3d printed tracker vs skywatcher star adventurer

[feverdreamer1]

Hi,

I've recently bought an ender 3 3d printer, and have been wondering, can a 3d printed tracker be better than, say a skywatcher star adventurer?

It may seem like a silly question, but I'm pretty new in AP. Also, how big of an error would they give off at say 200mm.

Thanks,

S

[happy-kat]

If you like the idea of a project and are happy with either Arduino or RaspPi could have a go, but I don't think you'd be able to as reliably image beyond 200mm which in it's self is an ask. 

One is a given for the money the other is a project.

[vlaiv]

I was thinking about 3D printed star tracker as well - as a project.

Still don't own a 3D printer, but it's on my shopping list (hence all the thinking about possible projects).

Estimating error of such tracker is not easy task. Size of it and how much parts you are going to print as opposed of using pre made metallic parts (such as shafts and bearings) contribute to precision.

Then there is a matter of type of drive used. Are you going for worm arrangement or belt drive? Parts for belt drive are much easier to print on 3D printer.

Let's see what sort of precision in tracking you want in the first place. We start by setting some basic constraints - image sampling rate, max exposure length and acceptable star eccentricity.
You mention 200mm FL and let's take very common pixel size - 4.5um. This gives ~4.6"/px as sampling rate, so let's put our constraint at 4"/px.

We want to be able to do 5min exposure and have our eccentricity less than say 30%? Well if our FWHM is about 6", then we want our error in RA over 5 minutes to be less than 6" * 0.3 = 1.8". Now we hit our first obstacle - we need to calculate what sort of error in mechanical design will give us such periodic error. We also need to know precision in our timing electronics - we need to time steps of motor such that total error over 5 minutes is less than 1.8" (or rather combined with periodic error).

Let's say that we want to have 5 steps per second. What sort of reduction do we need to achieve between motor and RA shaft?

If we use 1.8 degree per step motor (200 steps per revolution) with 16 micro steps and 5 steps per second timing, for 15"/s sidereal rate we have 3"/step or 0.1875"/microstep. That is quite good resolution - HEQ5 for example has 0.143617"/microstep. However, that requires quite large reduction. There is 360 * 60 * 60 = 1296000 arc seconds in full RA revolution, and there is 200 * 16 = 3200 micro steps per RA motor revolution. This means that one motor revolution corresponds to 3200 * 0.1875 = 600 arc seconds. Reduction ratio is therefore 1296000 : 600 = 2160 : 1

This can easily be achieved with one 60:1 reduction followed by one 36:1 reduction.

Belt system does not seem too far fetched for this I would say.

[david_taurus83]

It can be done.

https://www.reddit.com/r/Astronomy/comments/eb04dn/3d_printable_eqsplitring_style_tracking_and_goto/

[vlaiv]

Here is another example of full EQ type mount:

https://www.thingiverse.com/thing:2636470

 

[feverdreamer1]

2 hours ago, happy-kat said:

If you like the idea of a project and are happy with either Arduino or RaspPi could have a go, but I don't think you'd be able to as reliably image beyond 200mm which in it's self is an ask. 

One is a given for the money the other is a project.

I'm thinking of taking the barn door as a project and getting the skywatcher for imaging. If I end up doing this I'll compare them and post my results here

[feverdreamer1]

2 hours ago, vlaiv said:

I was thinking about 3D printed star tracker as well - as a project.

Still don't own a 3D printer, but it's on my shopping list (hence all the thinking about possible projects).

Estimating error of such tracker is not easy task. Size of it and how much parts you are going to print as opposed of using pre made metallic parts (such as shafts and bearings) contribute to precision.

Then there is a matter of type of drive used. Are you going for worm arrangement or belt drive? Parts for belt drive are much easier to print on 3D printer.

Let's see what sort of precision in tracking you want in the first place. We start by setting some basic constraints - image sampling rate, max exposure length and acceptable star eccentricity.
You mention 200mm FL and let's take very common pixel size - 4.5um. This gives ~4.6"/px as sampling rate, so let's put our constraint at 4"/px.

We want to be able to do 5min exposure and have our eccentricity less than say 30%? Well if our FWHM is about 6", then we want our error in RA over 5 minutes to be less than 6" * 0.3 = 1.8". Now we hit our first obstacle - we need to calculate what sort of error in mechanical design will give us such periodic error. We also need to know precision in our timing electronics - we need to time steps of motor such that total error over 5 minutes is less than 1.8" (or rather combined with periodic error).

Let's say that we want to have 5 steps per second. What sort of reduction do we need to achieve between motor and RA shaft?

If we use 1.8 degree per step motor (200 steps per revolution) with 16 micro steps and 5 steps per second timing, for 15"/s sidereal rate we have 3"/step or 0.1875"/microstep. That is quite good resolution - HEQ5 for example has 0.143617"/microstep. However, that requires quite large reduction. There is 360 * 60 * 60 = 1296000 arc seconds in full RA revolution, and there is 200 * 16 = 3200 micro steps per RA motor revolution. This means that one motor revolution corresponds to 3200 * 0.1875 = 600 arc seconds. Reduction ratio is therefore 1296000 : 600 = 2160 : 1

This can easily be achieved with one 60:1 reduction followed by one 36:1 reduction.

Belt system does not seem too far fetched for this I would say.

Belt system is what I originally planned to do it with, as it's cheaper and drives better. The calculations you provide are REALLY helpful and will aid me when I make the 3d tracker (if I end up deciding to do so). I will post my results here somewhere so that whenever you do get a printer you can check it out.

S

[feverdreamer1]

1 hour ago, david_taurus83 said:

It can be done.

https://www.reddit.com/r/Astronomy/comments/eb04dn/3d_printable_eqsplitring_style_tracking_and_goto/

Wow. That's a whole set of levels above me, but it sure is a challenge and I'll definitely give it a try (sometime in the future, when I get around the 3d printer).