NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Filter glass
From: John Rae
Date: 2005 Nov 10, 17:51 -0800
From: John Rae
Date: 2005 Nov 10, 17:51 -0800
Our fabricating shop (real boilermakers, for you
sports fans) uses mostly density 11 welding filters. But almost
nothing can be seen through this grade with normal light. So I have tried a grade 5 density filter, that I can actually
see through with outside light. They cost about $1.18 each in bulk.
At this price, and with the optical quality they seem to exhibit, this is a
bargain.
These filter glasses are 2 inches by 4.25
inches by 3 mm thick. ( a mixture of imperial and metric
dimensions!) The specified light transmission for grade 5 is stated to be
nominal 1.93%. In addition to protecting against light and heat, they are
also designed to protect against impact from flying chips, etc. As a
result of this, I have not yet learned how to cut these filter glasses using an
normal glass cutter, as they are hard to scratch, and fracture at odd
angles. Next move to try a spectacle grinding shop. Has anyone
suggestions about cutting this glass?
So my next project is to make a Bris device
from three full size filters (two clear and one number
5). This (50 x 108mm) size will certainly fully cover my 7 x 35
objective lens. These binoculars claim to have an 8 degree field of
view. So while looking at the horizon, as long as the sun is higher than
about 5 or 6 degrees, it is extremely easy not to even glimpse the direct
sun. Therefore, I was happy enough to use my sunglasses while looking at
the reflected images of the weak sun through clear glass. (But I wouldn't
do this in summer, or concentrate long enough to see tangency.) Even so,
after testing the grade 5 density, I might change to another density
filter.
Regrettably, I won't see a noon sun elevation of
even 40 degrees until about the Ides of March. So the Bris device should
be OK for those of us in northern latitudes.
