palgyulai2009
Mitglied
Beat,
you are right! My mistake! I did NOT make specific calculations now, just remembered that some time ago I calculated the refraction on an uncoated lens surface and I received 4.6% but and the refraction index of the glass was higher (optical glass, NOT fluorite). You are absolutely right, CaF2 has lower refraction index, so, refraction (per surface) must be lower ... but still, I believe my main idea is right: there is SIGNIFICANT refraction on the uncoated surfaces.
It is true that fluorite has lower interior light scatter than glass, but this problem is also very small in the case of fluorite glass (e.g. the light of a green laser crosses Schott FK-51 without practically any light scatter, while within normal glass, the beam is well visible) and even for "normal" glasses (like ZK7) this problem is minimal. It was years ago when I looked after this in the glass catalogues (hope my memory does not cheat me again
but I remember I declared the problem as "negligible" these days. If my memory is correct, transmission of normal wavelengths is higher than 99.5% (per inch of glass) for practically all glasses used in modern APO lenses, so, this is not a problem in practice (compared to e.g. the reflection on surfaces even using multi coatings, because no lens is thicker in practice than 5cm and this extreme thickness still means 1% loss only).
Only in the case of oil spaced lenses with modern multi coatings can we compare the internal glass loss to the reflection loss. Just an example: modern multi coatings provide about 0.3% light reflection per surface, so, an oil spaced lens (two air-to-glass surfaces) has about 0,6% loss on the front- and back surface. As the oil has very similar refraction ratio as the glass(es) used, the internal surfaces give practically zero reflection, but let's over-estimate it by 0,2% per internal surface (pair). There are two internal surface pairs, so, the total reflection loss of the lens is 0,3%+0,3%+0,2%+0,2%= 1%. Compared to the loss of 6.3% light on the fluorite member of the FC-100 this is significantly better.
But anyway, the refraction on the surfaces is not a real problem for modern lenses, there are other problems that are much more prominent, just I mentioned the uncoated surfaces of the FCT-100 to draw our attention to the fact that technology is advancing quickly, so, a scope that was considered the best a decade ago, now has significant drawbacks compared to modern optics. Even then, the FCT-100 might be an excellent telescope ... just in the technological meaning it is outdated now.
Pal
you are right! My mistake! I did NOT make specific calculations now, just remembered that some time ago I calculated the refraction on an uncoated lens surface and I received 4.6% but and the refraction index of the glass was higher (optical glass, NOT fluorite). You are absolutely right, CaF2 has lower refraction index, so, refraction (per surface) must be lower ... but still, I believe my main idea is right: there is SIGNIFICANT refraction on the uncoated surfaces.
It is true that fluorite has lower interior light scatter than glass, but this problem is also very small in the case of fluorite glass (e.g. the light of a green laser crosses Schott FK-51 without practically any light scatter, while within normal glass, the beam is well visible) and even for "normal" glasses (like ZK7) this problem is minimal. It was years ago when I looked after this in the glass catalogues (hope my memory does not cheat me again
but I remember I declared the problem as "negligible" these days. If my memory is correct, transmission of normal wavelengths is higher than 99.5% (per inch of glass) for practically all glasses used in modern APO lenses, so, this is not a problem in practice (compared to e.g. the reflection on surfaces even using multi coatings, because no lens is thicker in practice than 5cm and this extreme thickness still means 1% loss only). Only in the case of oil spaced lenses with modern multi coatings can we compare the internal glass loss to the reflection loss. Just an example: modern multi coatings provide about 0.3% light reflection per surface, so, an oil spaced lens (two air-to-glass surfaces) has about 0,6% loss on the front- and back surface. As the oil has very similar refraction ratio as the glass(es) used, the internal surfaces give practically zero reflection, but let's over-estimate it by 0,2% per internal surface (pair). There are two internal surface pairs, so, the total reflection loss of the lens is 0,3%+0,3%+0,2%+0,2%= 1%. Compared to the loss of 6.3% light on the fluorite member of the FC-100 this is significantly better.
But anyway, the refraction on the surfaces is not a real problem for modern lenses, there are other problems that are much more prominent, just I mentioned the uncoated surfaces of the FCT-100 to draw our attention to the fact that technology is advancing quickly, so, a scope that was considered the best a decade ago, now has significant drawbacks compared to modern optics. Even then, the FCT-100 might be an excellent telescope ... just in the technological meaning it is outdated now.
Pal
