CNC-Machined in the USA
Event Horizon EV1 — Dual-Speed 10:1 Crayford Focuser for GSO Newtonians
Our flagship manual focuser. A dual-speed Crayford drive with a 10:1 fine-focus knob and a 2.75″ GSO bolt pattern — a drop-in replacement for the stock focuser on Apertura, Zhumell and other GSO Newtonians.
Drop-in replacement for GSO focusers — no drilling
The Event Horizon EV1 uses the same base footprint as the stock GSO focuser: 2.75″ (69.85 mm) mounting-hole spacing on a 3.5″ (88.9 mm) diameter base. On a GSO-built Newtonian the existing holes line up, so the swap is four screws out, four screws in — no drilling, no adapter plate, no focuser board.
2.75″hole spacing
(69.85 mm)
(69.85 mm)
3.5″base diameter
(88.9 mm)
(88.9 mm)
8 lbload capacity
1.4 lbfocuser weight
Not sure what you have? Measure center-to-center between the mounting screws on your current focuser base. If it reads 2.75″, the Event Horizon EV1 bolts straight on.
A masterpiece of engineering and workmanship, the JMI Event Horizon EV1 is a low-profile, friction-drive Crayford focuser for the discriminating astronomer. The Crayford design presses a metal drive shaft directly against a machined metal drawtube held by bearings on the opposing side — a direct surface-to-surface drive that eliminates the backlash and drawtube shift of rack-and-pinion designs and allows impossibly fine adjustments. Zero image shift and zero backlash make it superb for both visual use and imaging.
Specifications
- ✓ Dual-speed drive with 10:1 fine-focus knob
- ✓ Rubber grip knobs and a compression ring for secure, mar-free holding of eyepieces and cameras
- ✓ Zero image shift, zero backlash — CNC-machined friction-drive Crayford design
- ✓ Drawtube length of travel is configurable at the time of order (see drop-down menu)
- ✓ Base diameter 3.5″ (88.9 mm) with 2.75″ (69.85 mm) mounting-hole spacing — matches the stock GSO focuser pattern
- ✓ Drop-in replacement on Apertura, Zhumell, Astro-Tech and other GSO-built Newtonians and astrographs — no drilling required
- ✓ Compatible with Newtonian telescope tubes 7.25″ in diameter or larger, as well as flat focuser boards
- ✓ Load capacity 8 lb · focuser weight 1.4 lb
- ✓ Individually assembled and tested in our own facility — 100% made in the USA
Will it fit my telescope?
Yes, if your Newtonian was built by GSO — which covers a large share of the imaging Newtonians and mid-to-large Dobsonians sold in the US. Confirmed drop-in fits include:
- Apertura Dobsonians — AD6, AD8, AD10, AD12 and the DT series
- Zhumell Dobsonians — Z8, Z10, Z12
- GSO imaging Newtonians and astrographs — 6″, 8″, 10″ and 12″ f/4 and f/5 tubes, including the Astro-Tech AT6IN / AT8IN / AT10IN and many GSO-built Orion imaging Newtonians
- GSO tubes sold under other badges — Revelation Astro, TS-Optics, Omegon, Bintel, Kson, Danubia and similar rebrands
- Any Newtonian or focuser board with 2.75″ mounting-hole spacing and a tube diameter of 7.25″ or larger
Telescopes built on a different footprint — many Sky-Watcher, Celestron and Synta-built Orion Dobsonians among them — use a different hole pattern, so drilling additional mounting holes in the tube or focuser board may be required. The opening in the tube is rarely the obstacle: the hole already cut for your existing focuser will almost certainly clear the Event Horizon EV1’s drawtube, so the work usually comes down to the mounting screws. Measuring your existing focuser base first takes thirty seconds and settles the question.
Fitting note: On a GSO-pattern telescope this is a true bolt-on swap. On any other Newtonian, drilling additional mounting holes in the tube or focuser board may be required — but the opening already cut for your existing focuser will almost certainly clear the Event Horizon EV1’s drawtube, so the job is normally limited to the mounting screws rather than enlarging the hole. Many publicly available tools and active community forums can help you plan it. Also check your tube balance afterward: at 1.4 lb the Event Horizon EV1 is light, and a lighter focuser can shift the balance point slightly on a Dobsonian.
