Make an autofocuser for a camera lens

Take your ultra-wide deep-sky imaging to the next level.

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Image Credit: 
Steve Richards
Our setup atop the completed dolly; the tripod plates should be as close to the wheels as possible
Credit: Mark Parrish

 

Written by Steve Richards

 

Modern camera lenses have built in autofocusers that connect to the camera’s central processor when attached to a matching DSLR camera.

These focus systems are designed for daytime use or well-lit night-time scenes and are unable to focus on stars, making them unsuitable for astronomy.

However, the lenses themselves can be excellent for ultra-wide deep-sky imaging, so finding a way to focus them accurately is a worthwhile exercise.

 


Components - Autofocus motor, connection cables, power supply, 12-tooth pulley, bore reducer (if required), high torque drive timing belt, dovetail bar, camera and lens 

Materials - Aluminium strip (2x30mm), 5mm stainless steel bolts, ¼”-20 bolt

Software - Autofocus controller software

Tools - Digital callipers, power drill with 4mm 0.25-inch bits, countersinking tool, fine-tooth metal hacksaw, fine-tooth flat file, needle file, bench vice, 5mm tap, Allen keys, crosshead screwdriver

Sundries - Masking tape, marker pen


 

A Bahtinov mask focused on a bright star can work, but the focus adjustment requires tiny movements.

That makes accurate focus difficult to achieve, and changing temperatures during an imaging session mean that refocusing must be done periodically.

The automated focus system we describe here allows for minute focus movements and for regular refocusing to take place automatically.

We already had an autofocus controller for our imaging telescope that we could use for this project so only an additional focus motor was required.

When used with a conventional telescope, the motor attaches to the focus shaft via a flexible coupling, but for use on a camera lens with a focus ring, a different method must be employed.

The simplest solution is to turn the motor sideways and couple it to the lens’s focus ring using a small timing belt.

 


You’ll need a 12-tooth pulley for your motor (left); use a reducer (right) to match its bore
Credit: Steve Richards

 

Flexible coupling

Remove the coupler and measure the diameter of the motor’s output shaft so that you can obtain a timing belt pulley that fits correctly.

Our shaft had a diameter of 5mm but the only suitable pulleys close to this size had a bore of 6mm, so we bought a 6mm to 5mm bore reducer to go with the 12-tooth high torque drive pulley selected for the project.

To determine the correct length for the timing belt, lay the motor alongside the camera lens and loop a dress-maker’s tape measure around the motor shaft and lens barrel.

Purchase the nearest size to this measurement, which in our case was 390mm with 130 teeth.

Ideally you should use a Losmandy-style dovetail bar to mount everything on as these have a wide top, but if your mount will only accept the much narrower Vixen-style bar, this will also work.

If your dovetail bar has a recessed bolt slot underneath, use a ¼”-20 bolt to attach your camera and lens to the bar. 

A slotted dovetail bar will allow you to accommodate different lenses.

If there is no slot then drill and countersink a 0.25-inch hole and use a countersunk ¼”-20 bolt for attachment. 

Bolt the focus motor to one end of the aluminium strip to form a bracket, with the drive shaft at a right angle to the strip.

Loop the timing belt over both the pulley and the camera’s focus ring and pull the motor away from the dovetail bar but level with its top.

This will allow you to determine the position of the aluminium strip on the top of the dovetail bar.

Mark both edges of the strip on the dovetail bar, and mark the far edge of the dovetail bar on the aluminium strip.

Remove the camera and lens and drill two 4mm holes in the dovetail bar, as far apart as possible, centred on the two marks.

Use a 5mm tap to thread the holes in readiness for mounting the motor bracket.

Cut off the excess aluminium strip at the edge mark you have just made then drill and slot two 5mm holes to match those on the dovetail bar.

Longer slots will allow different camera lenses to be accommodated.

Attach the motor bracket loosely to the dovetail bar, firmly attach the camera and lens to the dovetail bar with the lens set to manual focus and loop the timing belt round the pulley and focus ring again.

To complete the assembly, gently pull the motor and bracket outwards until the timing belt has no slack in it and tighten the two 5mm bolts to lock the bracket firmly in position.

Finally, install the ASCOM driver software on your computer and configure all the required parameters such as step size, focal ratio and your preferred focus exposure time.

You can now use the autofocus controller to adjust the focus of the camera lens either manually or automatically under software control.

 


 

Step-by-step

 

Step 1 - Remove the coupler supplied with the motor from the motor shaft by undoing the retaining grub screw with an Allen key. Measure the shaft diameter with a set of callipers. This will tell you what bore size you’ll need for selecting the correct pulley.

 

Step 2 - Attach the motor to a length of the aluminium strip, either directly or via the mounting bracket that it was supplied with. Attach the pulley to the shaft, using a bore reducer if necessary, seating it as close to the motor body as possible.

 

Step 3 - Attach the camera and lens combination to the dovetail bar with the timing belt looped around the focus ring and the pulley. Slide the motor outwards to tighten the belt and mark the position of the far edge of the dovetail bar on the aluminium strip.

 

Step 4 - Mark the edges of the aluminium strip on the dovetail bar. Cut off the excess aluminium and file the edge smooth. Drill two 4mm holes through the top of the dovetail bar in a line centred on the aluminium strip and thread them with a 5mm tap.

 

Step 5 - Drill and file two 5mm wide slots in the centre of the aluminium strip to match the positions of the two dovetail holes so that the timing belt can be adjusted for tension. With the timing belt in position, attach the aluminium strip with 5mm bolts.

 

Step 6 - Connect the motor to the autofocus controller and fill in all the parameters required by the software. The ‘step size’ will be difficult to determine as you can’t measure this; it will take a little trial and error under clear skies to ascertain the correct figure.

 

Steve Richards is BBC Sky at Night Magazine’s Scope Doctor

 

 

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