Takahashi has been the acknowledged leader in ultra-premium apochromatic optics for almost three decades. The new TOA and TSA Series of triplet refractor optical systems keep up that proud tradition.
Apochromatic means "free from spurious color" - a design that drastically reduces the faint violet halos of out-of-focus light that you see around the planets, the limb of the Moon, and all the bright stars in an ordinary achromatic (crown and flint glass) refractor. Orthoscopic means "visually correct," and is most commonly is associated with an eyepiece design widely regarded as having the most accurate and aberration-free images available.
All Takahashi TOA and TSA Series triplet refractors are true apochromatic optical systems, with virtually perfect correction of spurious violet color. Their colors are highly saturated, full of contrast, and true to life. They combine these vanishingly low levels of spurious color with equally low levels of optical distortion. Accordingly, they are called "Ortho-Apochromats" to distinguish them from ordinary apochromatic scopes. The term embodies their unrivalled high level of optical performance in both color correction and freedom from aberrations. It's an apt term that is well deserved.
The Takahashi ortho-apochromatic refractors use a newly developed lens design that uses three air-spaced lenses in three groups. A low dispersion crown glass element is positioned between two FPL-53 ED (Extra-low Dispersion) glass elements to produce images of very high quality . All lens surfaces are fully multicoated with state-of-the-art antireflection materials for maximum light transmission. Takahashi color correction and contrast equals, or exceeds, that of other triplet lens systems, regardless of cost or brand name, even costly oil-spaced triplet systems. Light loss is only about 0.5% at each multicoated lens surface in the Takahashi air-spaced system, versus about 2% at each surface in an oil-spaced triplet. Maintenance is less than oil-spaced designs, since there is no oil to potentially leak or become cloudy with age.
The simple, yet sophisticated, lens cells provide good stability for the optical system during the rigors of transport. The lens cells are fully collimatable for peak optical performance by using a simple optional Cheshire-type collimating eyepiece and the locking collimating screws on the lens cell. The lens cell design, combined with the tight spacing between the lens elements, allows the optics to quickly reach thermal equilibrium with changing ambient temperatures during the course of an evening's observing.
The Strehl ratio of a telescope is a numerical value that represents the percentage of the light of a star's image that actually falls into the Airy disk, compared to the theoretical maximum possible. A Strehl ratio of 0.95 is within 95% of perfection and is generally considered excellent. It equates to a 1/8th wave system accuracy. A Strehl ratio of 0.978 equates to a 1/12th wave accuracy. The Strehl ratio of the Takahashi triplet design is 0.992. This means that the Takahashi TOA and TSA objectives are within 99.2% of perfection. This compares with a Strehl ratio of 0.946 for a best-selling fluorite doublet system that has long been considered one of the very best telescopes available.
Previous apochromatic systems were optimized photographically for the state-of-the-art imaging media available at the time - the silver emulsion of Kodak Technical Pan 35mm film (TP-2415) and the small array/large pixel CCD sensors (less blue-sensitive than current models) that were just coming available to the amateur astronomer. The small residual blue halo seen around bright stars at high powers and off-axis in even the best apo systems of the time was not an issue.
Today, however, the imaging landscape has changed. Film imaging is becoming a dying art. Technical Pan film is no longer made. CCD sensors with larger arrays of smaller pixels demand tighter stellar images, both on and off-axis, to provide accurate and realistic images. The increased blue sensitivity of modern CCD sensors likewise demands a drastic reduction in the tiny residual blue halo around brighter stars that may not be visible to the eye, but glares like a searchlight onto the blue-sensitive CCD pixels.
The Takahashi triplets reduce the residual deviation from a flat line response over the blue to green portion of the visible spectrum of previous apo designs (even fluorite systems) by a third. The maximum deviation from all colors coming to a focus in precisely the same plane is no more than +/- 0.01mm from the blue end of the spectrum (436nm) to the H-alpha line at 656nm. The violet halo of chromatic aberration vanishes, and the tiny residual blue halation around bright stars at high powers essentially disappears. Stellar images are tight, with stars in the 12~20µm range, even at the very edges of the fully-illuminated image circle. CCD images are crisp and realistic, and visual observing is unparalleled in its clarity. Quite simply put, the Takahashi TOA and TSA optics have no equal.