AR102 4" f/6.5 achromatic doublet refractor optical tube

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This Explore Scientific 4” refractor has:

• big 4” aperture fully multicoated achromatic doublet refractor optics
• collimatable objective lens cell for fine-tuning the images
• big 8 x 50mm finderscope
• 2” dielectric star diagonal with 1.25” eyepiece adapter
• tube rings with carry handle and Vixen-style dovetail plate
• bright high contrast deep space views, plus excellent performance inside the solar system
• lifetime transferrable warranty

    This highly portable 4” achromatic refractor optical tube (only 25.5” long, with a weight of 11.3 pounds) is a sensational value! It has a 102mm (4”) diameter multicoated doublet objective lens (663mm focal length f/6.5). Its diffraction limited optics are sharp and well suited to serious deep space observing of binary stars, star clusters, and the brighter nebulas and galaxies, in addition to excellent high contrast planetary and lunar images. The objective lens cell is fully collimatable, using push/pull collimation screws, so you can optimize your image quality.
The 4” Explore Scientific is a great grab-and-go scope for quick looks at the sky when mounted on a suitable altazimuth mount. It’s good enough optically to be the heart of a reasonably-priced serious observing system when mounted on one of the many equatorial mounts and go-to altazimuth mounts from Celestron, Meade, iOptron, and others.
Some chromatic aberration is visible on stars and objects of 2nd or 3rd magnitude and brighter, as you would expect in an achromatic refractor with such a fast focal ratio. However, the spurious color is well controlled and not unreasonable, thanks to careful optical design, and has little effect on deep space observing. Most observers feel the relatively limited amount of chromatic aberration visible is a small price to pay for the scope’s many other virtues of portability, sharpness, contrast, and optical clarity.

Details About This Optical Tube . . .

  • Achromatic refractor optics: 4” (102mm) aperture, 663mm focal length, f/6.5 air-spaced crown and flint glass doublet lens. Tube diameter is 3.96” (100.7mm) and the tube length with dewshield installed is 25.5” (648mm). Scope weight is 11.3 pounds (4.67 kg).

  • Collimatable lens cell: The 4” Explore Scientific lens is precision collimated at the factory to give optimum performance. Although you may never need to adjust optical alignment, the ES102D allows for collimating the lens set should the telescope ever require it due to a fall or very rough handling.

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

  • Internal light baffles: Multiple internal knife-edge baffles in the optical tube mask and deflect unwanted stray light for improved contrast.

  • Dew shield: The removable 190.5mm (7.5”) diameter dew shield slows the formation of dew on the lens in cold weather 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.)

  • Dust covers: Front and rear dust covers are provided.

  • Dual-speed Crayford-style focuser: 2” focuser, with adjustable tension and lock knobs. The 2” eyepiece holder uses a non-marring brass compression ring to hold the supplied 2” star diagonal in place without scratching the diagonal barrel. The precision-made 2" focuser has dual-speed focusing. There are two coarse focusing knobs. The right knob also has a smaller concentric knob with 10:1 ratio reduction gear microfine focusing. This provides exceptionally precise image control during high power visual observing or critical DSLR or CCD imaging. The focus knobs have ribbed gripping surfaces so they are easy to operate, even while wearing gloves or mittens in cold weather.

  • Dielectric star diagonal: 90° viewing angle first surface mirror 2” star diagonal with 1.25” eyepiece adapter. The diagonal uses durable 99% reflectivity for maximum light transmission and long life.

  • Finderscope: 8 x 50mm straight-through achromatic design, in a dual-ring six-point adjustment bracket with a quick release mounting shoe. Focuses by loosening the trim ring behind the objective lens cell, screwing the lens cell in or out to focus, and tightening the trim ring to lock in the correct focus.

  • Dovetail tube mount: The optical tube comes with two split and hinged tube rings for installing the optical tube on your mount. The rings are connected to a dovetail mounting plate that slips into the standard dovetail groove on Astro-Tech Voyager, Explore Scientific, Celestron, Meade LX80, and Vixen mounts. A carrying handle is conveniently fitted to the top of the tube rings

  • Extended lifetime warranty: This individually serial numbered Explore Scientific telescope is backed by a limited one year warranty, which will be extended to an Explore Scientific transferable product lifetime limited warranty when its warranty card is registered with Explore Scientific within 60 days of purchase. Full details on this unique transferable lifetime warranty.
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.
166x
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.

663mm
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.

f/6.5
Resolution:
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.

1.14 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).

12.5
Aperture:
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.
4"
Weight:
The weight of this product.
11.3 lbs
Heaviest Single Component:
The weight of the heaviest component in this package.
1.3
 
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. 
Yes
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.  
Great
Planetary Observation:
Very Good
Binary and Star Cluster Observation:
Very Good
Galaxy and Nebula Observation:
Fair
Photography:
Yes
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.
Yes
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. 
Yes
Planetary Photography:
Yes
Star Cluster / Nebula / Galaxy Photography:
Yes
Warranty:
Limited Lifetime
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1. scott on 4/30/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
This is my second Explore Scientific telescope. I bought the 6" doublet a couple of years back and purchased this scope this last winter. These are finely crafted telescopes and a real joy to handle. But, most important the optics are excellent. These are hands down the best bang for the dollar that you can find. The optics are very clear and with absolutely no distortion. Each detail of the scope is obviously carefully constructed and tested. I'd highly recommend this Scott Helgeson
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Explore Scientific - AR102 4" f/6.5 achromatic doublet refractor optical tube

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Explore Scientific - AR102 4" f/6.5 achromatic doublet refractor optical tube
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Our Product #: ES102D
Manufacturer Product #: DAR102065-01
Price: $529.99  FREE ground shipping - Click for more info
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This highly portable 4” Explore Scientific achromatic refractor optical tube (only 25.5” long, with a weight of 11.3 pounds) is a great value for spur-of-the-moment grab and go astronomy . . .





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