Bausch & Lomb eyepiece ID

Can anybody tell me what the letters A S P on my oldish B & L 30 mm focal length eyepiece might mean? It is 1 1/4" diameter and is not a Huygenian (sp?) type, as it will focus an image on an an external surface. The image quality , when used in a telescope, appears to be just so-so. It also has at least some form of anti-reflection coating.

Thanks,
Dave K.

I always thought it meant ASsymmetric (meaning real) Plossl - but those who have actually taken it apart say differently...

David Kernberger said:

Can anybody tell me what the letters A S P on my oldish B & L 30 mm focal length eyepiece might mean? It is 1 1/4" diameter and is not a Huygenian (sp?) type, as it will focus an image on an an external surface. The image quality , when used in a telescope, appears to be just so-so. It also has at least some form of anti-reflection coating.

Thanks,
Dave K.

Plossls are made with two achromats back to back. They can be two different achomat lenses with the eye lens smaller than the field lens, or they can be two achros of the same design and even of the same diameter. It makes almost no difference to the on-axis or off-axis performance. There are advantages in cost and manufacturing for each of the above styles.

A classical Orthoscopic normally consists of a triplet field lens and singlet eye lens. Orthoscopic means that the field has no displacement distortion and thus a scale can be placed at the field stop and the divisions of the scale will look idential everywhere in the field.

Both Plossls and Orthos are inherently othoscopic and have very similar off-axis performance - both have off-axis astigmatism when designed for a flat field. They can be designed to have anywhere from 30 degree to 50 degree apparent fields with good supression of astigmatism and field curvature depending on the scope's focal ratio.

Generally the design should be matched to the focal ratio of the telescope that they are being used with in order to get the most performance. The longer the focal ratio, the larger can be the apparent field. Generic Plossls and Orthos are designed with some middling f-ratio in mind and will work best with F8 or longer scopes.

Rolando