Explore Scientific ED102 Essential Series 102mm f/7 Air-Spaced Triplet APO Refractor Telescope

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Product Overview The Explore Scientific ED102 Essential Series is a 102mm air-spaced triplet apochromatic refractor with a focal length of 714mm at f/7. The centre element is genui ne HOYA FCD1 extra-low-dispersion glass, and Explore Scientific applies proprietary multi-layer coatings aimed at holding transmission high while keeping chromatic aberration low. Published figures are a limiting magnitude of 12.5 and a resolution of 1.14 arcsec. The tube is 30.5″ (774mm) long with the dew shield extended and 23.75″ (603mm) with it retracted, 119mm in diameter, and weighs 12 lb (5.4 kg). It rides on a Vixen-pattern dovetail and focuses through a precision 2-inch rack-and-pinion focuser supplied with extension tubes. What ships is the telescope tube with its dew shield and the focuser extension tubes — no mount and no eyepieces. Who It's For Deep-sky imagers: 714mm at f/7 is a widely used astrophotography focal length — large enough for the brighter galaxies and globulars, short enough to frame the big nebulae and to keep guiding demands reasonable. Visual observers who want colour-free views: the triplet design with FCD1 ED glass removes the violet halo an achromat leaves on Venus, Jupiter and the lunar limb. Observers who travel: this is a good match if you want an apochromat that packs down to 23.75″ with the dew shield retracted and weighs 12 lb. Owners of mid-capacity equatorial mounts: this is a good match if your mount is rated comfortably above the loaded weight of a 12 lb tube plus rings, diagonal, finder and camera. Double-star and lunar observers, working near the published 1.14 arcsec resolution. Key Features & Design 102mm air-spaced triplet: three elements with air gaps between them rather than cemented pairs. Genuine HOYA FCD1 ED glass in the centre element, chosen for how closely it brings widely separated wavelengths to a common focus. 714mm focal length at f/7: a balanced focal ratio for both wide-field imaging and medium-to-high power visual work. Proprietary multi-layer coatings applied to maximise light transmission alongside the colour correction. Precision 2-inch rack-and-pinion focuser supplied with extension tubes, which is what makes reaching focus with a diagonal or a camera straightforward. Retracting dew shield: 30.5″ (774mm) extended, 23.75″ (603mm) retracted. Vixen-pattern dovetail on a tube 119mm in outside diameter. Published performance: limiting magnitude 12.5, resolution 1.14 arcsec, weight 12 lb (5.4 kg). Optical Design An achromatic doublet brings two wavelengths to a common focus and leaves the rest slightly out, which shows as a violet halo around bright objects at high power. A triplet apochromat adds a third element so three widely separated wavelengths converge together, and the residual error falls below what the eye picks up in normal use. The glass in the centre element is what makes that practical: extra-low-dispersion types spread the colours less to begin with, so the design has less to correct. HOYA FCD1 is the type used here, and Explore Scientific specifies it as genuine FCD1 rather than an equivalent. At f/7 the light cone is moderate. That has a direct effect on imaging: field curvature and off-axis aberrations are easier to correct than in a fast astrograph, and focus tolerance is looser, which makes a night of unattended imaging less fragile. The 2-inch rack-and-pinion focuser is the mechanical half of that story — rack-and-pinion drives resist slipping under load in a way that a lightly tensioned Crayford does not, which matters when a camera train hangs off the drawtube pointed at the zenith. Explore Scientific does not publish a corrected image circle or a backfocus distance for this tube, so those rows are absent from the specifications rather than estimated. Send us the camera and flattener you have in mind and we will work out the spacing with you before anything ships. Recommended Uses Wide-field and medium-field deep-sky astrophotography at 714mm. Visual observing of the Moon, planets and double stars where colour correction sets the ceiling. Deep-sky visual work on clusters, nebulae and the brighter galaxies. Portable dark-site sessions, given the 12 lb weight and 23.75″ retracted length. Solar observing with an appropriate front-mounted filter, and outreach sessions where a refractor's zero-maintenance optics matter. Compatibility and Accessory Notes Mount: not included. The published tube weight of 12 lb is before rings, diagonal, finder and camera. For visual use a mount comfortably above