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Old 16-12-2019, 04:48 PM
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Merlin66 (Ken)
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Join Date: Oct 2005
Location: Junortoun Vic
Posts: 8,927
Ignition of paper using lenses??

OK, we've all heard about the dead ants resulting from youthful use of magnifying glasses....
My issue at the moment (!) is trying to determine when paper when subjected to a focused solar beam (i.e. from a telescope objective) will burst into flames.

As you probably know I build and use solar telescopes...some solar observers are keen to mention that various telescopes/ filters can cause paper held at the focus to smoulder (never seems to burst into flame though??!).
Seeing this sort of thing happen puts doubt in their mind as to the safety of solar telescopes.

There's a critical ignition temperature for paper -around 246 deg C - at this temperature the paper will ignite.
Knowing the solar irradiation is 1000W/m^2 - what sized objective (this determines the input power - solar irradiation * area of the objective), what focal length (this determines the solar image area = 1/100 the focal length)
would give sufficient Heat Flux to ignite paper, and how long would it take??
Any ERF filters etc would reduce the power throughput...

I found the only "auto ignition" formula was based on work done on the ignition of wood products :
https://www.researchgate.net/publica...rnal_heat_flux

and the formula on page 38 gets bandied around as the one to use..
Unfortunately it doesn't work!!
(According to this formula a piece of paper lying in sunshine will burst into flames after 54 min!!!! -Obviously not true)
Has anyone got any creditable information/ knowledge/ documentation on the ignition of paper in a focused solar beam???

The attached spreadsheet image shows some of the factors I've been looking at.
Attached Thumbnails
Click for full-size image (Solar ignition_no DERF_01.JPG)
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