TMB-92 Signature Series 92mm f/5.5 apochromatic refractor optical tube

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This TMB Optical TMB-92 refractor has:

• 92mm f/5.5 apochromatic triplet optics using FPL-53 ED glass
• collimatable lens cell for peak imaging and optical performance
• 3” dual-speed Feather Touch rack and pinion focuser
• ultra-short 13.25” length for travelling
• removable extension tube for binoviewer use without a Barlow
• rotating eyepiece holder for quick camera/eyepiece orientation
• 2” and 1.25” compression ring eyepiece holders
• retractable dew shield, dust covers, and hard case
• sales limited to 100 scopes per year to maintain the value of your investment
• normally available from stock – with no multi-year waiting list

    The TMB-92 Signature Series 92mm f/5.5 triplet apo is a very compact refractor for the serious observer and traveler. Measuring only about 13.25” long with the focuser and dewshield retracted for storage and travel, the TMB-92 optically and mechanically fills the void left by the discontinued 90mm f/5 Astro-Physics Stowaway (a legendary scope that sells for up to $6000 on the used market).

    The TMB-92 puts exceptionally high optical and mechanical quality in a truly tiny package. It is ideal for the serious observer who will accept nothing less than unsurpassed imaging and visual observing performance in a supremely portable body you can take with you anywhere in the world.

    The TMB-92 is an exceptional grab and go scope for observing and imaging – both within the solar system and out to the furthest reaches of infinity. Its images of subtle low-contrast lunar and planetary details are razor-sharp, with realistic and highly saturated color. Those images are free from chromatic aberration of any type, thanks to the ED (Extra-low Dispersion FPL-53 glass) element in the scope’s triplet optics.

    The latest antireflection multicoatings and knife-edge internal baffles assure the maximum image contrast possible. Image excellence is limited only by the seeing conditions, not by any aberration or short-coming in the telescope.

    How does the TMB-92 perform in the real world? A customer emailed to tell us “I have a 92 from the first batch. Visual performance defies the laws of physics.” While we can’t claim that the TMB-92 really does defy the laws of physics, that kind of comment is typical of the enthusiastic response the TMB-92 has be generating among knowledgeable astronomers. And it’s one of the reasons the TMB-92 was chosen a readers’ choice “Gear of the Year” product by a vote of the members of the forums on Cloudy Nights.

    An extended four-page test report in the March 2009 edition of Sky & Telescope magazine gave the TMB ratings of 4½ stars out of 5 for optics, 5 stars for mechanics, and a 4½ star overall rating. The review compared the 92mm f/5.5 TMB-92 against a later model 90mm f/7 Astro-Physics Stowaway, rather than the Stowaway’s original f/5 version. (A longer focal length scope should have intrinsically better correction than a shorter f/ratio version of the same scope, all other things being equal, leading to a perhaps not exactly “apples to apples” comparison of the f/5.5 TMB optics to the f/7 A-P optics.) Nonetheless, the review said “The new TMB fared very well indeed against this remarkable Astro-Physics apo. The TMB-92 presented images of bright stars and planets that were as free of chromatic aberration (false color) as the Stowaway. It’s fair to say that the TMB had no visible chromatic aberration, stunning for such a fast instrument . . . If I were to rate the Stowaway with 5 stars for optical perfection, then the TMB-92 earns 4½. Considering that the Stowaways have been known to command $5,000 to $6,000 on the used market, $2,000 for a new TMB-92 is an excellent bargain.

   ”Adding to its value,’ the review stated, “the TMB handily beat the Stowaway when comparing their tube assemblies . . . just 15 inches (38cm) long with the dewcap retracted, the TMB is as short as some 80-mm apo refractors. And that’s not all. You can make the scope even shorter by removing a 2¼-inch-long tube section that connects the focuser to the main tube. Removing this piece adds the several inches of extra ‘focus travel’ that binoviewers demand. As such, you can enjoy two-eyed panoramic views through the TMB at low powers, without the need for a Barlow lens in the system to make the binoviewer reach focus . . . with a pair of TeleVue 24-mm Panoptic eyepieces, the 3°-wide views of Milky Way starfields . . . were spectacular.

    “The beauty of the compact TMB scope is its adaptability for both visual use at all magnifications with and without a binoviewer and for astrophotography,” the test report continued. “I used it extensively to take deep sky images with a DSLR, a task well suited to its fast f/5.5 optics . . .

