Disassembling a JMI NGF Mini2M motorized focuser

Note: I will shortly publish updated instructions

I take no responsibility nor do I claim the instructions given below will yield success. The instructions below are only provided for help. Please do not misuse the information provided on this page.

Motivation for the disassembly

In order to adapt a JMI NGF Mini 2M focuser to my TAL-200K visual back, I had to disassemble the focuser in order to avoid metal chips marring fragile parts while machining. WARNING: do not attempt at disassembling this focuser without understanding what you do. Make sure you know how the procedure works, and make sure you understand you may break things. The pressure of the friction axis on the focuser drawtube is critical. Applying too much pressure will damage the focuser drawtube by imprinting the axis on it. You have been warned.

Before the operation

Removing the focuser knob close to the cogwheel, and the cogwheel

The focuser knobs and the cogwheel are secured to a flattened section of the stainless steel axis that controls the focuser movement. Their placement is secured with a recessed screw requiring a small Allen wrench. Give the securing screw a half turn to losen the knob. Carefully slide it off the axis. Then repeat the operation with the cogwheel:

After removal of the cogwheel:

Removing the motor assembly

The motor assembly is secured by 2 bolts: one that actions the 'clutch' (left in picture), and one that provides a pivot for the clutching effect (right in picture). The former has a plastic end and unscrews easily. The latter requires an Allen wrench. Be very prudent when unscrewing both bolts, as the motor will likely fall out of the focuser frame. Remove the plastic washer too.

Views from the motor assembly:

The blade spring for the clutch action can be seen in the photo below: it is secured with a bolt requiring an Allen wrench. Unscrew the blade spring.

Loosen the friction axis

The friction axis (from which the focuser knobs and the cogwheel were removed) runs in a small bearing and a bronze bushing. This axis is pressed onto the focuser drawtube by means of 2 recessed screws (again, requiring an Allen wrench):

Remove the focuser drawtube

Unscrew the 2 recessed screws, and gently but firmly press the axis away from the focuser drawtube. You should be able to gently remove the drawtube. You may have to untighten both screws a bit more, and to rotate the focuser knob a bit to allow the focuser drawtube to be lifted entirely from the focuser frame:

Remove the friction axis and the remaining focuser knob

Note that this step can also precede the previous step (actually, doing so is highly recommended). Make sure in any case that the axis is removed in such a way that the bearing remains on the axis. This implies you have to remove focuser knob and the bronze bushing first.

First remove the focuser knob (unlock the recessed bolt with an Allen wrench), remove the 2 black plastic washers and spacer, and then untighten both recessed bolts that secure the friction axis by its bushing and its bearing:

Remove the 4 bearings that keep the drawtube in place

A flat screwdriver is required for unscrewing the 4 small bearings that secure the drawtube orientation. Those screws are fragile.

Overview of all focuser parts

Close-up of motor assembly:

Close-up of friction axis. Parts (left to right: focuser knob, washer, spacer, washer, bronze bushing, bearing, washer, cogwheel, focuser knob):

Remove tilt adjustment screws from focuser frame

The base of the focuser can be "collimated" with the help of 4 recessed screws. They can be removed with an Allen wrench:

Bottom view of disassembled focuser baseplate

In this picture the focuser is entirely disassembled. Some tapped holes are still protected with plastic "shields" that can be removed with some amount of patience. If not required, you can leave them.

Focuser frame on stripped visual back of the TAL-200K

The image below shows how the JMI focuser will be positioned with respect to the 6 collimation screws of the TAL-200K telescope.

Jorrit's picture

Vraag over TAL-modificatie

Beste Olivier,
Met veel interesse lees ik uw bericht op uw blog en op Cloudy Nights over de modificatie van uw TAL 200K. Ik ben zelf ook bezitter van zo'n TAL, maar niet erg tevreden over de focusser. Veel speling, niet erg robuust en daardoor ongeschikt voor astrofotografie.
Ik zou graag een zelfde soort ombouw uitvoeren van mijn telescoop, en zie op Cloudy Nights dat ene 'Chriske' van Urania destijds een plaat voor u heeft vervaardigd. Zou ik hem kunnen benaderen met de vraag of hij nog zo'n plaat zou kunnen maken - uiteraard tegen vergoeding? Mogelijk heeft hij de tekeningen er nog van liggen. In dat geval kunt u mij misschien met hem in contact brengen.
Complimenten overigens voor uw website: zeer leerzaam!
Met vriendelijke groet,
Jorrit van Broekhoven
Arnhem, Nederland

Olivier Biot's picture

Re: Vraag over TAL-modificatie

Dag Jorrit,
Ik heb nog altijd een "halffabrikaat" thuis liggen. Het bestaat uit een ijzeren plaat en een ijzeren bus die aan de plaat wordt bevestigd om de plaat in de draaibank op de juiste afmetingen af te draaien. De 6 grote collimatiegaten zijn reeds uitgeboord. Ik zal eens horen Chris die 2de plaat kan draaien. De afmetingen moet ik terug opzoeken, of opmeten van de oorspronkelijke TAL focuser.

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