How the Event Horizon focusers compare
| Model | Drive | Load capacity | Best for |
|---|---|---|---|
| EV3 | Single-speed friction drive | 4 lb | Visual observers who want zero backlash at the lowest cost |
| EV2 | Single-speed, rubber grip knobs | 8 lb | Heavier eyepieces and camera trains, focused by hand |
| EV1 (this focuser) | Dual-speed with 10:1 fine focus | 8 lb | Critical manual focus — planetary, high power, imaging |
| EVRZ | Single-speed + ZWO EAF mount (motor not included) | 8 lb | ASIAIR, N.I.N.A. and SGP autofocus routines |
Every Event Horizon Newtonian-base focuser shares the same fitment: 2.75″ mounting-hole spacing, 3.5″ base diameter, 1.4 lb, 2″ drawtube with compression ring, configurable travel, CNC-machined and assembled in the USA.
Frequently asked questions
Will this focuser fit an Apertura or Zhumell Dobsonian?
Yes. Apertura and Zhumell Dobsonians are built by GSO, and the Event Horizon EV1 matches the stock GSO focuser footprint exactly — 2.75″ mounting-hole spacing on a 3.5″ diameter base. The existing holes in the tube line up, so no drilling or adapter plate is required.
How do I confirm my telescope has the GSO focuser pattern?
Measure center-to-center between the screws holding your current focuser to the tube. A spacing of 2.75″ (69.85 mm) means the Event Horizon EV1 is a direct bolt-on replacement. Your tube also needs to be 7.25″ in diameter or larger, or you can mount the focuser to a flat focuser board.
Does it work on GSO imaging Newtonians and astrographs?
Yes. GSO f/4 and f/5 imaging Newtonians — including tubes badged as Astro-Tech AT6IN, AT8IN and AT10IN, and many GSO-built Orion imaging Newtonians — use the same 2.75″ focuser pattern as the Dobsonians, so the Event Horizon EV1 drops straight in.
What if my telescope is not a GSO?
You may need to drill additional mounting holes in the tube or focuser board to match the 2.75″ spacing of the Event Horizon EV1. The hole already cut in your tube for the existing focuser will almost certainly clear the drawtube, so the fitting work is normally limited to the mounting screws rather than enlarging the opening.
What is the difference between a Crayford and a rack-and-pinion focuser?
A rack-and-pinion focuser drives a toothed rack with a gear, and the tooth clearance produces backlash and image shift. A Crayford presses a smooth metal drive shaft directly against the drawtube, with bearings on the opposite side. That surface-to-surface contact has no gear lash, so the Event Horizon EV1 has zero backlash and zero image shift.
How much drawtube travel does it have?
Travel is configurable at the time of order — choose the length of travel from the drop-down menu on this page. Picking the right travel matters most on imaging Newtonians, where the camera sits far outside the tube.
Where is it made?
The Event Horizon EV1 is CNC-machined, individually assembled and tested in our own facility in the United States.
Why does a fast Newtonian need a 10:1 fine-focus knob?
The faster the focal ratio, the shallower the depth of focus. On an f/4 or f/5 Newtonian the in-focus zone is only a few tens of microns deep, and a single-speed knob moves the drawtube too far per degree of rotation to land inside it reliably. The 10:1 reduction on the EV1 divides each turn by ten, which turns critical focus into a repeatable adjustment.
Is the EV1 good for planetary imaging?
Yes. Planetary imaging at high magnification is exactly where dual-speed fine focus and a zero-image-shift drive pay off, because nudging focus during a capture run does not walk the planet across the sensor.
Can the EV1 be motorized?
The EV1 is the flagship manual focuser. For automated focusing, the EVRZ is built to carry a ZWO EAF motor, and the EVRP is the motor-ready alternative.
Optional Optical Adapters
- JMI Combination 1.25″ Eyepiece & T-Thread Adapter (ADPT2-1.25T) — for camera use. Mounts a CCD or other T-threaded camera into a 2″ drawtube, and doubles as a 1.25″ adapter. Uses a compression ring and includes 48 mm filter threads on the focuser end.
- Farpoint 2″ Focuser to 1.25″ Eyepiece Adapter (FAV100) — a visual 2″-to-1.25″ eyepiece adapter with a non-marring compression ring. Threaded for both 2″ and 1.25″ filters, with a stop to keep long eyepieces from damaging filters.
New to Crayford focusers? Read the Event Horizon Focusers guide. Want autofocus? See the motor-ready EVRZ and EVRP versions. Prefer single-speed? See the EV2. Have a Schmidt-Cassegrain instead? See our SCT Focusers.
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