The Meade UHTC Group

Technologies recently developed at the Meade Irvine coatings facility, however, including installation of some of the largest and most advanced vacuum coating instrumentation currently available, have permitted the vacuum-deposition of a series of exotic optical coatings precisely tuned to optimize the visual, photographic, and CCD imaging performance of Meade telescopes. These specialized, and extremely advantageous, coatings are offered here as the Meade Ultra-High Transmission Coatings (UHTC) group, a coatings group available optionally on many Meade telescope models.

In Meade catadioptric, or mirror-lens, telescopes (including the ETX-90EC, ETX-105EC, and ETX-125EC; LX10, LX90, and LX200GPS Schmidt-Cassegrains; and LXD55-Series Schmidt-Newtonians) before incoming light is brought to a focus, it passes through, or is reflected by, four optical surfaces: the front surface of the correcting lens, the rear surface of the correcting lens, the primary mirror, and the secondary mirror. Each of these four surfaces results in some loss of light, with the level of loss being dependent on the chemistry of each surface's optical coatings and on the wavelength of light. (Standard aluminum mirror coatings, for example, typically have their highest reflectivity in the yellow region of the visual spectrum, at a wavelength of about 580nm.).

Mirror Coatings: Meade ETX, Schmidt-Cassegrain, and Schmidt-Newtonian telescopes equipped with the Ultra-High Transmission Coatings group include primary and secondary mirrors coated with aluminum enhanced with a complex stack of multi-layer coatings of titanium dioxide (TiO2) and silicon dioxide (SiO2). The thickness of each coating layer precisely controlled to within +/-1% of optimal thickness. The result is a dramatic increase in mirror reflectivity across the entire visible spectrum; at the important hydrogen-alpha wavelength of 656nm. — the predominant wavelength of emission nebulae — reflectivity is increased from 89% to over 97%.

Correcting Lens Coatings: Meade telescopes ordered with the UHTC group include, in addition, an exotic and tightly-controlled series of coatings on both sides of the correcting lens or correcting plate, coatings which include multiple layers of aluminum oxide (Al2O3), titanium dioxide (TiO2), and magnesium fluoride (MgF2). Per-surface light transmission of the correcting lens is thereby increased at the yellow wavelength of 580nm., for example, to 99.8%, versus a per-surface transmission of 98.7% for the standard coating.

The importance of the UHTC group becomes apparent when comparing total telescope light transmission, or throughput, caused by the multiplier, or compounding, effect of the four optical surfaces. With each optical surface contributing significantly to telescope light throughput, the effect of all four surfaces combined is indeed dramatic, as demonstrated by the graphs on the facing page, as well as by the table of the brightest nebular emission lines. At the H-alpha wavelength of 656nm., total transmission increases from 77% to 93%, an increase of 93/77 or 21% at all three nitrogen-III and sulfur-II wavelengths of 655nm. and 673nm.— prominent lines in certain galactic nuclei and in supernova remnanats such as the Crab Nebula— transmission increases by 21%; ; at the helium wavelengths of 588nm. and 469nm. — strong emission lines in hot planetary nebulae — total telescope transmission increases by 18% and 19%, respectively; at the two nitrogen II lines of 655nm. and 658nm. and at the sulfur II line of 673nm., transmission is increased by 21%. Averaged over the entire visible spectrum (450nm. to 700nm.), total light transmission to the telescope focus increases by about 20%.

Effects on CCD Imaging: While the human eye loses sensitivity to light beyond wavelengths of about 700nm., CCD imaging chips remain sensitive to about 750nm. and longer, wavelengths at which the reflectivity of an aluminum coating is near its lowpoint. Importantly, however, the UHTC's total light transmission at 750nm. is 83%, vs. 72% for standard coatings, an increase of 83/72, or 15%.