JMI EV3-C Standard Event Horizon Telescope Focuser - Cassegrain

Product Overview

The JMI Event Horizon EV3-C is the standard single-speed Crayford focuser in the Event Horizon Cassegrain line. It threads onto the rear port of a Schmidt-Cassegrain telescope and adds a second, independent focus stage: a friction drive with 0.565″ (14.3 mm) of travel, a 4 lb load capacity, and a 2″ drawtube with a compression ring. The body is CNC-machined and each unit is assembled and tested individually in JMI's own facility in the United States. At 1.4 lb it adds very little to what the rear cell already carries.

Who It's For

This is a good match if the reason you want a Crayford on your SCT is the mechanism itself — zero backlash, zero image shift while focusing — and the back of the scope carries a visual train or a light camera. It is the most direct version of the Event Horizon: one speed, a compression ring, and nothing else to pay for.

The 4 lb capacity is what separates it from the rest of the line. A diagonal and an eyepiece, or a compact camera, sit inside that comfortably. A full imaging train with a filter wheel and an off-axis guider is the EV2-C's job, and a 10:1 fine knob for high-magnification work is the EV1-C's.

Key Features & Design

• Single-speed friction drive: one knob, direct feel, and none of the backlash a rack-and-pinion focuser gives on reversal.
• Compression ring: holds eyepiece and adapter barrels securely without marking them.
• 0.565″ (14.3 mm) of fine-focus travel: enough range to cover focus drift through a night once the mirror has set rough focus.
• 4 lb load capacity: sized for a visual train or a compact camera on the back of the scope.
• Standard SCT rear thread: threads directly to the rear cell of Cassegrain, R-C and Maksutov telescopes that use standard SCT rear threads.
• Low-profile body: adds the focus stage without pushing the eyepiece or camera far off the back of the telescope.
• 1.4 lb focuser weight: a small addition to the back end, and it sits close to the rear cell rather than out on a lever arm.
• CNC-machined and made in the USA: individually assembled and tested in JMI's own facility.

Mechanical Design

A Crayford focuser has no rack and no pinion. A polished steel drive shaft presses directly against the flat of a machined metal drawtube, and bearings on the opposite side carry the load and hold the tube square. Because the drive is a friction contact rather than meshing teeth, there is no tooth-to-tooth clearance to take up when you reverse direction — the drawtube starts moving when the knob does, and it moves along its own axis rather than tipping. That is what zero backlash and zero image shift describe in practice: a star stays on the same pixel as you rack through focus.

On a Schmidt-Cassegrain this matters twice over, because the telescope's own focus knob moves the primary mirror, and mirror movement is where SCT image shift and focus backlash come from. The Event Horizon threads onto the rear cell and adds a second, independent focus stage behind it. Rough focus happens once on the mirror; every critical adjustment after that happens on the Crayford, across 0.565″ (14.3 mm) of travel.

Recommended Uses

• Visual observing on an SCT, Maksutov, classical Cassegrain or R-C, with a diagonal and eyepiece.
• Lunar and planetary observing where mirror-flop image shift makes touching focus a nuisance.
• Manual focusing with a compact camera, on a Bahtinov mask or a live-stacked frame.
• Adding a fine-focus stage to a telescope whose mirror knob is the only focus control it has.
• Locking the primary mirror and doing all focusing behind it, on telescopes that have a mirror lock.

Compatibility and Accessory Notes

The base threads to the standard SCT rear port, so it fits Schmidt-Cassegrain, classical Cassegrain, Ritchey-Chrétien and Maksutov telescopes that use standard SCT rear threads. The drawtube is 2″ and holds eyepieces and camera adapters with a compression ring, so nothing gets a set-screw mark on the barrel.

It adds length to the imaging train. Because it sits between the rear cell and everything behind it, a visual train reaches focus comfortably, while a camera train — especially one behind an SCT reducer or a corrector with a fixed back focus figure — needs its spacing budget checked before the focuser goes on. Focal reducers and field flatteners are unaffected optically: the Event Horizon is a mechanical stage with no glass in it.

