Although the subject was already the object of a detailed
the capacity of astrosolar to filter the UV and IR radiations
is frequently discussed.
I thus realized a small experiment concerning the near
UV to estimate what can reach up to the eye of the
Everybody knows the photochromic glasses, these
lenses which darken under the influence of the solar light. This reaction
is provoked by the near UV part of the spectrum (300 - 400
nm) which is exactly the one which is questioned for the filter. It
does not there need to explain the photochromic glasses right
in the sun light to obtain a reaction; They darken even in diffuse
light - for example in a well enlightened room or
under a parasol - in a brilliant atmosphere where we can stay for a
long time without solar glasses and without lack of comfort.
Having glasses of this type, I thus used it as detector
The test was made with my Newtonian telescope
equipped with a density 5 astrosolar filter. The useful aperture
was 185 mm and the eyepiece - a 12.4 mm f.l.
- allows to get a small diameter exit pupil while keeping
the light of all the solar disk, thus a maximal concentration of the
beam. One of the photosensitive glasses is unsettled
then fixed at the level of exit pupil of the eyepiece. The telescope
is pointed towards the Sun and the mount follows without problem.
A dark and opaque tissue recovers all the eyepiece and eyepiece-holder
arrea. The telescope followed the Sun during at least
10 mn. The height of the Sun was about 40°.
The other glass was partially covered with a black mask and was placed
on a table, in the shadow of a parasol. It served as comparison
with the "test glass".
The control glass with
some adhesive black lightweight felt serves as mask. A small
aperture allows to have a "spotlight" more in touch
with the dimension of exit pupil of the telecope.
The control glass was "unmasked". The
test glass was removed from the eyepiece, having verified that
the Sun was always well centred. During this time (approximately
30s) the partof glasses not exposed already began to darken.
We can notice however that the spot left by the beam going
out of the telescope is lighter than that resulting from an
exposure in the indirect ambient light.
The box in the low left corner is a simple adjustment of contrast and brightness.
The quantity of near UV radiation
going out of a telescope equipped with a full aperture astrosolar
D=5 filter seems to be at very low level, in any cases - for this
optical configuration - sharply lower than that of the ambient diffuse
This is only an approximative experiment but strengthens the idea that UV crossing the astrosolar D5 do not constitute a threat for eyes during solar observations.
This test can be easily reproduced if you have