general info on prisms

[womblemaster]

I dunno if this is accurate, but it is interesting info.

`BaK4 (Barium crown) has no advantage over BK7 (Borosilicate) when used in in a situation where the incoming light beam angle is very narrow. However in wide field applications such as low power binoculars there are advantages - increasing the brightness of the edge of field while having no effect on the on-axis image. There would be no advantage to BaK4 over BK7 to any part of the image for instance in a Binoviewer because of the narrow beam angle where generally they are slow systems without wide fields.

The disadvantages of BaK4 is that it introduces more spherical aberration into the optical path than BK7. BaK4 also introduces more chromatic aberration than BK7. You may not notice this if your telescope optics are poor to begin with.

BK7 is the clearest, most defect-free optical glass available for prisms. BaK4 is close, but not quite as good. That's ok for low power applications of a typical binocular, but not for high power low contrast situations seen in telescopes. For a high powered high f# telescope, with a very narrow light beam, BK7 would be a better choice for a prism. Similar applications are found in binoviewers.`

Any views on this?

[ronin]

Never understood why everyone quotes BaK4. When building optical systems the vast majority of the components are Bk7. Have been for the last 30 years and more.

If BaK4 were better then the components would be made from that.

For our applications Bk7 has a lower dispersion so less CA is added when putting a prism in the optical path.