hi everyone today's video is a bit of a
collaboration with Cody from Cody's lab
and he's made a video about an
interesting telescope of his own but
today I wanted to tell you the story
about a telescope which I visited back
in 2013 which has a very interesting
history so this is a photo that I took
in Texas at the McDonald Observatory
this is a photo of the outside of the
dome of the Harlan J Smith telescope I
spent a few nights there with an
astronomer who was helping to identify
extrasolar planets so we were doing
follow-ups to discoveries made by the
Kepler spacecraft which was out there
looking for planets going around other
stars now this is a picture from inside
the dome of the telescope itself you can
see it's quite a big one but what's
really interesting is something that I
saw one night when it was particularly
cloudy and there wasn't really much for
astronomers to do other than to kind of
sit around and wait for it to clear and
what we did was we went out into the
dome and the astronomer I was with moved
the telescope so that I could see right
down through it into the base into the
primary mirror and this is what I saw so
this is the primary mirror with several
holes here and these are indeed bullet
holes now that's a very interesting
thing to see on a telescope I haven't
seen that on any other telescope since
so in 1970 a very disgruntled member of
staff at the observatory after firing
one shot towards the supervisor fired
seven shots straight into the telescope
I guess in an effort to damage or
destroy it
the mirror though which was made out of
32 centimeter thick fused silica glass
didn't shatter it just has these bullet
holes which bear a mark of the incident
the mirror and the telescope also
survived a few blows with a hammer in I
guess an attempt to make a bit more
damage now luckily no one was hurt in
this incident and the person responsible
quickly arrested and taken away to get
some mental-health help now the
telescope in question is a cassegrain
reflecting telescope and what that means
is that it looks like this we've got a
mirror at the back in this case it is
2.7 meters wide and the light comes in
reflects off this primary mirror which
is concave up to the secondary mirror
which is convex and then is brought to a
focus down here behind the telescope
where you would put your camera or any
other instrument so the primary mirror
here is actually a doughnut shape that's
got a big hole in the middle where the
light is focused through now in some
preliminary reports from the sheriff
about the shooting incident he noted
that the telescope was completely
destroyed because there was a big hole
in the mirror so maybe it was just the
case that he wasn't so familiar with the
telescope design now to put the record
straight
the observatory came out and released a
statement which cleared up how much
damage had actually been done and I'll
read that to you so the harm suffered by
the mirror from his bullets and his
several preliminary blows with a
hammer was extraordinarily small the
damage is limited to small craters about
three to five centimeters in radius
which reduced the light collecting
efficiency by about 1% and introduced a
very small amount of diffraction
astronomical observations of all types
are essentially unimpaired by this
tragic episode the telescope resumed its
observing program the following night
producing some of the best photographs
of quasar fields so far obtained with
this instrument in its first year of use
so these bullet holes which were back
here on this primary mirror don't cause
any black spots on the image in much the
same way that any specks of dust on
either of the mirrors or any other
obstructions inside the telescope
including this mirror here or the big
hole in the middle of this one they don't
show up either even with an obstruction
the light that is still here unblocked
is enough to give us the full picture it
is true that any obstruction will reduce
the total amount of light coming into
the telescope and making its way to your
instrument so the image will be a little
bit dimmer but in this case these minor
you know obstructions have caused such a
low loss of you know light-collecting
efficiency that it doesn't really matter
that much and this telescope that's
still there is still able to take
parallel rays of light and bring them to
a focus the telescope is focused out at
infinity so anything that was so close
to either of the mirrors would be so
blurred and out of focus that you just
wouldn't be able to see it it would
become a problem if say one of the
mirrors was completely covered in dust
because then you'd have an issue with
the dust starting to scatter a lot of
the light but as it were the astronomers
at McDonald just took the bullet holes
that were left after the incident and
blacked out some of the edges of the
holes so that you didn't have so much
light scattering and went on to use the
telescope for research grade astronomy
for nearly ten years after the incident
the telescope reflected a laser off of
mirrors left on the moon by Apollo
astronauts and this helps to refine
measurements of the distance to the moon
and also provide a test of Einstein's
theory of general relativity this
telescope also went on to study a lot of
the planets in our solar system and
helped with spacecraft that were
exploring those planets the telescope
has also looked at the composition of
stars, the motions of galaxies and as I
mentioned before the search for
extrasolar planets it continues to be
used on every clear night of the year
and has even just celebrated its 50th
birthday now Cody has made his own video
about a telescope this is kind of the
inverse case of mine he's taken his
telescope and put a cap over the top and
made a few holes in that cap so instead
of having our view possibly obstructed
by a few
bullet holes this time we only have a few
holes with which to see an image so go
and check out Cody's video if you want
to see a very practical demo of some of
these ideas with telescope optics and
thank you for watching
