Spikes with Astro-Amici Prisms

I see spikes on bright objects - is my prism defective?

Concerning Amici prisms, there are two factors which can limit the use for astronomy. The first one is the optical quality, i.e. how good is the alignment of the optical surfaces. Most Amicis are made for terrestrial use, where our atmosphere limits the useable magnification to ca. 60x or 70x, anyway. So, it's fine if they don't show a double image up to ca. 100x, as such high magnifications can't be used.

Those prisms which are designed for astronomical use are designed for those magnifications which we can use for the night sky – ca. 300x or even more. This high precision makes our Astro-Amici star diagonals so expensive.

But all Amici-prisms – no matter how good they are – suffer from one design feature: There is a line going through the center of the image, where both sides of the prism surfaces com together. This may cause reflections or spikes. To some extent, this will always happen when a bright object like a star is viewed in front of a dark background like the night sky. This should be obvious only with very bright stars or planets, while dimmer stars are no problem. In this case, simply move the telescope a little bit so that the image of the bright light source isn't centered on the prism edge any longer.

To be honest, there is one way to reduce these reflections: by destroying the prism edge. Even a sharp, perfect prism edge with a width of only 1/100 mm will produce reflections. But if you polish it round, there will be no spikes! Unfortunately, in the final image in the eyepiece, there will be a streak with a width of ca. 1 mm where contrast will suffer and details will no longer be visible, because they drown in the multitude of image errors introduced by this wide "bar" cutting the image in two halfes. We decided not to sacrifice the over-all contrast by giving the prism a blunt edge – even if that means that bright objects must be placed a little bit away from the center to avoid spikes.

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