that loaded figure works well; for imaging, more headroom again. Dovetail: Vixen pattern, fitting Vixen saddles and dual-fit saddles. Eyepieces: not included. At 714mm, a 26mm eyepiece gives 27x, a 20mm gives 36x, a 12mm gives 60x and a 6mm gives 119x. Diagonal: the 2-inch focuser takes 2-inch diagonals, and the supplied extension tubes are what bring the focal plane out to meet one. We stock dielectric 2-inch diagonals. Imaging train: a field flattener is the standard companion for deep-sky work. Since no corrected image circle is published, we will confirm the flattener match against your camera rather than quote a figure. Focal extenders: a 2x or 3x extender on 714mm brings lunar and planetary work into a useful magnification range for a 102mm aperture. Good to Know Before You Order This is an optical tube assembly, not a complete telescope. A mount, a tripod and at least one eyepiece are needed alongside it. We can quote the package as a whole if that is simpler — tell us what you already own. The 30.5″ length is with the dew shield extended. Retracted it is 23.75″, which is the figure to measure a case or a vehicle against. No corrected image circle or backfocus figure is published for this tube, so those rows are left out below rather than filled with an estimate. If you are planning an imaging train, send us the camera and flattener and we will work the spacing out with you. A triplet takes longer to reach ambient temperature than a doublet of the same aperture. Setting the tube outside ahead of the session is what lets the optics settle before high-power observing begins. At 12 lb it is portable but not featherweight. One person handles it comfortably, and the mount underneath it wants capacity to spare once the diagonal, finder and eyepiece are added. We are glad to check your mount against a loaded figure. Frequently Asked Questions Is it difficult to set up? The optics need nothing — a refractor holds its collimation, so the tube is ready as soon as it is in the rings and on the mount. At 12 lb it is a comfortable one-person lift, and it goes on most easily with the mount levelled and the counterweights roughly positioned first. The extension tubes take a moment to fit for whatever is going in the focuser. What mount will carry it? For visual use, one rated above the 12 lb tube weight with room for the diagonal, finder and eyepiece on top. For imaging, more margin, since tracking accuracy falls off well before a mount reaches its rated capacity. Tell us the model you own and we will give you a straight answer. Does it come with eyepieces or a diagonal? No. The tube arrives with its dew shield and the focuser extension tubes; the diagonal, eyepieces, finder and rings are chosen separately and we can assemble a set that matches. Will I see colour fringing? An air-spaced triplet using HOYA FCD1 ED glass is designed to eliminate it in normal use. On Venus, Sirius and the lunar limb at high power the image stays neutral where a two-element achromat would show violet. Can I image with it? Yes — 714mm at f/7 is a well-liked deep-sky imaging focal length. A field flattener is the usual addition, along with an equatorial mount and guiding. How does it compare to the 127mm models? We stock three ED127s and they are easy to confuse, so it is worth being precise. All three are 127mm at 952mm and f/7.5. The ED127 Essential uses HOYA FCD1 glass — the same type as this ED102 — in an 18 lb tube with a 2-inch two-speed rack-and-pinion focuser. The ED127 FCD100 steps up to HOYA FCD100 glass and a 2.5-inch dual-speed HEX Crayford focuser, also 18 lb in the painted aluminium tube. The ED127 FCD100 Carbon Fiber pairs those same optics and focuser with a carbon tube at 14 lb, which is the one to look at if your mount is the limiting factor. Against any of them, this ED102 gathers less light and resolves less detail — 127mm is roughly 55% more aperture area — but it is 12 lb rather than 14 or 18, it is shorter, it asks considerably less of a mount, and at 714mm it gives a wider field per eyepiece. When people are choosing between them, the mount they own or plan to buy usually settles it faster than the optics do. Ask us and we will work through it with you. Bottom Line In short: a 102mm f/7 air-spaced triplet apochromat with genuine HOYA FCD1 ED glass, a 2-inch rack-and-pinion focuser with extension tubes, a Vixen dovetail and a published weight of 12 lb — a 714mm focal length that suits wide-field imaging and high-power visual work equally well. If you are matching it to a mount or planning an imaging train around it, send us the details and we will confirm the fit with you before you order. Show more