Explore Scientific FCD100 Series 127mm f/7.5 Air-Spaced Triplet ED APO Refractor Telescope

Product Overview

The Explore Scientific FCD100 Series 127mm is a five-inch air-spaced triplet apochromatic refractor with a focal length of 952mm at f/7.5. The objective is built around Hoya FCD100 extra-low-dispersion glass, and Explore Scientific publishes a limiting magnitude of 13 and a resolution of .9 arcsec for it. The tube is aluminium, finished in white, and measures 41.75″ (1060mm) with the dew shield extended, 33.25″ (849mm) with it retracted, over a 130mm outside diameter, at a published 18lb (8.2kg).

It is supplied as an optical tube assembly. The published package covers a 2.5-inch focuser rated to carry up to 10 lbs of imaging gear, an integrated retractable dew shield, a cradle ring with a carry handle, and a Vixen-style dovetail plate. Enhanced multi-layer coatings are applied to suppress aberrations. No mount and no eyepieces are listed in the vendor's package.

Who It's For

• Deep-sky imagers who want a five-inch refractor with a focuser rated for a real imaging train rather than a token one. The 10 lb focuser rating is the figure that matters when a cooled camera, filter wheel, off-axis guider and rotator are all hanging off the back.
• High-power visual observers: at f/7.5 and 952mm this is a long enough refractor to hold correction on the Moon, planets and double stars, with .9 arcsec of published resolution to work with.
• Observers stepping up from an 80mm or 100mm ED tube who want more aperture without moving to a mirror system and the collimation and cool-down that comes with one. This is a good match if you already run a mount with headroom well above 18 lb.

Key Features & Design

• 127mm air-spaced triplet objective: three elements in the objective cell, separated by air gaps rather than cemented.
• Hoya FCD100 ED glass: the low-dispersion element in the group, which is what pulls the design into apochromatic territory.
• 952mm focal length at f/7.5: a moderate focal ratio for an apochromat, which eases the correction burden on the objective and keeps the field better behaved than a very fast triplet.
• 2.5-inch focuser rated to 10 lbs: a load rating Explore Scientific publishes specifically for imaging gear.
• Integrated retractable dew shield: it slides back over the tube, which is where the 8.5-inch difference between the two published tube lengths comes from.
• Enhanced multi-layer deposition coatings on the optical surfaces, applied to raise transmission and hold contrast.

Optical Design

A two-element achromat brings two wavelengths to a common focus and leaves a visible residual — the secondary spectrum — around bright objects. A three-element apochromat adds a degree of freedom: with one element made of extra-low-dispersion glass, the designer can bring three widely separated wavelengths to essentially the same focal plane instead of two. The practical result is that the violet and deep-red ends of the spectrum land close enough to the visual focus that colour fringing on Venus, Jupiter and the lunar limb stops being a feature of the view.

FCD100 is the ED glass type in this objective. Its job is dispersion control: it bends the different wavelengths by more similar amounts than ordinary crown or flint glass does, so the two mating elements have less residual colour to correct. Air spacing rather than cementing gives the design six optical surfaces to shape instead of four, and it removes a cement layer as a thermal variable in a five-inch lens — a real consideration in a lens this size cooling down through an evening.

Explore Scientific does not publish a corrected image circle or a backfocus distance for this tube, so those rows are absent from the specifications below rather than estimated. If you are assembling an imaging train, send us the camera and flattener you have in mind and we will work the spacing out with you.

Recommended Uses

• Deep-sky astrophotography on a suitably rated equatorial mount, with a flattener matched to your sensor.
• High-magnification visual work on the Moon, planets and double stars, where .9 arcsec of published resolution sets the ceiling.
• Lunar and planetary imaging with a focal extender ahead of a planetary camera.

Compatibility and Accessory Notes

• Mount: not included. The published tube weight is 18lb (8.2kg) bare, before a diagonal, finder, camera or eyepiece, so a mount rated comfortably above that is the sensible starting point — and more again for imaging, where tracking accuracy rather than raw capacity is the constraint.
• Dovetail: the supplied plate is Vixen-style, so it seats in any Vixen saddle and in the dual-format saddles that take both Vixen and Losmandy plates.
• Eyepieces and diagonal: not listed in the published package. The 2.5-inch focuser takes 2-inch accessories with the appropriate adapter, and 2-inch is the format most observers build around at this focal length.
• Magnification: 952mm ÷ eyepiece focal length. A 30mm gives 32x, a 20mm gives 48x, a 9mm gives 106x, a 5mm gives 190x.
• Focal extenders: a 2x extender takes the system to 1904mm at f/15, which is planetary imaging territory in steady air.
• Imaging train: the 10 lb focuser rating covers most cooled-camera trains, and we can go through yours item by item if you would like a second opinion on the load.

Good to Know Before You Order

• This is an optical tube assembly, not a complete telescope. A mount, a tripod, a diagonal and at least one eyepiece are separate items. We put packages like this together regularly — tell us how you plan to use it and we will price the whole thing out.
• It is a 41.75-inch tube with the dew shield extended. That length is worth measuring against your case, your car and your observing space before ordering; the shield retracts to bring the tube down to 33.25 inches for transport.
• The published 18lb is the bare tube. Loaded with a diagonal, finder and a 2-inch eyepiece, or with a full imaging train, it sits meaningfully higher, which is why mount headroom matters more than a bare-minimum rating.
• No corrected image circle or backfocus figure is published for this tube, so we quote neither. Send us your camera and we will tell you what we can establish about the spacing.

Frequently Asked Questions

Is it difficult to set up?

The optics need nothing from you — a refractor holds its collimation, so the tube is ready the moment it is on the mount. The work is physical rather than technical: 18lb and nearly 42 inches is a two-handed lift onto a saddle, and it goes more smoothly with the mount already levelled and the counterweights roughly positioned first.

What mount does it need?

One with capacity well above the 18lb bare tube weight, since the diagonal, finder and eyepiece or camera all add to it. For imaging, the useful figure is usually around half a mount's visual rating. Tell us what you own and we will give you an honest read on whether it will carry this tube.

Will I see false colour?

An air-spaced triplet using FCD100 ED glass is built specifically to remove it. On the brightest targets at very high power some residual is inherent to any refracting design, but it is not the limiting factor in this telescope the way it is in an achromat.

Does it come with eyepieces or a diagonal?

Neither is listed in the vendor's published package. The tube arrives with its 2.5-inch focuser, dew shield, cradle ring and Vixen dovetail. We can supply a diagonal and a starting eyepiece set alongside it.

Bottom Line

In short: a 127mm f/7.5 air-spaced triplet apochromat on FCD100 ED glass, 952mm focal length, with a 2.5-inch focuser rated to 10 lbs of imaging gear, a retractable dew shield, a cradle ring with carry handle and a Vixen dovetail — five inches of low-maintenance refractor aperture that serves visual and photographic use equally. If you are matching it to a mount or planning the imaging train, send us the details and we will work through the fit with you before you order.