Explore Scientific AR102 4" f/6.5 achromatic doublet refractor optical tube


Availability: In stock

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 . . .
Our Product #: ES102D
Explore Scientific Product #: DAR102065-01

Product Description

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
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, 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 and tube rings: 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 industry standard Vixen-style dovetail groove on altazimuth and equatorial mounts from Astro-Tech, Explore Scientific, Celestron, iOptron, and others. 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 Life of the Product Limited Warranty when its warranty card is registered with Explore Scientific within 60 days of purchase.

Tech Details

Aperture 4"
Binary and Star Cluster Observation Very Good
Focal Length 663mm
Focal Ratio f/6.5
Galaxy and Nebula Observation Fair
Heaviest Single Component 1.3
Highest Useful Magnification 166x
Lunar Observation Great
Weight 11.3 lbs
Planetary Observation Very Good
Resolution 1.14 arc seconds
Visual Limiting Magnitude 12.5
Warranty Limited Lifetime


Review by:
Great scope I use it on my new mount and will use it
for Astrophotograhy. I got it when it went on sale. (Posted on 10/23/2019)
Review by:
Since I wrote the earlier review my AR102 has been used most clear nights. It is being used with an ATIK414 camera and doing high level photometry. When you use photometric filters then the CA ceases to be an issue. It has become one of my favorite scopes of all times. I still get out the Celestron 6" refractor from time to time especially when Mars is at opposition but the AR102 is easier to use and balance. Still a great bang for the buck! (Posted on 7/24/2019)
Review by:
I purchased this scope as an upgrade from a similar sized 102mm refractor that was of less quality. I'm very glad I did. This has been a great scope. I generally use it to observe while I am taking photos with other scopes, and I use it at public star parties. There is some aberration, but I expected that and it isn't very bad. I use a fringe killer filter that helps quite a bit. The only reason I am giving it 4 stars is due to the compression ring where the diagonal attaches to the scope. I often have a hard time getting the diagonal to go in at all. Several times the set screws come out all together when I am attempting to get them loose enough to work. I will probably eventually replace the compression ring with a Baader click-lock. As I said, this is a great scope, I would highly recommend it for beginners or a grab and go. My criticisms are minor. (Posted on 7/24/2019)
Review by:
Bought this for a white light solar scope (herschel wedge) and it works great. Small enough and light enough to mount on my Nextstar mount. Comes with a nice 2" diagonal. Fit and finish was nice. Great little scope for the price. (Posted on 1/14/2019)
Review by:
My original intention was to buy the 5" or 127 mm version of this scope. However, budget and size dictated the 102mm or 4" version. This has been a "Big Bang for the Buck!" My original intention was to buy this telescope as a grab and go scope. When it arrived the clouds played havoc with trying to test it for several months. When it finally cleared I had a chance to look at Jupiter and several winter objects. Wow! The star tests were very good as well as the chromatic aberration being much better controlled than I anticipated. It was still there but visually could be controlled with a Minus V filter. I finally settled on a 2" Wratten #8 to attenuate the worst of the CA. This telescope works quite well even for color imaging, especially on planets. I have a 6" F/8 refractor that I use for planetary imaging but it had to have the focuser and finder modified to use with a camera. Over the years I find myself using the 6" less and less and the 4" ES more and more! Using a Celestron Minus V filter when imaging planets seems to eliminate the CA in the images, but leaves some in long exposure star images if a brighter star is in the field. If you were going to use this telescope for long duration color images you would probably want to get an ADP or ED version but you still can work with this one, with a little care. If you use RGB color filters on a mono camera you eliminate the effects of CA entirely on your images. Way cheaper than buying an APO scope. You also can use Photoshop to eliminate the worst of the CA around bright stars. I currently use this scope as my main imaging telescope using a 3.5 pound SBIG ST-8E and the dual speed focuser handles that weight quite well! Overall it is one of my favorite telescopes to use and the best buy I ever got on a new scope! (Posted on 1/11/2019)
Review by:
I use this scope both visually and for astrophotography. As stated in other reviews, visually the optics are excellent, with little chromatic aberration even on very bright objects. Aberration is even tolerable photographically except on highly stretched images. Construction is quality. Great scope for the cost! (Posted on 12/13/2018)
Review by:
I use this as a grab n go scope along with my Stellarvue SV80ED. It actually rivals the Stellarvue in contrast. The ES is an excellent buy and I would recommend it for both the beginner and the seasoned amateur astronomer. (Posted on 10/16/2018)
Review by:
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 (Posted on 8/4/2017)
How do you rate this product? *