BullsEye DIY Focuser Assembly

Last Updated on 16th November 2023 by nightowl

Introduction

In this post I will describe how I assembled together the BullsEye DIY Focusers introduced here. The procedure for putting together both the NEMA17 and the NEMA11 variations is just about the same. There isn’t much extra space inside either of the enclosures so order of assembly and wire lengths do matter. You’ll find links to the products I’ve used in the text below.

Stepper Driver

I started the assembly with the TMC2209 SilentStepStick driver. First thing to do is to solder a jumper/bridge between UART and Pin 6. I soldered a small piece of wire onto the jumper pads to do this.

You’ll definitely want to add a heasink onto the stepper driver as it can get hot especially with higher motor currents. I used a 9x9mm 5mm thick aluminium self adhesive heatsink and put it on the other side of the PCB.

Then I soldered the capacitor C3 (100uF 16V) and all the wires coming in and out from the stepper driver. Care must be taken to ensure correct polarity of the C3. I’ve put some guidance on the wire colours in the schematic but it’s up to you whether you want to follow them. I used these wires from ebay. There’s basically four groups of wires:

1. Wire within the board. There’s only one ground wire running from pin 9 to pin 15.
2. Input motor voltage wires. These should be about 12cm long.
3. Wires to and from Arduino. These should be about 18cm long.
4. Wires to the stepper motor. These should be about 10cm long.

I used small pieces of heat shrink tubing to group the wires together which helps both the assembly further down the line and also to identify the wires from each other.

Arduino

Before soldering the Arduino wires to and from the stepper driver it’s best to solder the odd few passive components directly in the Arduino pins together with any wires that should go in the same pins. Here’s how I soldered R3, R4, C2 together with VIN and GND wires (10 cm) that go to the 12V connector in the front panel. Be aware of the C2 polarity if you’re using a tantalum capacitor like I am!

Next I soldered capacitor C1 (10uF) together with the GND wire followed by the red wire (+5V / pin 27) and brown wire (D12 / pin 15). Be sure to put the C1 the right way around. Also be sure to read the design notes regarding C1 here. These three wires (about 12cm) go to the 3.5mm stereo jack used for the temperature sensor. After this I soldered the brown and orange wires (about 12cm) from D10 and D11 to the dual red/green LED cathodes.

Before soldering the rest of the wires to and from the stepper driver I soldered R2 in place together with the orange wire on pin 8 (D5). This orange UART wire goes to pin 6 of the stepper driver.

Now is a good time to solder the wires on the legs of the reset switch SW1 which is a 6mm x 6mm x 4.3mm miniature tactile switch. I cut off the unused legs of the switch so they don’t get in the way when placing the switch inside the enclosure. The other end of the green wire I soldered to the reset pin on the Arduino (pin 3). The black GND wire I soldered onto one of the connectors in the front panel.

Next you can solder the rest of the wires to the Arduino Nano following the schematic. It looks a little messy at this stage but there should be enough space for the wires inside the enclosure if you guide them carefully.

After this you can install the DC jack (J1), the 3mm dual common anode LED (D1) with a collar and the 3.5mm stereo jack for the temperature sensor (U1) onto the enclosure lid. I soldered the resistors R1 and R5 to the legs of these components along with the wires to and from the stepper driver and Arduino.

Preparing The Enclosure

I used M3x10 machine screws to secure the lid onto the enclosure. While you could screw them directly to the plastic, I found it’s better to use v-coil thread inserts for durability. You can get a kit that includes everything you need to do the task. The procedure is fairly simple. First you drill the screw holes to the correct size using the drill bit included in the kit.

Then you tap the holes with the threading tap in the kit. I used a drill to do this but you have to be careful and go very slow. It’s very easy to strip the threads if you go too far. You can also melt the plastic if you go to fast! I suggest to do this with hand tools if you haven’t done anything like this before.

Then you use the installation tool to put the thread inserts at correct depth into the tapped holes. Just below the surface of the plastic is good.

Finally you use the break tool to break off the little tang used for installation.

Putting It Together

Unfortunately I don’t have many photos of how I put all the bits together but the procedure is basically this:

1. Place the reset switch SW1 in it’s holder inside the enclosure.
2. Place the stepper driver into it’s slot with the heatsink up.
3. Connect the stepper motor wires and temporarily secure the motor in it’s place using a couple of screws.
4. Slide the Arduino in it’s slot carefully guiding the wires so that they don’t get squashed.

The Temperature Sensor

I wanted to be able to screw the temperature sensor onto the existing M4 threads on my focuser so I designed a 3D printable enclosure for it that houses the sensor and a short M4 bolt directly in contact with the sensor. For the cable I used an audio cable with 3.5mm stereo plugs which I cut in half so I could make two sensors for both of my focusers.

The photo above shows the wires soldered onto the sensor and insulated with some heat shrink tubing. The red wire is +5V and goes to the tip of the plug at the other end. Black wire is GND and goes to the sleeve of the plug. The white wire is the data from the sensor and goes to the ring of the plug.

The blue sleeve screws onto the sensor housing over strain relief piece which locks the cable in place. For this to work properly the cable needs to be about 3.5mm in outside diameter.

Final Installation

Obviously every telescope is different and I can’t possibly cater for all of them. In the design files I’ve included an L-Bracket that fits a William Optics Fluorostar FLT-91 which uses the NEMA17 version of the focuser. There is also a bracket to fit the NEMA11 version onto a Baader Diamond Steeltrack NT focuser.

Please do let me know if you are designing brackets for other telescopes and I can link them here.

Do let me know in the comments below if you’re struggling with anything and I will amend this post accordingly.