There was a recent discussion and some speculation on the Meopta forum as to the type of mirrors used in the Meostars.
Dielectric mirrors for binoculars with Schmidt-Pechan prisms were introduced around the turn of the century and because they can boast reflectivity in excess of 99%, and perhaps because it is the newer technology, there is a tendency, at least amongst binocular users, to regard them as superior to metallic coatings.
Though not an amateur astronomer, I have followed recent developments and here the trend seems to be going in the opposite direction. Amateur astronomers are concerned with obtaining the highest contrast and making the most of the resolution provided by their telescopes when observing moon or planetary detail at high magnifications. Most are unwilling to accept an upside-down image when viewing on axis and settle for an upright left/right reversed image provided by a 45° erecting prism or mirror.
A few years ago one of the contributors to Cloudy Nights conducted tests on various erecting prisms and mirrors. His conclusions were that erecting prisms (with total internal reflection) gave the best detail and contrast and that silver or enhanced aluminium mirror coatings were superior to dielectric coatings. In contrast to binoculars, where the dielectric coatings are behind the glass surface, an erecting mirror has up to 140 (!!!) layers on a quartz or sittal substrate, so it's not really surprising that tolerances in the coating thickness can accumulate to degrade the flatness of the top surface. In addition, and this would also apply to binoculars, there is a lateral shift in the partial reflections from different levels as here: https://en.wikipedia.org/wiki/Dielectric_mirror#/media/File:Dielectric_mirror_diagram.svg.
Now the state of the art in erecting mirrors seems to be silver with a dielectric protective coating. Baader Planetarium have even gone so far as to put a silver coating on an erecting prism. Why they would want to do this escapes me, unless they for some reason wanted to avoid the polarization associated with total internal reflection.
The interference of polarized light from the two roof prism surfaces in binoculars results in a loss of contrast and can be compensated by phase coating. But what if the roof prism surfaces had a silver or enhanced silver coating? There would be a small sacrifice in transmission but an attendant gain in contrast as there would be little or no polarization at the reflecting surfaces.
Could it be that Meopta have taken this radical step? When checking out the phase coating on a TFT screen with a polfilter all my other monoculars, bins and scopes exhibit a colour shift across the roof edge (e.g. blue/yellow). Perhaps recent Meostars have a very effective phase coating but the tested example (2016 build) goes progressively from a clear view to an almost uniform violet.
John
Dielectric mirrors for binoculars with Schmidt-Pechan prisms were introduced around the turn of the century and because they can boast reflectivity in excess of 99%, and perhaps because it is the newer technology, there is a tendency, at least amongst binocular users, to regard them as superior to metallic coatings.
Though not an amateur astronomer, I have followed recent developments and here the trend seems to be going in the opposite direction. Amateur astronomers are concerned with obtaining the highest contrast and making the most of the resolution provided by their telescopes when observing moon or planetary detail at high magnifications. Most are unwilling to accept an upside-down image when viewing on axis and settle for an upright left/right reversed image provided by a 45° erecting prism or mirror.
A few years ago one of the contributors to Cloudy Nights conducted tests on various erecting prisms and mirrors. His conclusions were that erecting prisms (with total internal reflection) gave the best detail and contrast and that silver or enhanced aluminium mirror coatings were superior to dielectric coatings. In contrast to binoculars, where the dielectric coatings are behind the glass surface, an erecting mirror has up to 140 (!!!) layers on a quartz or sittal substrate, so it's not really surprising that tolerances in the coating thickness can accumulate to degrade the flatness of the top surface. In addition, and this would also apply to binoculars, there is a lateral shift in the partial reflections from different levels as here: https://en.wikipedia.org/wiki/Dielectric_mirror#/media/File:Dielectric_mirror_diagram.svg.
Now the state of the art in erecting mirrors seems to be silver with a dielectric protective coating. Baader Planetarium have even gone so far as to put a silver coating on an erecting prism. Why they would want to do this escapes me, unless they for some reason wanted to avoid the polarization associated with total internal reflection.
The interference of polarized light from the two roof prism surfaces in binoculars results in a loss of contrast and can be compensated by phase coating. But what if the roof prism surfaces had a silver or enhanced silver coating? There would be a small sacrifice in transmission but an attendant gain in contrast as there would be little or no polarization at the reflecting surfaces.
Could it be that Meopta have taken this radical step? When checking out the phase coating on a TFT screen with a polfilter all my other monoculars, bins and scopes exhibit a colour shift across the roof edge (e.g. blue/yellow). Perhaps recent Meostars have a very effective phase coating but the tested example (2016 build) goes progressively from a clear view to an almost uniform violet.
John
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