    “All in all,” the Sky & Telescope report concluded, “I’m impressed with this new telescope from TMB. It combines superb optics with solid, well-crafted mechanics for a versatile, compact instrument. What more could an apo-lover want?”

    Read the entire TMB-92 review on the Sky & Telescope website.

This Telescope’s Optical and Mechanical Systems . . .

  • Apochromatic triplet ED refractor optics: 3.6” (92mm) aperture, 506mm focal length, f/5.5 triplet lens. It uses an FPL-53 ED (Extra-low Dispersion) glass element for a sharp and flat field and images that are high in contrast and free from spurious color (chromatic aberration).

  • Multicoated optics: Fully multicoated on all surfaces with the latest antireflection materials for exceptionally high light transmission and excellent contrast. Light transmission is so high that the optics seem to virtually disappear when viewed from head on.

  • Collimatable lens cell: The lens cell has three sets of push/pull collimating screws that will let you tweak the optical collimation after rough handling or extensive travel. This assures that optimum visual and photographic performance is only a few moments away.

  • Internal light baffles: Contrast-enhancing knife-edge light baffles in the optical tube and focuser provide truly dark sky backgrounds and high terrestrial contrast. In addition, the edges of the objective lens are blackened to eliminate contrast-reducing stray internal reflections.

  • Retractable dew shield: The self-storing retractable dew shield/lens shade slows the formation of dew on the lens to extend your undisturbed observing time. It also improves visual and photographic contrast by shielding the lens from off-axis ambient light (the neighbor’s yard light, moonlight, etc.)

  • Dual-speed microfine #3025 3” Feather Touch rack and pinion focuser: The precision-made 3” diameter drawtube non-vignetting focuser is a genuine Feather Touch from Starlight Instruments, a company known world-wide for making the finest and most reliable focusers available to the amateur astronomer. The focuser has dual-speed focusing. There are two coarse focusing knobs. The right knob also has a smaller concentric knob with a 10:1 reduction gear microfine focusing ratio. This provides exceptionally precise image control during very high magnification visual observing, and during critical 35mm or CCD imaging. All focus knobs are ribbed, so they are easy to operate, even while wearing gloves or mittens in cold weather. Multiple internal baffles in the focuser drawtube assure high contrast.

  • Rotating eyepiece holder: The entire focuser can be rotated a full 360° without disturbing the focuser drawtube position. This lets you rotate the eyepiece/accessory holder to line up a camera in either a landscape or portrait orientation (or any orientation in between) without disturbing the focus, as well as put a star diagonal and eyepiece into the most comfortable observing position. Three large locking knobs on the focuser body let you temporarily lock your camera or accessories in the desired orientation, then change that orientation with a simple twist of the wrist.

  • Focusing scale: The 65mm (2.65”) travel focuser drawtube has a scale marked in 1mm increments so you can note individual focuser positions for easy return to the correct focus when switching between visual use and photography. A large lock knob on top of the focuser lets you lock in your photographic focus.

  • Extension tube: The minimum length of the TMB-92 is 13.25” with the dew shield and focuser retracted for traveling and for long equipment train CCD imaging. A 2.25” extension tube is supplied to provide the proper back focus for convenient visual use and 35mm/webcam/Deep Sky Imager type astrophotography. This installs between the optical tube and focuser. A groove in the extension tube engages the brass thumbscrews in the optical tube (that normally control focuser rotation) to hold the extension tube firmly to the optical tube. Three nylon-tipped metal thumbscrews at the opposite end of the extension tube now hold the focuser in place and allow focuser rotation, just as the brass thumbscrews in the optical tube do when the extension tube is not used. A Teflon insert in the focuser end of the extension tube assures smooth focuser rotation. When the extension tube is not used, the available back focus allows you to reach focus with virtually any combination of photographic accessories (CCD camera, filter wheel, electric focuser, adaptive optics system, etc.). When the supplied 4” diameter by 2.25” long extension tube is threaded into place between the focuser and the optical tube, the maximum overall scope length is increased to 20.75”, the proper length to achieve visual focus with a star diagonal and eyepiece.

