NP101Is 4" f/5.4 Nagler-Petzval apochromatic refractor

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This updated and improved TeleVue refractor optical tube has:

• 101mm f/5.4 four-element Nagler-Petzval apochromatic optical system
• 2.4” rack and pinion focuser with 2” accessory adapter
• Focusmate 10:1 ratio dual-speed fine focus made by Starlight Instruments
• stainless steel lens cell for improved thermal stability
• longer retractable dew shield for improved contrast
• state-of-the-art visual and imaging capabilities
• 5-year warranty

    The Backyard Astronomer’s Guide said of the original TeleVue NP101, “optics don’t get any better than this;” while Sky & Telescope gave the NP101 top honors (May and June 2002) and selected it as the test instrument for its landmark review of the SBIG STL11000 CCD camera (July 2004). Featuring a new enlarged 2.4” focuser and redesigned optics, this new TeleVue NP101is (Imaging System) improves and extends the photographic capability of the original NP101.

The new TeleVue NP101is sets new optical and mechanical quality standards in a moderately small and highly portable apochromatic refractor for the serious observer and astrophotographer. TeleVue first reinterpreted the incredible design flexibility of the time-proven Petzval portrait lens into an astronomical objective over 20 years ago. They’ve been refining the design ever since. Nagler-Petzval optics permit the optical designer greater freedom to simultaneously correct for coma, astigmatism, field curvature, secondary spectrum, spherical aberration, and spherochromatism than any other commercial optical design.

This newest 4” four-element air-spaced Nagler-Petzval system has a 101mm aperture ED doublet at the front of the scope, with a second field-flattening/aberration-correcting ED doublet at the rear. This combination of materials and design gives the TeleVue NP101is diffraction-limited apochromatic (spurious color-free) performance across the entire visual spectrum. In addition to exceptional visual performance, both inside the solar system and beyond, it has been optimized for deep space imaging, particularly with CCD cameras having large format detector chips with diagonals up to 50mm across. Every optical and mechanical element has been optimized to work as a single integrated unit, with new features to make imaging easier and more flexible.

For visual use, the scope’s 540mm focal length is long enough, using a 1.25” TeleVue 2.5mm Nagler eyepiece, to provide 216x for high magnification observations of the Moon, planets, and binary stars; yet it will also produce a huge 4.1° field at 15x, using a 2” TeleVue 35mm Panoptic eyepiece for wide field observing.

Its image quality, easy set-up, and airline portability make it a scope you will use often. Manufacturing advances in lens finishing, including interferometric monitoring of all surfaces, and TeleVue’s stringent made-in-the-USA telescope assembly and quality control, insure uniformly excellent results with every telescope, and a level of manufacturer-direct service that’s all too rare.

Whether you are interested in terrestrial nature studies, astronomical rich-field or planetary observing, or photography of the earth and skies, just put in an optional star diagonal and eyepiece or attach a camera and the TeleVue NP101is will satisfy your observing desires right out of its case.

This Telescope’s Optical and Mechanical Systems . . .

  • Nagler-Petzval apochromatic refractor optics: 4” (101mm) aperture, 540mm focal length, f/5.4 apochromatic four-element air-spaced optical system. ED (Extra-low Dispersion) glass elements in both the front and rear doublets of the system provide images that are free from spurious color and other optical aberrations.
    The newly-redesigned optics feature a field-flattening rear lens group that is larger than that in the original – and still available – NP101. The larger second optics group minimizes vignetting in photographic formats having up to a 50mm diagonal. In addition, for CCD cameras using a detector chip with a 35mm or larger diagonal, an optional large field corrector is available so that the stars in the newly brightened non-vignetted corners of the image are virtually as tightly-focused and sharp as those in the center of the field.
    The new front lens cell is made of stainless steel. This provides better thermal stability in extremes of heat and cold and when taking your scope from a warm house outdoors to observe on a cold winter’s night.

