Re: SEMI-APO???

david goldman said:

Can someone explain to me what the term SEMI-APO means?.

In reality, often scopes that are termed "semi-apo" are gussied up achromats... The Orion Express/Antares Sentinel was one such scope. Some vendors sold it as a "semi-apo" some sold it as an achromat...

The WO "semi-apo" Petzval would have better color correction than a similar achromat because the focal length of the Petzval objective is greater.

Examples of "true" "semi-apo" telescopes would probably be worthwhile???

Jon

Examples of "true" "semi-apo" telescopes would probably be worthwhile???

There are only a few glass combinations that can result in true semi-apo. These glasses are called abnormal dispersion glasses because their partial dispersions lie off the Abbe Normal line. These glasses are quite a bit more expensive, they are made in small quantities, small strip widths, and generally have poor internal homogeneity (i.e. they produce astigmatism). They are almost always glasses that do not weather very well and are very sensitive to acids (such as found in atmospheric moisture).

Almost all other glasses, the vast majority in fact, lie on the Abbe Normal line and result in normal secondary chromatic aberration (also known as longitudinal color). That means with respect to the green wavelength of 550nm, the red and blue at 630nm and 480nm will be at slightly longer focus of approximately 1 part in 1800.

A semi-apo lens woudl need to have red and blue focus at approximately 1 part in 3600 to be qualified as semi-apo. By contrast a simple ED doublet, with the least expensive ED glass and simple mating element can provide anywhere from 1 part in 5000 to 1 part in 8000 color error from red to blue. The violet of course still goes quite out of focus in such a lens.

Whereas correction from red to blue can be handled by a doublet ED, and for visual use that's quite enough, the violet quickly gets out of hand in objectives much more than 4" aperture. A doublet using fluorite or Ohara FPL53 glass can correct somewhat in the violet, it will not be a fast lens, nor will the correction be really all that good for an imaging lens. This is where a triplet comes into play, where the correction for wavelengths from near IR to red, yellow, green, blue, violet and near UV can be as good as 1 part in 20,000 or better.

Rolando