S.T.E.M. for the Classroom
Hubble, bubble,
toil and trouble
Vocabulary
• Focal Length: he distance between a curved mirror and its Focal Point
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• Focal Point: The position where all reflected light inside the telescope focuses to a point of light
• Focal Surface:The place where the Focal Point converges to
• Primary Mirror: The larger of the two curved mirrors that reflect the incoming light to the Secondary Mirror
• Secondary Mirror: The smaller of the two curved mirrors that direct reflected light from the Primary Mirror
to the Focal Surface
Narrative
The Hubble Space Telescope (HST) is celebrating its
25th year in space. Launched in 1990, the HST fit snugly
within the confines of the Space Shuttle’s Payload Bay,
which had a length of 18 meters, or almost 60 feet. The
HST overcame its initial flaws and went on to become
a historic telescope, thanks to the intrepid high–flying
Shuttle repair crews and the folks on the ground. Indeed,
after the fixes, the effective Focal Length of the HST lived
up to its design of a whopping 57.6 meters (189 feet)!
Wait. The Space Shuttle Payload Bay was 18 m long,
and the HST has a focal length of almost 58 m long.
That’s greater than the entire length of the Shuttle itself
at 37 m (122 ft)! So how did the HST fit insid e the Space
Shuttle?
Enter the Cassegrain Telescope, a design that allows
for a magnification factor! The HST is aRitchey–Chrétien
variant, which uses hyperbolic mirrors. A hole in the cena
ter of the Primary Mirror (M ) llows the reflected light from
1
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the smaller Secondary Mirror (M ) o pass through to the
2
Focal Surface (F) located behind the Primary Mirror.
By extending the lines made from the reflected light
a
w
bouncing off of M2 b ckwards, e can see how this magnification effect works:

The M mirror simply shortens the time needed to get to
2
position F, the Focal Surface. The Effective Focal Length
of a Cassegrain telescope (eFt) is:
eFt = Fp * q / p where,
Fp = Pimary Mirror (M ) ocal Length
r
1F
p = the system Focal Point (F) to Secondary
Mirror (M2) distance
q = the M surface to the focal point
2
m
of the M irror distance
1
So le ’s check those Effective Focal Length figures,
t
shall we?
Example
Diagram of a Cassegrain telescope which uses two reflecting mirrors.
Credit: htp://en.wikipedia.org/wiki/File:Diagram_Reflector_RitcheyChretien.svg
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For a more in-depth treatment of this high
school project by Joe Maness & Rich Holtzin
visit www.stemfortheclassroom.com.
Looking up the specifications of the HST, we find that
the Focal Length of the Primary Mirror (Fp) is 5.52 m, the
mirrors are separated by a distance (Md) of 4.91 m, and
the Focal Surface (F) is 1.5 m behind the Primary Mirror.
So,
q = Md + F
= 4.91 + 1.5 = 6.41 m