Technical Tid-Bit #3 – Welder’s Glass

     By Aaron Clevenson, AL-Cor

 

By now I am sure that all of you have heard, but it bears repeating…  The only dangerous activity that astronomers participate in is solar observation.  With solar eclipses and the recent Venus Transit it is critical that solar observations be done in a way that protects our eyes.  Looking at the sun can quickly and permanently damage your eyes.  It has happened to some rather famous astronomers.  Looking at the sun through unprotected binoculars or a telescope will permanently damage your eyesight IMMEDIATELY.  Don’t do it.

 

Okay.  So you want to observe one of these solar events.  How can you do it safely?  Well for a telescope, you should buy a full aperture professional solar filter, preferably made of coated glass.  But today, we want to talk about observing with no magnification.  You can buy a pair of “eclipse glasses” or you can use #15 Welder’s Glass.  Well I went out to buy some, and #15 is very hard to find.  The people at the welding shops did not understand how you can combine two or more pieces to still get a safe value.  The astronomy literature says you can’t do it, but you can do it if you understand the formula behind the glass.  You can NOT just add the numbers.

 

First the short form of the solution, for those not mathematically inclined.  What you need to do is buy two pieces that total #16, or three that total #17.  For every additional piece, add 1.

 

#15 Welder’s Glass blocks 99.9999 % of the light.  This also means it only lets 0.0001 % of the light through.  This is the level of transmission (T) that is quite safe.  So this is the level we want.  To calculate the Shade Numbers (SN) use this formula:

 

SN = 1 + ( 7 / 3 ) *  (-log ( T ))

 

If we start with the SN values, then the formula is:

 

T = 10 ^{( - ( SN – 1 ) * 3 / 7 )}

 

If you do the entire math, you will see that two pieces of glass of SN = 8 and 8 equal the same T value as one piece of SN = 15, or three pieces of SN = 5, 6, and 6.

 

Have fun, but be safe.  Next month – Telescope Solar Filters.