Three optional adapters are made for the 2″ drawtube. Ontario Telescope does not stock them at the moment — if you want one alongside the focuser, send us a note and we will bring it in with the order.

• JMI ADPT2-1.25T combination 1.25″ eyepiece and T-thread adapter: mounts a T-threaded camera into the 2″ drawtube and doubles as a 1.25″ eyepiece adapter. Compression ring, with 48 mm filter threads on the focuser end.
• Lumicon LA1060 2″ male to SCT male adapter: drops into the 2″ drawtube and presents male SCT threads (2″ × 24 tpi) for SCT-threaded accessories such as an SCT diagonal. Includes 2″/48 mm filter threads.
• Farpoint FAV100 2″ to 1.25″ eyepiece adapter: a visual adapter with a non-marring compression ring, threaded for both 2″ and 1.25″ filters, with a stop that keeps long eyepieces clear of the filter.

Ask us if you are not sure it fits. The one thing worth checking before you order is whether your telescope's rear port is a standard SCT thread and whether you have the back focus to spare. Tell us the telescope and what you plan to hang off the back — diagonal and eyepiece, or camera, filter wheel and off-axis guider — and we will work it out with you.

Good to Know Before You Order

• Load capacity is 4 lb. That covers a diagonal and eyepiece or a compact camera. For a full imaging train with a filter wheel and off-axis guider, the EV2-C is the 8 lb version of the same focuser — tell us what is on your back end and we will say which one fits.
• It is single-speed. There is no 10:1 fine knob. For high-magnification planetary and double-star work where the last micron matters, the EV1-C adds that reduction on the same mechanism.
• It is a manual focuser. There is no motor and no electronics. For computer-controlled autofocus, the EVRZ (ZWO EAF) and EVRP (Pegasus FocusCube 3) versions of the same focuser are the ones to look at.
• The travel is fine-focus range, not full range. 0.565″ (14.3 mm) covers critical focus and a night's thermal drift. Rough focus still happens on the telescope's own mirror knob, which is how the focuser is meant to be used.
• Adapters are sold separately. The 2″ drawtube takes eyepieces and 2″ accessories directly; a T-threaded camera or a 1.25″ eyepiece needs one of the adapters listed above.

Frequently Asked Questions

Is it difficult to set up?

No. It threads onto the SCT rear port the same way a visual back does, and there is nothing to align, shim or calibrate. Set rough focus on the mirror once, then use the focuser from there.

How do I know whether 4 lb is enough?

Add up what hangs off the back: diagonal, eyepiece, adapters, or camera, filter wheel and guider. A 2″ diagonal with a big eyepiece is usually around 2–3 lb and fits; a full mono imaging train usually does not. Send us the list and we will total it with you.

Does this replace my telescope's focuser?

No, it works with it. The mirror knob still sets rough focus and covers the full focal range; the Event Horizon handles the critical adjustment at the end, without moving the mirror.

What does the Crayford design get me over the stock SCT focuser?

The stock knob moves the primary mirror, which tips and shifts as it travels. The Crayford moves only the drawtube, along its own axis, with no backlash on reversal — so the target stays put while you focus.

What is the difference between the EV3-C and the EV2-C?

Same mechanism and the same 14.3 mm of travel. The EV3-C is the standard model rated to 4 lb; the EV2-C adds rubber grip knobs and is rated to 8 lb.

Does it fit a Newtonian?

No — this is the Cassegrain base, which threads to an SCT rear port. JMI makes Newtonian-base versions of the Event Horizon; ask us and we will point you at the right one.

Bottom Line

In short: the straightforward Event Horizon — a single-speed friction-drive Crayford rated to 4 lb, with 0.565″ (14.3 mm) of zero-backlash travel behind the SCT rear cell. It suits a visual train or a compact camera, and it takes the mirror out of the focusing.