Once you have or plan to have AstroLink device you can select among few options of controlling focus position of your imaging setup using solutions with stepper motor.

1. What is the precision required?
2. What is the power required to move focuser?
3. Use commercial focuser motor solution
4. Connection of geared stepper motor to main focuser shaft
5. Connection of  stepper motor to microfocuser shaft
6. Connection of high torque stepper motor to main focuser shaft (with microstepping module)
7. Final thoughts

What is the precision required?

Focuser is precise tool – it must allow to adjust imaging train position with high accuracy. But what is the exact accuracy? Well, it depends – mainly on focal ratio of instrument. There is a quality called Critical Focus Zone that, simply saying, describes the distance where the image is sharp. You can see in the table below how CFZ varies depending on telescope focal ratio:

Focuser must provide at least 3-4 steps for the distance of CFZ. So if you have for example f/4 instrument with CFZ 40um, your focuser step should be smaller than 10-15 um. This is not so hard to achieve. Most Crayford type focusers have shaft with 6mm diameter, so about 18mm circumference. That gives 18,000 / 10 = 1800 steps required per shaft revolution. It can be achieved with simple 200 steps motor and 1:10 gearbox. If you have 1/10 instrument, then you can use for example 400 steps motor coupled with main focuser shaft without any additional reduction.

What is the power required to run focuser?

Power is not the most precise term – to move focuser shaft we need quantity called torque. Torque is expressed in N*m or N*cm units. Imagine 10cm long lever attached to your focuser knob. The force you need to press the end of this lever to move focuser is the torque it requires. If you need 0.5N (it is more less the weight of 50g), then torque is 0.5 * 10 [N * cm] = 5 N*cm.

For most of focusers torque required to rotate microfocusing knob is about 1-2 N*cm, and for main focuser knob required torque is typically around 10 N*cm. But it is always better to be on the safe side, assume you need two times more torque for some unexpected circumstances. When you use stepper with reduction gear box, torque required to rotate main shaft can easily be obtained, because for example 1:20 gear box slows rotation speed 20 times, but in the same time increases torque 20 times (assuming 100% efficiency). When you couple microstepping motor to main focuser shaft, then microstepping torque may not be enough to reach desired precision. Lets take for example quite large stepper motor that gives 40 N*cm torque. And for main shaft you need like 10 N*cm to move. Sounds good. But torque for microstepping is less. For 1/64 microstep the torque is 2.5% of initial one, for 1/32 step it is 5%, for 1/16 it is 10% and for 1/8 step it is 20% of the full step torque. So when you have 40 N*cm stepper working at 1/64 microstep, it effectively achieves resolution of 1/8 step, when the torque reaches the one required to move the shaft.

Commercial focuser motors

AstroLink 4 mini device is compatible with many commercial focuser motors (Robofocus, Moonlite, USB Focus, Pegasus Astro and with small adaptations to many others, like FeatherTouch or Seletek for example). If you have focuser that can be equipped with such dedicated focusing motor, then this is the most convenient, although usually most expensive solution. However if you would like to have neat and well manufactured device, it may be a reasonable option to purchase such dedicated motor. If you have any questions or doubts if the device you have is compatible with AstroLink – please contact with me.

Geared stepper motor to main focuser shaft

This solution is most widely used in commercial focusers. It consists of relatively small stepper motor connected with proper gear box with ratio in the range 1:10 to 1:200. Shaft from the gearbox is connected to main focuser shaft. Backlash is present, as it is intrinsic quality of each mechanical gearbox, but it can be easily compensated with proper settings of focuser driver. This solution is also more expensive than other two, because there is additional cost of gearbox.

Example stock solutions that are available for this solution:

• Nema 16 9V 0.6A motor with 1:14 gearbox
• Nema 17 12V 0.4A motor with 1:27 gearbox
• cheap 35BY412 motors with 1:42 gearbox available on amazon, ebay or aliexpress (I will test these motors soon)
• low power stepper motors available with gearbox at different retailers (like https://www.mclennan.co.uk/product/p535-series-geared-stepper-motor – can be pretty expensive)

Drawbacks of this solution are price and backlash. Advantages are: low power consumption, no power required when idle, compact size, high resolution and compatibility with many commercial solutions (Robofocus, Moonlite, USB Focus, Pegasus Astro, AstroLink and other).

