TSA-120 4.7" F/7.5 Ortho-Apochromat triplet refractor OTA

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Takahashi triplet lens
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.

This Takahashi refractor optical tube has:

• 120mm (4.72”) f/7.5 Ortho-Apochromat ED triplet optics
• 2.7” rack and pinion focuser with 1.25” eyepiece adapter
• retractable dew shield
• 227.5mm of back focus for CCD and film imaging
• state-of-the-art optical performance, exceeding even fluorite scopes
• 5-year warranty

    The Takahashi TSA-120 refractor provides exceptionally high optical and mechanical quality for the serious visual observer and astrophotographer who needs portable optics. The TSA-120 is exceptional for observing and imaging within the solar system. Its images of subtle lunar and planetary details are sharp, with realistic and highly saturated color. They are free from chromatic aberration’s violet haze of spurious color, thanks to the scope’s ED triplet optics. The latest broadband antireflection multicoatings and numerous knife-edge internal baffles assure the maximum image contrast possible.
This 4.72” Takahashi has excellent light transmission and diffraction-free true color images that make it superb for deep space observing and imaging, as well. Binary stars and globular star clusters are particularly well-resolved and vivid, with the contrasting colors of many binary systems showing nicely. The brighter nebulas and galaxies stand out against a very dark sky background.
The f/7.5 focal ratio and 900mm focal length of the TSA-120 are long enough, using a Takahashi 2.8mm ED eyepiece to provide 321x, for high magnification observations of the Moon and planets, yet it will also produce a huge 2.72° field at 18x, using a 2” Takahashi 50mm eyepiece.

This Telescope’s Optical and Mechanical Systems . . .

  • Apochromatic triplet ED refractor optics: 4.72” (120mm) aperture, 900mm focal length, f/7.5 Ortho-Apochromatic triplet lens using an FPL-53 ED (Extra-low Dispersion) glass element between two crown glass elements for images that are free from spurious color (chromatic aberration). For more details, click on the “Takahashi triplet lens” icon above.

  • Multicoated optics: Fully coated on all surfaces with multiple layers of broadband antireflection materials for high light transmission and good contrast.

  • Retractable dew shield: Slows the formation of dew on the lens to extend your undisturbed observing time. Also improves visual and photographic contrast by shielding the lens from off-axis ambient light (the neighbor’s yard light, moonlight, etc.) For transport, the retractable lens shade keeps the overall length of the optical tube to a very manageable 28” when it is retracted and the visual extension tube removed.

  • Rack and pinion focuser: 2.7” focuser, with a 2” eyepiece/accessory holder that threads into the 2.7” focuser drawtube. A 1.25” compression ring eyepiece/star diagonal holder is also supplied. The non-marring soft nylon compression ring of the 1.25” eyepiece holder won’t scratch the barrels of your accessories or star diagonal, as ordinary thumbscrews can. Dual focusing knobs provide precise image control with either hand. The large focus knobs are easy to operate, even while wearing gloves or mittens in cold weather. A large thumbscrew locks the focuser tube in place without image shift for photography.
    A useful photographic accessory would be the Takahashi #CAA102 camera angle adjuster, shown below. This attaches to the 2.7” focuser drawtube in place of the 2” eyepiece holder. It allows the photographic accessory train to be rotated to the most appropriate angle to frame the object being photographed (a landscape format, portrait, or any angle in between) without having to loosen the camera adapter and perhaps lose the correct focus. Once the camera is oriented correctly, a large knob on the side of the adjuster can be tightened to hold the camera in place during the exposure. It is also very handy for visual applications to position the star diagonal or binoviewer at the most comfortable viewing angle.

  • Finderscope mounting point: No finderscope is supplied when this optical tube is bought alone. A flat boss with two mounting bolt holes is provided on the top of the focuser body for mounting an optional Takahashi 7x50mm 6.2° field finder and bracket finderscope.

  • Long back focus: Back focus is a very good 227.5mm, allowing the stacking of a wide variety of CCD cameras, color filter wheels, electric focusers, camera angle adjusters, and film cameras.

