar
e av
ailabl
e f
o
r
other t
e
lesc
opes, such as equat
orial
mounting.
USE THE SPECIFIC
A
TIONS T
O
C
A
L
CULA
TE
THE MA
GNIFIC
A
TION OF Y
OUR EYEPIECE
The pow
er of a t
e
lesc
ope is how much it
magnifies objects. Y
our 25mm e
yepiec
e
magnifies an object 28 times. Y
our 9mm
e
yepiec
e magnifies objects 78 times.
But if y
ou obt
ain other e
yepiec
es, y
ou c
a
n
c
a
lculat
e how much magnific
ation the
y hav
e
with y
our t
e
lesc
ope. Jus
t divide the f
o
c
a
l
12
length of the t
e
lesc
ope by the f
o
c
al l
ength of
the e
yepiec
e
.
F
o
c
al Length of the T
e
lesc
ope
÷
F
o
c
al Length of the Ey
epiec
e
=
Magnific
ation
Look at the specific
ations. Y
ou will see that
the f
o
c
al l
ength of y
our sc
ope is 700mm.
Let’
s say that y
ou hav
e obt
ained a 13mm
e
ye
piec
e. Y
ou c
an t
ell that what the f
o
c
a
l
length of y
our e
yepiec
e is as it is al
ways
ed on the side of an e
yepiec
e. Divide:
700 ÷ 13, which equals 53.8. Round this off
to
the near
es
t whol
e number and y
our new
e
ye
piec
e magnifies objects 54 times.
If y
ou use y
our Barl
o
w
l
ens with one of y
our
e
ye
piec
es, it doubl
es the magnific
ation of
your e
yepiec
e. Other types of Barl
o
ws c
a
n
tripl
e or further incr
ease the pow
er of an
e
ye
piec
e. T
o
find out how much y
our
magnific
ation is when y
ou use a Barl
o
w
,
multipl
y y
our e
yepiec
e
’s
magnific
ation by
two.
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