Stepper motor to microfocuser shaft

This is quite easy solution that requires low power stepper motor, because torque to move microfocuser shaft is pretty low. All you need to do is to find suitable stepper motor for 9-12V. It can be both unipolar or bipolar motor, and it needs to provide torque at level 2-3 Ncm or more. Motor shaft needs to be connected to microfocusing shaft using a shaft coupler with proper diameter.

Example motors that will work for this solution:

• Nema 8 Bipolar 1.8deg 0.3A 12V 3Ncm motor
• Nema 14 Bipolar 1.8deg 0.4A 12V 14Ncm motor
• Nema 14 Bipolar 1.8deg 0.4A 10V 5Ncm motor
• Round Nema 14 Bipolar 0.9deg 0.5A 8.5V 5Ncm motor
• Nema 16 Bipolar 1.8deg 0.4A 12V 21Ncm motor 
• Nema 17 Bipolar 1.8deg 0.3A 12V 16Ncm motor

This kind of connections is quite simple and inexpensive. The drawbacks of this solutions are: microfocusing knob is not available anymore for manual focusing (however you can use shaft coupler to rotate it manually), and when microfocusing gear is not adjusted well, there can be some slip at its mechanism.

High torque stepper to main focuser shaft

This solution requires high torque oversized stepper motor that is coupled directly to main focuser shaft. Stepper motor must be bipolar and works in microstepping mode, so precision may be increased. In this solution motor needs to be high power due to two reasons. First – it rotates directly main focuser shaft, so depending on the focuser load it must be able to move it in any orientation. Second, and actually more important – it works in microstepping mode, and in this mode holding torque is getting lower when microstepping resolution is increased. Torque for moving shaft between 1/64 microstep position is about 2.5% of torque required to move motor shaft between full steps (source). Also in microstepping mode stepper motor must be constantly powered to keep the right position. The advantage of this solution is that in theory backlash is eliminated. In practice backlash can still be present when torque required to rotate focuser shaft is larger than torque that motor provides between microsteps (see section about torque below).

Example motors that will work with microstepping module (I recommend at least Nema 16):

• Nema 16 Bipolar 1.8deg 0.65A 4.55V 18Ncm motor
• Nema 17 Bipolar 1.8deg 0.85A 5.4V 36Ncm motor

Solution with microstepping controller and oversized motor requires larger motor and microstepping controller. Motor requires more power and needs to be powered even when idle. Some heat is generated near the focuser, and this is not desirable condition. Large and constant power consumption also make this solution not suitable for battery operation in the field.

There are some other amateur solutions as well – like with tooth belt for example, but it becomes then more complicated from mechanical point of view, and less popular.

Final thoughts

I have seen commercial focusers that offer step resolution well below 1um. It is pointless rat race. Even for f/3 instrument step you do not need step resolution better than 3-4um.

If you do your own motor adaptation consider compatibility with some standard (Robofocus/Moonlite). Nothing lasts forever. Some day you will want to sell it and such construction will find new owner much easier.

Think twice before you implement microstepping solution. It is both incompatible with common standards, and also has quite a number of drawbacks (resolution dependent torque, variable backlash, constant power requirement, heat generation).

In the table below I sum up all main characteristics of presented solutions. In my opinion the most recommended is solution with geared stepper motor (and commercial focuser motors as well). It provides high precision, low power (no additional heat, do not drain battery when imaging in the field) and what is also quite important – it is compatible with large amount of commercial solutions. Connecting stepper motor to microfocusing shaft provides the same qualities as geared stepper motor, but requires microfocusing gearbox to be present at focuser. Last solution – microstepping  – is mechanically easy to implement, however requires specific controller and oversized motor that drains lot of power, even when idle. Plus it is not compatible with other commercial products.

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