"André P." wrote in message . ..
Hi all !

Since I have only Newton type télecope, I don't have experience about
refractors. Here my question:

Witch is the best between prism diagonal and miror diagonal and why ?


Thanks and clear sky !

A prism diagonal works by with two refracting surfaces and one surface
where a total internal reflection takes place. A refracting surface has
1/4 the wavefront damage than a reflecting surface from the same surface
defect (with typical glass n=1.5). So a prism diagonal has 1.5 times
the surface damage as a mirror diagonal, assuming 3 surfaces of equal
quality. A mirror only has one surface that is optically accurate.

The total internal reflection reflects more light than all be the best
exotic coatings. and glass absorbs light. The net result is a prism
diagonal can deliver ~95% light throughput. A typical mirror diagonal
will reflect 88%, a diagonal with an enhanced aluminum coating can
reflect 96%. Exotic Bragg dielectric reflectors can reflect 98%.
A prism secondary can last forever, a mirror will last 5-20 years
before recoating becomes necessary (assuming scope is used regularly).

Refractor objectives are color corrected for no glass in the optical
path. A prism diagonal inserts a considerable chunk of glass into the
optical path 1.5"-2.2". This glass-path can add astigmatism due to the
converging light cone from the objective. Binocular objectives are
corrected for the prisms in the optical path, telescope objectives
are not.