  • Optional photographic accessories: Two accessories are available to provide a variety of image scales and sky coverage: a field flattener and a focal reducer. The field flattener (#TOA130FF) reduces the focal ratio slightly (to f/7.3), but gives pinpoint stars to the very edges of a 40mm (2.6°) image circle for virtually unvignetted 35mm/DSLR/large format CCD images.
    The focal reducer (#TOA130R) effectively cuts the focal length to 672mm and the focal ratio to f/5.6. The photographic field is now 3.4°, spread over a 40mm image circle for virtually unvignetted 35mm/DSLR/large format CCD images. A variety of photographic coupling attachments and eyepiece projection photography adapters are also available.

  • Mounting rings: No mounting rings are supplied for the 125mm diameter 15.4 pound optical tube. Several options are available. One is the Takahashi clamshell-type tube holder (#120TH) shown below that is designed specifically for mounting the TSA-120 on a Takahashi EM-11 or larger equatorial mount. The second option is the Takahashi double ring tube holder with bridge (#120DTH) shown below. It can be mounted on a non-Takahashi mount, such as a Celestron CGEM or CGE Pro or Losmandy GM-8 or G-11, by using an optional dovetail plate. This double ring tube holder can also be used to mount the TSA-120 on a Takahashi EM-11 or larger equatorial mount by adding optional Takahashi #TMP106 double ring tube holder mounting plate. A third option is a pair of Parallax split mounting rings (#PFS106), also shown below, for mounting on a non-Takahashi mount, using an optional dovetail plate.

The image above shows the scope with a 7 x 50mm finderscope, mounting bracket, and eyepiece. These accessories are not provided as standard equipment with the optical tube, but are available as optional accessories.

Highest Useful Magnification:
This is the highest visual power a telescope can achieve before the image becomes too dim for useful observing (generally at about 50x to 60x per inch of telescope aperture). However, this power is very often unreachable due to turbulence in our atmosphere that makes the image too blurry and unstable to see any detail.

On nights of less-than-perfect seeing, medium to low power planetary, binary star, and globular cluster observing (at 25x to 30x per inch of aperture or less) is usually more enjoyable than fruitlessly attempting to push a telescope's magnification to its theoretical limits. Very high powers are generally best reserved for planetary observations and binary star splitting.

Small aperture telescopes can usually use more power per inch of aperture on any given night than larger telescopes, as they look through a smaller column of air and see less of the turbulence in our atmosphere. While some observers use up to 100x per inch of refractor aperture on Mars and Jupiter, the actual number of minutes they spend observing at such powers is small in relation to the number of hours they spend waiting for the atmosphere to stabilize enough for them to use such very high powers.
Focal Length:
This is the length of the effective optical path of a telescopeor eyepiece (the distance from the main mirror or lens where the lightis gathered to the point where the prime focus image is formed). Focallength is typically expressed in millimeters.

The longer the focallength, the higher the magnification and the narrower the field of viewwith any given eyepiece. The shorter the focal length, the lower themagnification and the wider the field of view with the same eyepiece.

Focal Ratio:
This is the ‘speed’ of a telescope’s optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera.

Fast f/4 to f/5 focal ratios are generally best for lower power wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to higher power lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either.

An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio – so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.

This is the ability of a telescope to separate closely-spaced binary stars into two distinct objects, measured in seconds of arc. One arc second equals 1/3600th of a degree and is about the width of a 25-cent coin at a distance of three miles! In essence, resolution is a measure of how much detail a telescope can reveal. The resolution values on our website are derived using the Dawes’ limit formula.

Dawes’ limit only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as the planets. For example, Cassini’s Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5” telescope – which has a Dawes’ limit of 1.8 arc seconds!

The ability of a telescope to resolve to Dawes’ limit is usually much more affected by seeing conditions, by the difference in brightness between the binary star components, and by the observer’s visual acuity, than it is by the optical quality of the telescope.

0.97 arc seconds
Visual Limiting Magnitude:
This is the magnitude (or brightness) of the faintest star that can be seen with a telescope. The larger the number, the fainter the star that can be seen. An approximate formula for determining the visual limiting magnitude of a telescope is 7.5 + 5 log aperture (in cm).

