An extender optical adapter presents some additional challenges to the design process. In addition to a well corrected optical system that compensates for some of the SCT aberrations, the Apus 2.5X is also manufactured with a reticule quality surface polish of 20-10 (scratch-dig). This feature reduces the amount of scattered light, which will result in an increase of contrast for subtle planetary surface features.

 

With an SCT of f/10 focal ratio, the Apus 2.5X will extend the f-number to f/25. Additional increase in the magnification will not result in an increase in resolution. At f/25 and considering a perfect optical system, Airy’s disk at 550 nm is already over 30 microns in diameter and would be sufficiently over sampled by the CCD array (7 micron pixels) to give the maximum resolution.

 

The Apus 0.36X WEB camera adapter uses a three-element design to reduce the focal ratio of an f/10 SCT to f/3.6. This is the smallest f-number that can be used with most WEB cameras without vignetting. This unit cannot be used on other types of telescopes or other focal ratios without some loss of performance.

The spherochromatic aberration (different colors coming to focus at different points along the optical axis) and field curvature that afflicts the common Schmidt Cassegrain (SCT) telescopes is difficult to eliminate entirely. The design goal for the Apus 0.36X compressor system was to eliminate lateral color completely and produce similar size blur spots for the entire field. This would result in the uniform appearance of stars throughout the entire image field. As can be seen from the geometric encircled energy diagram, 95% of the stellar profile is contained within a 5 by 5 pixel array of 7 micron pixels. In addition, the stellar profiles as indicated by the spot diagrams do not have any pronounced asymmetric patterns that would make it difficult for an accurate centroid to be computed or non-round stellar images. The design goal was achieved with the use of expensive low dispersion glass for one of the elements. Another element uses a high index crown in order to keep the surface radii large in relationship to the power of the element. See APUS SCT 0.36X performance diagram.

For comparison purposes, an Edmund Scientific TECH SPEC achromatic lens (stock number 32-496) was used as a compressor optic with a SCT. The placement of the lens was made to give the same magnification as the Apus 0.36X. As can be seen from the Edmund performance diagram, the on-axis and nearly on-axis blur spots are slightly smaller than the Apus 0.36X unit. However, the off-axis performance is considerably worse with lateral color exceptional bad. Color fringing of off-axis stars would be objectionable with this system. This type of standard optic is normally designed for best on-axis performance at the expense of off-axis correction.