  • Compression ring eyepiece holders: The focuser drawtube ends in a 2” eyepiece holder that uses a non-marring soft brass compression ring to hold a 2” star diagonal and accessories in place. The compression ring won’t scratch the barrel of your star diagonal and accessories as an ordinary thumbscrew can. Also supplied is a 1.25” eyepiece holder that slips into the 2” compression ring holder to let you use a 1.25” star diagonal or image erecting diagonal. Like the 2” eyepiece holder on the eyepiece holder rotation mechanism, the 1.25” adapter also uses a non-marring soft brass compression ring to hold 1.25” star diagonals and accessories in place. The 2” accessory holder can be removed from the drawtube to allow the installation of custom A-P type photo accessories available directly from Starlight Instruments.

  • Finderscope mounting point: No finderscope is provided with the TMB-92, but two finderscope mounting points are located at the 2 o’clock position on the Feather Touch focuser body. Only one of the mounting points is needed to install an optional Astro-Tech #ATF multiple reticle finder. The finder’s mounting stalk can be attached to the scope using either of the screws normally threaded into the mounting points to prevent the entrance of dust. If you prefer to use an existing 50mm finder, you can mount it on either of the scope’s mounting points by means of an optional Astro-Tech #ATFB mounting base and optional Losmandy #LQRB quick-release finder bracket, as shown in the feature image below.

  • Tube finish: The 114mm o.d. optical tube is finished in a durable baked-on automotive white paint with a black-anodized focuser, trim, and 2.25” extension tube.

  • Shipping/storage case: The TMB-92 comes standard in an aluminum frame 22.25” x 12.75” x 8” locking hard case with a carry handle for transportation and general protection purposes. TMB Optical is one of the very few manufacturers to provide a case at no charge for protection during shipping and as a storage convenience when the scope is not in use. Unfortunately, FedEx, UPS, and the Postal Service are very good at treating packages roughly. Occasionally, your scope can arrive in perfect condition, but with the walls of the shipping case dented in transit from rough handling, or the aluminum frame sprung, rendering the appearance of the case less than pristine. Damage to the shipping case in such instances is not covered by warranty.

  • Other supplied accessories: A slip-on metal dust cap is standard. As mentioned previously, threaded holes for installing a finderscope bracket are located on the upper right side of the focuser body.

  • Two year warranty: As an expression of the confidence TMB Optical has in the quality of their products, the TMB-92 is protected by a two-year limited warranty against flaws in materials and workmanship (shipping case excluded).
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.
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).

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.

1.26 arc seconds
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.
8.5 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:
2 years
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1. Terry on 3/11/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
In my opinion It is easily the best refractor made for under $2,000.00 and comparable to similar size refractors that cost over double the price. Built like a tank and with the Feather Touch focuser it is as solid as a rock.
I use mine solely for imaging, the optics are sharp as a tack and far exceeding my expectations. It has a very quick f/ratio of F5.5 making it suitable for any DSLR or CCD up to a full frame chip (providing a field flattener is used). I used this scope for my recent APOD
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General Accessories
Finderscopes (2)
65mm quick release finderscope bracket
by Losmandy
Extra mounting bracket for Astro-Tech multiple reticle finder
by Astro-Tech
Tube Rings (1)
114mm tube rings for TMB-92 refractor, pair
by TMB
Photographic Accessories
Tele-Compressors (Photo/Visual) (1)
2" Field flattener for imaging with Astro-Tech and TMB refractors
by Astro-Tech
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TMB-92 Signature Series 92mm f/5.5 apochromatic refractor optical tube

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TMB-92 Signature Series 92mm f/5.5 apochromatic refractor optical tubeTMB-92 shown with lens shade extended.Image showing the Feather Touch focuser on the TMB92.Image showing the extension tube for visual use installed between the TMB92 body and the Feather Touch focuser.Image of the extension tube for visual use, showing the Teflon insert for smooth focuser rotation.Image showing how an optional #ATFB mounting base and #LQRB quick release bracket can be combined to attach a 50mm finder to the TMB-92.
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Our Product #: TMB92
Manufacturer Product #: TMB92
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Clear skies,

If you are looking for the best fast focal ratio/ultra-compact apo refractor legendary optics designer Tom Back could create, then look no further. This 13.25" long TMB-92 fills the void left by the discontinued 90mm Astro-Physics Stowaway (a legendary scope that sells for up to $6000 on the used market). A customer wrote to tell us “I have a 92 from the first batch. Visual performance defies the laws of physics” . . .

. . . our 36th year