Focal Point Mystery Cache
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Difficulty:
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Terrain:
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Size: 
(small)
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When we were kids we used a magnifying glass to concentrate the sun's rays on one spot to start a fire - I'm not sure we ever succeeded, but we at least made smoke. Last night we tried an experiment in the kitchen. We turned out the lights, Bubble shone a torch at the fridge and Squeak put the magnifying glass between the torch and the 'fridge. She moved it to the right spot and yes! the torch light concentrated into one bright spot, leaving the rest of the fridge in shadow (all the light had been bent into the centre)
What if we were able to put a big magnifying glass on the Parliament House flag pole (GZ) with the centre of the lens at the tip of the pole. It would concentrate the light from the Mount Ainslie beacon (S35 16.206 E149 09.496) on to one spot. Where would that spot be? (see schematic below)
Yes, it does depend on the shape of the lens. So we've cunningly manufactured it so that it focuses exactly on the only possible point where the beam hits the earth's surface. The cache is in a small cairn, you will need to climb through the fence into the nature reserve. The altitude of the cache is 647 meters.
Additional Hints
(Decrypt)
Bar bs gur guvatf nobhg n yraf vf gung gur yvtug gung cnffrf guebhtu gur prager bs gur yraf vf abg orag ng nyy. Bayl gur yvtug gung uvgf gur pheirq cneg bs gur yraf oraqf vajneqf. Guvf zrnaf gung vs lbh jrer fvggvat ng gur cbvag jurer gur prager bs gur yraf (gur gvc bs gur Cneyvnzrag Ubhfr synt cbyr) naq gur Zg. Nvafyvr ornpba jrer va n fgenvtug yvar, lbh'q or fvggvat ba gur pnpur!