Regarding a DIY Star Tracker. NOT Barn Door Style.

Hello Everyone, My name's Tushar. I'm from India.

I am interested in making a "Star Tracker". It would follow the rotation of earth but in opposite direction. This way the sky will look static. I wish to capture some shots of Milky way with my phone.

So I was skimmimg the internet for a setup that can rotate my phone, which sits in a phone stand, on a tripod, at a rate of 0.0007 RPM or 1 rotation per day.

My phone weighs 380 grams and the phone holder is 170 grams, so i need a way to move this ~700 grams setup at the rate of movement of stars. Not only this, I am also saving up to buy a small telescope, and the one i have in mind is 1.4 kilograms. So if i couple the telescope with my DSLR camera, the weight goes somewhere around 2 kilograms. So i wish to make a tracker that can have the payload capacity of at least 3 kilograms so that the gears or the motor don't have to over-exert themselves. I know it would be overkill for my current setup but it will be perfect for the future setup i wish to have.

I tried programming the arduino with a DC motor of 200 RPM (have 3 of those). I was sending pulses for 10 milliseconds, after every 10 milliseconds. This isn't in any way precise speed of tracking for any object. I was just trying out the codes and checking the torque. I soon realized that the torque was so low that if held between thumb and index finger, the shaft stopped. So now i know that the DC motors i have, wont work for the desired output at such slow RPMs.

Some online forums say its wise to use a stepper motor in a configuration that's called a "Barn door Tracker". But I am not interested in that design.
Some forums say its okay to use a 180 servo because they have high torque, at slow RPMs.

I was hoping that, now that you guys know my weight and type of motion requirements , you could give me some insight on which motor should i use for the desired output and how to write the arduino code for it.

Waiting For Your response,
Tushar.

But I am not interested in that design.

What mechanical design does interest you? That decision is the first step.

I agree. The first thing is for you to post a diagram of the mechanical system you have in mind.

As far as the weights are concerned a lot will depend on whether the axis of rotation is through the centre of mass. If it is then the weights are of little relevance provided you have a good quality low-friction bearing to support the weight.

A cheap 28BYJ-48 stepper motor (which comes with a gearbox) may be sufficient. However stepper motors move in steps which might cause some vibration of the camera.

Another possibility is one of the small DC N20 gear motors. Some of them are available with very large gear reductions. The downside with a DC motor is that you will need to add some means for the Arduino to detect the speed so it can stay at whatever speed you choose.

...R

sorry for the delay.

I wish to work on something like this. https://www.instructables.com/id/Two-Axis-Star-Tracker/

I like the design of this model. Its sleek and the guy who posted has explained how it works.

But i have to make it slightly larger so that the future setup of mine can also work with it.
Thats why i asked about larger motors and larger weight/Torque capabilities.

Also, The instructables link has a link that takes us to a github repositiry. I downloaded all those files and changed their extension to .ino from .cpp so that i can read and manipulate them on arduino. But there are modular programs in there, different file for stepper, different file for real time clock..... How do i convert those codes in a single file that can be uploaded to the board???

Was i wrong when i converted the extension to .ino from .cpp?? Does it harm the code in any way??

From a quick look it seems that the code in GitLab was not written to work with the Arduino IDE and I don't know what changes would be needed to make it work with the Arduino IDE.

Basically that design has one stepper motor for the vertical axis and another fixed so it rotates with the vertical axis and whose shaft is the horizontal axis. The idea can be adapted to motors of any size but you will probably need to fabricate brackets and it may not be wise to rely on the motor bearings to carry heavy loads along the shaft axis.

...R

If tried,
What kind of motor would you suggest for the desired weight/Torque and slow RPM??

Servos or Steppers??

Also,

The guy in the Instructables post uses an Arduino Pro Mini.

I get the basic difference is of operating voltage and frequency, but i was curious, is the coding of both these different too??

tusharbhardwaj, you are still going about this the wrong way.
You yourself have said you need a bigger setup than the instructables example.
Robin2 has pointed out that using the motor shaft as the weight bearing structure is bad.

Go find a better mechanical solution.
Google is your friend.

tusharbhardwaj:
If tried,
What kind of motor would you suggest for the desired weight/Torque and slow RPM??

If you design the mechanical system properly I suspect the small 28BYJ-48 stepper motors will do the job perfectly well.

The key design requirements are to NOT use the motors to carry the weight of the camera and to make sure that the weight is suitably balanced to minimise the torque required from the motors.

Maybe think about it this way - design a system that can hold the camera and allow it to rotate around the vertical and horizontal axes without any motors. If it is in balance then the camera will remain pointed in any position that you move it by hand. Then add the motors.

...R

Thanks a lot guys.

I'll try making a balanced structure first. Also, I will try to find a mechanical solution for weight distribution.

Will reach out again when done.

Wish me luck.... :slight_smile:

tusharbhardwaj:
Wish me luck… :slight_smile:

Good googling!

I had no problems in moving a 70mm (36.4g) lens with two 28BYJ48 stepper PT camera system:

I don't know why wanting to use a smartphone for that, astro people on Raspberry forum use new HQ camera for that. I added Raspberry HQ camera with 35mm lelephoto lens (109g in total) onto the stepper PT camera system as well:

With half-stepping 28BYJ48 gives more than 4000 steps per rotation, so may be good enough for the task.

Two basic approaches are an equatorial mount and a Dobsonian mount. The latter requires concerted non-linear movement from two motors to track the earth's rotation, but is simpler to build and requires less mechanical engineering to make it viable. An equatorial mount only needs a single motor and once aligned parallel to the earth's axis of rotation just needs a constant rotation rate from the motor (it used to be common for small telescopes to use a clockwork motor to run one of these).

The Dobsonian requires calibration rather than alignment, and you'll need some processor like an Arduino to do the trigonometry to determine the motor movements.