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

  • Dual-speed rack and pinion focuser: The newly-designed focuser has a non-vignetting internally-tapered drawtube with a 3” entrance aperture and a 2.4” exit aperture. This tapered design prevents any restriction of the larger converging light cone from the scope’s larger field-flattening rear lens group. The drawtube terminates in an end ring that has a tilt capability to correct for any residual focal plane tilt. This is particularly important with the more stringent focus requirements of CCD cameras. Four lock knobs on the end ring hold the supplied 2” accessory adapters firmly in place with no possibility of tilting under the weight of camera or accessories. The end ring also accepts a “position stop” for an optional digital micrometer that allows repeatable focus measurements with a 0.0001” accuracy.
    The imaging adapter is threaded to accept TeleVue’s new Imaging System accessories – including large diameter extension tubes, optical accessories, and camera and CCD attachments, all with mating threads. The separate visual adapter has a 2” non-marring brass clamp ring with two lock knobs to hold a 2” star diagonal or other visual accessories firmly in place.

  • Fine focus system: The focuser is equipped with a Focusmate 10:1 reduction ratio dual-speed fine focus system on the right focus knob. This precision focuser is made for TeleVue by Starlight Instruments, makers of ultra-premium Feather Touch focusers.While the left focus knob still provides TeleVue’s standard focusing speed, the Starlight Instruments Focusmate on the right knob provides extremely sensitive 10:1 ratio fine focusing for imaging. Focus changes are measurable with an optional Digital Micrometer Kit to better than 1/10,000” for precisely repeatable focusing. Both the standard focus and the Focusmate use large knurled focus knobs that are easy to operate, even while wearing gloves or mittens in cold weather.
    An optional Focusmate Driver permits pushbutton electronic motor control of the focusing process. The Focusmate Driver has directional pushbuttons and adjustable speed. A tap on a pushbutton permits vibration-free focusing in steps as small as approximately 0.0005”. The Focusmate Driver can be remotely computer controlled with software under development.

  • Optional photographic accessories: Optional photo accessories include the following: a Digital Indicator Kit for digital readout of the focuser position for repeatable focus points; a Focusmate Driver electronic vari-speed motor control for the Focusmate fine focus mechanism; a Large Field Corrector for optimized edge performance with large diagonal CCD chips; extension tubes of various lengths to bring various CCD and 35mm cameras to focus; an adapter to connect the SBIG STL11000 CCD camera; an adapter to connect a standard 35mm camera using a T-Ring; an adapter to connect a Canon EOS or digital SLR using a wide mouth Canon T-Ring; and a 0.8x focal reducer for wider field imaging.

  • Sliding captive dew shield: The new, longer retractable dew shield slows the formation of dew on the lens to extend your undisturbed observing time. The longer dew shield 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 manageable 25.5” when it is retracted.

  • Mounting rings: No mounting rings are supplied for the 101mm diameter optical tube. The optional TeleVue #MRS4 dual 4” mounting ring set is designed specifically for installing the NP101is on wide variety of equatorial mounts and mounting adapters.

  • Finderscope mounting point: No finderscope is supplied. However, dovetail accessory mounting slots for a finderscope and other accessories (such as a piggyback camera mount) are provided on the optional TeleVue #MRS4 dual mounting ring set.

  • Carrying case: A custom foam-fitted locking hard shell case is standard. The case has a large accessory compartment built into its lid.
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.

1.15 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.
11 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:
Very Good
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
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General Accessories
Tube Rings (1)
Mounting ring set for 4" refractors
by TeleVue
  • Lens shade
  • Dust cover
  • Hard case
  • Visual adapter ring
  • Imaging adapter ring
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TeleVue - NP101Is 4" f/5.4 Nagler-Petzval apochromatic refractor

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TeleVue - NP101Is 4" f/5.4 Nagler-Petzval apochromatic refractorImage showing scope with all supplied accessories – hard case, thread-in dust cover, Focusmate, and visual and imaging adapters.
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Our Product #: NP101is
Manufacturer Product #: NPI-4057
Price: $3,995.00
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MSRP: $5580.00

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This TeleVue NP-101is is a superb and highly portable apochromatic refractor for both visual observing and imaging . . .

. . . our 38th year