This is the formula that we use with all of the telescopes we carry, so that our published specs will be consistent from aperture to aperture, from manufacturer to manufacturer. Some telescope makers may use other unspecified methods to determine the limiting magnitude, so their published figures may differ from ours.

Keep in mind that this formula does not take into account light loss within the scope, seeing conditions, the observer’s age (visual performance decreases as we get older), the telescope’s age (the reflectivity of telescope mirrors decreases as they get older), etc. The limiting magnitudes specified by manufacturers for their telescopes assume very dark skies, trained observers, and excellent atmospheric transparency – and are therefore rarely obtainable under average observing conditions. The photographic limiting magnitude is always greater than the visual (typically by two magnitudes).

This is the diameter of the light-gathering main mirror or objective lens of a telescope. In general, the larger the aperture, the better the resolution and the fainter the objects you can see.
The weight of this product.
15.4 lbs.
Based on Astronomy magazine’s telescope "report cards", scopes of this size and type generally perform as follows . . .
Terrestrial Observation:
Observing terrestrial objects (nature studies, birding, etc.) is usually possible only with refractor and catadioptric telescopes, and convenient only when the scope is on an altazimuth mount or photo tripod. Most reflectors cannot be used for terrestrial observing. Scopes with apertures under 5" to 6" are generally most useful for terrestrial observing due to atmospheric conditions (heat waves and mirage, dust, haze, etc.) that degrade the image quality in larger scopes. 
Lunar Observation:
Visual observation of the Moon is possible with any telescope. Larger aperture scopes will provide more detail than smaller scopes, thereby getting a higher score in this category, but may require an eyepiece filter to cut down the greater glare from the Moon's sunlit surface so small details can be seen more easily. Lunar observing is more rewarding when the Moon is waxing or waning as the changing sun angle casts constantly varying shadows to reveal craters and surface features by the hundreds.  
Planetary Observation:
Very Good
Binary and Star Cluster Observation:
Very Good
Galaxy and Nebula Observation:
Terrestrial Photography:
Photographing terrestrial objects (wildlife, scenery, etc.) is usually possible only with refractor and catadioptric telescopes, and convenient only when the scope is on an altazimuth mount or photo tripod. Most reflectors cannot be used for terrestrial photography. Scopes with focal ratios of f/10 and faster and apertures under 5" to 6" are generally the most useful for terrestrial photography due to atmospheric conditions (heat waves and mirage, dust, haze, etc.) that degrade the image quality in larger scopes.
Lunar Photography:
Photography of the Moon is possible with virtually any telescope, using a 35mm camera, DSLR, or CCD-based webcam (planetary imager). While an equatorial mount with a motor drive is not strictly essential, as the exposure times will be very short, such a mount would be helpful to improve image sharpness, particularly with webcam-type cameras that take a series of exposures over time and stack them together. Reflectors may require a Barlow lens to let the camera reach focus. 
Planetary Photography:
Star Cluster / Nebula / Galaxy Photography:
5 years
Reviews from Cloudy Nights (www.cloudynights.com)
These reviews have been written by astronomers just like you and posted on the Cloudy Nights astronomy forums . . .
Takahashi TSA-120

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General Accessories
Finderscopes (2)
7 X 50mm straight-through finder
by Takahashi
Mounting bracket for 7 x 50mm Takahashi finderscope
by Takahashi
Tube Rings (1)
Takahashi FSQ-106ED (not older FSQ-106) and TSA-120 rings, 125mm ID, pair
by Parallax
Photographic Accessories
Camera Adapters (1)
Camera angle adjuster for TSA-102S, TSA-120, FS-128N, TOA-130S (2.7" focuser)
by Takahashi
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Takahashi - TSA-120 4.7" F/7.5 Ortho-Apochromat triplet refractor OTA

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Takahashi - TSA-120 4.7" F/7.5 Ortho-Apochromat triplet refractor OTA
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Our Product #: TSA120
Manufacturer Product #: TSA0120
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This Takahashi TSA-120 4.72” ED triplet refractor puts true-color “super apochromat” optical excellence in a portable 15.4 lb. package . . .

. . . our 38th year