Event Horizon EVRZ ZWO EAF Robotic Telescope Focuser - ASCOM - Cassegrain Adaptation

Product Overview

The Event Horizon EVRZ is a Crayford focuser machined to carry a ZWO EAF focus motor, threading onto the rear port of a Schmidt-Cassegrain telescope. It is the focuser mechanics and the EAF mounting adaptation — the ZWO EAF motor itself is not included. The drive is a CNC-machined friction Crayford with 0.565″ (14.3 mm) of travel, an 8 lb load capacity, a 2″ drawtube with a compression ring, and a published calibration of 2.54 µm of drawtube travel per EAF motor step. Each unit is assembled and tested individually in JMI's facility in the United States, and the focuser weighs 1.4 lb.

Who It's For

This is a good match if you already run a ZWO EAF, or plan to, and want it driving something better than the telescope's mirror. Bolting an EAF to the stock SCT focus knob motorises the mirror — and the mirror is exactly where SCT backlash and image shift come from. The EVRZ gives the motor a friction-drive Crayford to turn instead, so an autofocus run that reverses direction gets the travel it asked for and the star stays on the same pixel.

It suits remote and automated rigs in particular: an observatory SCT that refocuses on a temperature delta, or a filter-change autofocus sequence that has to land on the same focus point every time. If you run a Pegasus FocusCube 3 instead, the EVRP is the same focuser with the Pegasus mounting; if you focus by hand, the EV1-C, EV2-C and EV3-C are the manual versions.

Key Features & Design

• Machined for the ZWO EAF: the motor mounts with two screws using the couplers already included in the EAF kit — no bracket, no adapter, no fabrication.
• ASIAIR and ASCOM control: focus is driven from ASIAIR and from any ASCOM client, so it drops into an existing workflow.
• Autofocus in ASIAIR, NINA and Sequence Generator Pro: and other packages that speak ASCOM.
• Calibrated at 2.54 µm per motor step: a published figure, so autofocus step size is a calculation rather than a guess.
• 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 together.
• Standard SCT rear thread: threads directly to the rear cell of Cassegrain, R-C and Maksutov telescopes that use standard SCT rear threads.
• CNC-machined and made in the USA: individually assembled and tested in JMI's own facility; 1.4 lb.

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 the motor reverses direction — the drawtube starts moving when the shaft does, and it moves along its own axis rather than tipping. For an autofocus routine that is the whole game: the curve it measures on the way in still describes the mechanism on the way back out.

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 EVRZ 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

• Automated autofocus on an SCT, classical Cassegrain, R-C or Maksutov with a ZWO EAF.
• Filter-offset autofocus sequences in NINA or Sequence Generator Pro that must return to a repeatable focus point.
• Temperature-compensated refocusing through a night in a remote or observatory rig.
• ASIAIR-based imaging where focus is driven from the tablet rather than by hand.
• Locking the primary mirror and giving the EAF a mechanism that holds focus, 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.

Motor side. The focuser is machined to take a ZWO EAF, which mounts to it with two screws using the couplers already supplied in the motor kit — there is no separate bracket to buy and no adapter to fabricate. Once mounted, focus is controlled through ASIAIR and ASCOM interfaces, so autofocus routines in ASIAIR, NINA and Sequence Generator Pro drive it directly.

Step size. This focuser is calibrated at 2.54 µm of drawtube travel per motor step. That figure is what an autofocus routine needs to size its step interval, and JMI publishes a free Autofocus Step-Size Calculator with this focuser's calibration already loaded — pick your telescope and camera and it returns the number to enter.

It adds length to the imaging train. Because it sits between the rear cell and everything behind it, a camera train with a fixed back focus figure — behind an SCT reducer or corrector — is worth adding up before the focuser goes on. Optically it changes nothing: 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, the camera and what else is in the train — filter wheel, off-axis guider, reducer — and we will work the spacing out with you.

Good to Know Before You Order

• The ZWO EAF motor is not included. This is the focuser mechanics with the EAF mounting adaptation. You supply the EAF and its couplers, which come with the motor kit. We stock the EAF — ask and we will put the two together on one order.
• 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, tell us what is on it and we will say whether this is the right focuser.
• Adapters are sold separately. The 2″ drawtube takes 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. The focuser threads onto the SCT rear port like a visual back, and the EAF mounts to it with two screws using the couplers in your EAF kit. From there it is the normal EAF setup — connect in ASIAIR or ASCOM and run a calibration.

Why not just put the EAF on my SCT's focus knob?

Because that knob moves the primary mirror. Motorising it automates the backlash and the image shift rather than removing them. The EVRZ gives the motor a friction-drive Crayford instead, so the movement is repeatable and the image stays put.

What autofocus step size should I use?

Start from the published 2.54 µm per motor step and size the interval to your telescope's critical focus zone. JMI's free step-size calculator does the arithmetic with this focuser's calibration already entered.

Does it work with NINA and Sequence Generator Pro?

Yes — the EAF presents as an ASCOM focuser, so NINA, Sequence Generator Pro, Ekos and other ASCOM clients drive it, as does ASIAIR directly.

Can I still focus it by hand?

The EVRZ is built around the motor mounting. If hand focusing is what you want, the EV1-C, EV2-C and EV3-C are the manual versions of the same Crayford mechanism.

What is the difference between the EVRZ and the EVRP?

Only the motor they are machined for. The EVRZ takes a ZWO EAF and is calibrated at 2.54 µm per step; the EVRP takes a Pegasus FocusCube 3 at 2.63 µm per step. The focuser itself is the same.

Does it fit a Newtonian?

No — this is the Cassegrain base, which threads to an SCT rear port. There is a Newtonian-base EVRZ; ask us and we will get you the right one.

Bottom Line

In short: a friction-drive Crayford machined to carry a ZWO EAF on the back of an SCT, with 0.565″ (14.3 mm) of zero-backlash travel, an 8 lb capacity and a published 2.54 µm per motor step. Bring your own EAF, and the autofocus routine finally drives a mechanism that holds focus.