Introduction

Although the subject was already the object of a detailed document, 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 observer.

Method

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 of UV.

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".

Results

The control glass with its mask: 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.

Conclusion

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 light.
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 photochromic glasses.