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