Icarus750............Optical Design

Hello, I'm Michael Pankratz, thanks for stopping by!

This site discusses the optical details of the new ICARUS (ik'-uh-rus) 750....a compact, 30-inch all-spherical optical design.   Although the design is proprietary, I'm happy to offer it to any other ATM's who are interested in building the telescope for personal, non-commercial use.







I began with the following design goals in mind for my next scope project:

One design which had been in the back of my mind for quite a while is the Jones-Bird, an all-spherical design which uses an f/4 primary along with 2 sub-aperture corrector lenses, placed just ahead of the diagonal, which bring the final speed to f/6. The design performs quite well in smaller sizes, but when scaled up to the aperture I had in mind it fails in 2 of the above criteria, eyepiece height and field correction (ie. excessive levels of astigmatism and chromatic aberration).

The new design replaces the Jones-Bird's air-spaced doublet with an oil-spaced triplet, thereby improving the color correction to apochromatic levels, and adds an air-spaced doublet late in the optical train --just before the focusser -- to take care of astigmatism, coma and lateral color, as well as providing just a bit more focal amplification.

The spherical primary is f/2.133, and the EFL is 4800mm, or f/6.4. Secondary obstruction is 20%.

Since the triplet is no longer air-spaced, leaving the coma-correction for later, this design may more properly be considered a descendant of the Brixner (Sky & Telescope, Aug '66). The field corrector near the eyepiece is, as far as I'm aware, unique within this family of designs. The nearest relative might be the Schalck Newtonian (Telescope Making #31), although that design has no primary focal amplification.

The system is well corrected for all aberrations, and is diffraction-limited in green light over nearly the entire FOV of the 35mm Panoptic. Even with the above-mentioned oculars, eyepiece aberrations will dominate. Field curvature remains, as appears to be inherent in most designs containing strongly amplifying elements, but the modest radius of about 300mm -- concave toward the eye -- is not objectionable visually, requiring only 0.3 diopter accommodation by the eye at the edge of the 35mm Panoptic's field, and dropping rapidly to indistinguishable levels as higher-powered oculars are used. At any magnification, the residual off-axis aberrations will be virtually invisible to the eye, delivering tack-sharp images to the edge of the field.


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