JMI EV1-C Dual-Speed Event Horizon Telescope Focuser - Cassegrain

Product Overview

The JMI Event Horizon EV1-C is a dual-speed Crayford focuser that threads onto the rear port of a Schmidt-Cassegrain telescope and gives it a second, independent focus stage. It is the dual-speed model in the Event Horizon Cassegrain line: a friction drive with a 10:1 fine-focus knob, 0.565″ (14.3 mm) of travel, an 8 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 you focus a Schmidt-Cassegrain by hand and want the last few microns under finer control than the mirror knob gives. The 10:1 reduction is the reason to choose the EV1-C over the single-speed EV2-C: at high magnification on the Moon, a planet or a tight double, coarse and fine focus live on separate knobs, so you can creep up on best focus without overshooting.

It also suits imagers who focus manually — on a Bahtinov mask, or by eye on a live-stacked frame — and anyone who wants the mechanical benefits of a Crayford stage without adding a motor. If autofocus is the goal instead, the EVRZ and EVRP versions of the same focuser take a ZWO EAF or a Pegasus FocusCube 3.

Key Features & Design

• Dual-speed drive with a 10:1 fine-focus knob: coarse and fine focus on separate controls, for the finest adjustment available by hand.
• Friction-drive Crayford mechanism: no rack and pinion, so no backlash on reversal and no drawtube tilt through the stroke.
• 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.
• 8 lb load capacity: carries a camera, filter wheel and off-axis guider, or a 2″ diagonal and a heavy wide-field eyepiece.
• Rubber grip knobs and a compression ring: positive grip with gloves on, and no set-screw marks on eyepiece or adapter barrels.
• Standard SCT rear thread: threads directly to the rear cell of Cassegrain, R-C and Maksutov telescopes that use standard SCT rear threads.
• 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

• High-magnification visual observing — lunar, planetary and double stars, where the 10:1 knob earns its place.
• Deep-sky visual observing with heavy 2″ eyepieces, held by the compression ring rather than a set screw.
• Manual focusing for imaging, on a Bahtinov mask or a live-stacked frame.
• Any SCT, classical Cassegrain, R-C or Maksutov where mirror-flop image shift makes precise focus tedious.
• 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

• 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.
• It lengthens the imaging train. It sits behind the rear cell, so a camera train with a fixed back focus figure — behind a reducer or corrector — is worth adding up before you order. Send us your setup and we will check the numbers with you.
• Load capacity is 8 lb. That covers a full imaging train on most SCT back ends. If yours is heavier than that, tell us what is on it and we will say whether this is the right focuser.
• 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.

What does the 10:1 knob actually change?

It reduces the drawtube movement per turn by a factor of ten on the fine knob, so a small hand movement becomes a very small focus change. At 300× on a planet, that is the difference between stepping over best focus and settling onto it.

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.

Will it fix mirror flop?

It removes mirror movement from the focusing process, which is where most SCT focus-time image shift comes from. Mirror shift caused by slewing across the meridian is a separate matter, and a mirror lock is the tool for that.

Can I use it for imaging?

Yes. Zero image shift and zero backlash are exactly what a camera wants, and the 8 lb capacity carries a normal imaging train. If you want the focus routine automated rather than done by hand, look at the EVRZ or EVRP.

What is the difference between the EV1-C, EV2-C and EV3-C?

Same Crayford mechanism and the same 14.3 mm of travel throughout. The EV1-C adds the 10:1 dual-speed knob and carries 8 lb; the EV2-C is single-speed with rubber grip knobs and also carries 8 lb; the EV3-C is the standard single-speed model rated at 4 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: a dual-speed friction-drive Crayford that bolts a proper fine-focus stage onto the back of an SCT. It brings 0.565″ (14.3 mm) of zero-backlash travel, a 10:1 fine knob and an 8 lb capacity, and it leaves the mirror where you set it — which is the whole point of putting a focuser behind one.