Any optical imperfections, such as surface roughness and zonal errors on the mirrors, adds to the problem. We tested a commercial scope where the central obstruction, optical errors and surface roughness were large enough to cause the first diffraction ring and central Airy disc to have almost equal brightness (with a 35% obstruction, theoretically there should be at least a 4 to 1 difference). Even so, this scope "tested" very well on the star test - it had quite similar inside and outside Fresnel patterns and might be judged textbook perfect by the star test. Yet it was a very poor performer on all but the most steady of nights, when the seeing was essentially perfect. The slightest motion in the atmosphere would cause cotton ball stars. This is one reason I have not been a major fan of the "star test" to evaluate the actual performance of a telescope. The only unbiased way to measure an optic is with interferometry, with an MTF (modulation transfer function) test, or with a PSF (point spread function) test, which measures the relative strength of the Airy disc vs. the diffraction rings with the image IN FOCUS.

The 10" Mak-Cass optics are fabricated by skilled opticians of the Aries Instrument Company in the Ukraine, in close co-operation with the Crimean Astrophysical Observatory. The optics are hand figured to 1/10 wave P-V using a green light interferometer in double-pass mode. We supply the precision-annealed blanks of Corning fused silica for the mirrors, and solid blanks of high-quality Ohara BK7 for the correctors. That way, there is no question of internal homogeneity, striae or strain. They are expensive, but they free the optician to concentrate on making very accurate and smooth curves on the optical surfaces without having to compensate for local discontinuities in the glass, or worrying about the mirror warping during polishing. Fused silica was chosen both for its high stability during polishing and figuring, and for its thermal performance under actual night-sky conditions. The end result is an optical system that does what it’s supposed to do - produce unparalleled views of the planets and deep-sky objects.

Mechanical Design and Execution

Mirror cell being machined on the Mori Seiki turning center

Our tube assemblies were designed right from the start with trouble-free precision as the major goal. This means that collimation is set at the factory and does not change for the life of the instrument. To achieve this, all optics are accurately centered, including the mirror backs. All mounting components are made from solid billets - no fragile die castings anywhere! The critical mirror cell starts out as a solid 12" diameter aluminum slug weighing in excess of 30 lb. It is then precisely machined to very tight tolerances on our Mori-Seiki SL300 turning center. The finished mirror cell with its ventilation cutouts weighs a mere 2 lb. The other 28 lb. in the form of chips is, of course, recycled. Similar care is taken with the corrector cell and other mounting components. The result is a scope you can count on to produce consistently excellent images any time, any place.

Price: $9,800.00 USD

Digital Images

These are some of the first images taken in December with the 10" Mak using a Starlight Express HX516 Camera. Images by R. Christen.