Backstory:

My Newtonian telescope came with a cheap focuser which does not allow me to focus on a DSLR. This issue is common with entry-level Newtonians and can be solved with either photographing through a lens (eypiece with camera adapter / Barlow / specially designed lenses) which "resets" the focal plane or with a better focuser which goes deeper into the telescope. I wanted to photograph with no additional lenses (aka "prime focus") but slimmer focusers are expensive (may be worth it, though).
I was afraid that with a traditional rack and pinion focuser design, I could not achieve a fine enough level of control of the focus distance so I chose a helical design, which is just a glorified screw. In retrospective, it would have been fine with a traditional design which has some big advantages:

• The camera retains the same orientation.
• Easier to use.
• Faster to travel large distances, needed when I use eyepieces.

Design parameters

The outer diameter on my telescope is 14 cm. The original focuser was attached with 3 M4 screws arranged as a triangle. I had to match that.

The camera (Canon EOS 1300D) with T2-1.25 in adapter needs to go 54 mm deeper than eyepieces. The focus plane range is about 2-3 cm, between stars and close objects. Therefore, my focuser needed to travel at least 7 cm and still have 2 cm for support and 1 cm just in case (adjusting collimation moves the focal plane). I made it 10 cm.

The focuser needs to solidly support 500 grams of expensive camera and / or eypieces even 10 cm out of the telescope tube. It needs to move easily enough but with no backlash.

I wanted some quick way to measure how far into the telescope is the focuser. I cut a slot in the screw and placed some markings with a marker.

An easy way to add a grippable surface at the front of the screw was to use a gear. Due to the holes for the screws, it can't actually be used as a gear if you want to motorize the focuser (not easily).

Issues with the current design:

Due to the design, I have to turn the focuser 27 times to change focus between camera and eyepieces. Also, the camera rotates with the focuser. Once I am close to focus, I still need to rotate it a lot to fine-tune it. This make the focuser quite awkward to use. For the next versions, I may increase the pitch from 2 mm (current) to maybe 10 mm / turn. 2 or 3 parallel threads maybe?

You will have to check tolerances. I expected this to be a problem and I made a pair of tester objects. I eventually set the outer diameter parameter of the screw to 32 mm - 0.25 mm and the inner diameter parameter of the shell to 32 mm + 0.25 mm.

I made the screw a bit to long and it hit the secondary mirror, so I sanded down about 5 mm out of the end of the screw. I also need to sand the interior of the focuser because my camera adapter fitted too tightly. As a result, eyepieces now fit to easily and I need to secure them with 3 M3 screws. I used long screws to use them as handles.

I did not measure correctly the placement of the screws which attach the focuser to the telescope. I had to use a drill to slightly enlarge the holes and some persuation to fit 3 M4 screws.

Next iterations / improvements

I don't intend to do another version anytime soon. My priorities are improving the mount and the motorization. In the far future, I may do a complete redesign with motorized control, camera rotation, and faster manual focusing. If I mange to do a good EQ mount, camera rotation may not be needed.

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