Starting a Fire With a Telescope

Yes, you read the title correctly.  Today I'm going to discuss how you use a telescope to start a fire.  Please, please, please do not do this experiment without supervision and/or don't allow your kids to do this without your supervision.  To start you need a telescope.  I recently took my astronomy class outside during the day to do a bit of solar observing.  We saw a few sunspots on the Sun which was super cool!  Afterwards I spoke a bit about the telescope and then did a simple demonstration showing why the solar filter on the telescope is VERY important.  The solar filter blocks 99+% of the sun's light, allowing one to view sunspots without going blind.

When the filter is removed, however, the telescope mirror collects MUCH more light.  I have an 8 inch diameter telescope where I work and if you hold you hand a few inches away from the eyepiece, the collected light from the Sun will very quickly warm your hand.  If you move your hand closer, it will quickly burn your hand leaving you with the feeling that someone just jabbed a pencil in your hand.  I then took a piece of paper and held it an inch or two from the eyepiece and the concentrated light quickly lit the paper on fire!  Next, I took a second piece of paper and burned a smiley face into it!

Okay, so maybe you have to squint a bit to see the smiley face, but it's there!  We also tested this with the finder scope on the telescope.  The finder scope is simply a smaller telescope.  This one had a diameter of about 1 inch, meaning it collected 64 times less the light of the telescope itself.  We tried burning the paper but with no luck.  You wouldn't want to observe the Sun with the finder scope as it collects enough light to seriously damage your eyes, but it wasn't enough light to burn a piece of paper.  

There you have it.  Solar observing and fires.  I'd call that a successful astronomy class!