Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

Arduino polar alignment aid


kens

Recommended Posts

When I thought I had fried my DMD-1 controller I decided to try to make a replacement with Arduino. As it turns out the controller was fine but my interest in Arduino was piqued. Having a pretty basic scope I thought something to help with alignment or even tracking would be useful. It would need a 3-axis accelerometer and magnetometer, plus ideally a GPS. At first all the devices I could find had insufficient resolution but I think I've now found one that could do the job. Its a breakout board based on a LSM9DS0 which has a resolution of 0.061 milli-g/LSB and 0.08 milli-gauss per LSB. Another breakout board has a GPS unit based on the MTK3339 chipset (not that it means anything to me - but just in case anyone is interested)

I had to order the bits from the States as there was no local supplier so hoping it will arrive in a couple of weeks.

Not sure whether it will work or not but should be fun to build. I also ordered a 2.8" TFT display to show the outputs and if I start to look at using it for tracking I want to use an IR remote control. 

Has anyone else tried anything like this?

Link to comment
Share on other sites

There have been thoughts on tracking the end of the telescope via a 3-axis gyro or magnetometer to tel the dome where to rotate to base don the position of the scope.

Creating your own hand controller using the arduino should be a good project, the power drivers are cheaply available on ebay, so it comes down to getting the code doing what you want.

There's also a kit IR hand remote on ebay for a couple of pounds - just include the interface on your shield.

I'm not sure what you intend the gyro unit to do though.

Good luck

Mike

Link to comment
Share on other sites

I wasn't really planning to do anything with the gyro unless some bright idea presents itself. It just came as part of the integrated unit along with the magnetometer and accelerometer. I chose those based on their resolution which is way better than anything I had seen before. Largely because they are 16bit rather than 10bit or 13bit.

My scope is an old Vixen (c 1983) on a Super Polaris mount. I've got motor drives on each axis but they are different to later models. They are unipolar steppers with 1:300 gearing. I've worked out how to drive them with the Arduino and can now control their speed through the whole range versus the 1x, 2x and 8x options on the DMD-1 controller.

I think where I'll head is to see if I can get reasonable tracking using the sensors but I haven't thought it through yet. Maybe just apply periodic corrections to adjust for any alignment errors or something like that.

My rough calculations say that the inclinometer can resolve down to 0.003 arc-seconds but the magnetometer can resolve only to about 40 arc seconds.

Getting the code to do what I want is what I do best. That's what I like about Arduino as it makes the interface to the hardware so easy.

Other ideas pretty much revolve around making things so that I don't have to bend so much - I'm not as young as I was when I first bought the scope. :smiley:

Link to comment
Share on other sites

I'm astonished if the inclinometer gives you that accuracy, and the magnetometer wobbles all over the place due to the changing metal distribution of the mount and you moving nearby presence.

I would expect the gyro to be able to point you at 30' accuracy though.

Mike

Link to comment
Share on other sites

I agree. The resolution is really theoretical based on the number of bits but the only way to find out for sure one way or the other is to build it. I'm still learning about the gyro.One blurb says "the gyro can be used to very accurately track rotation on a short timescale, while the accelerometer and compass can help compensate for gyro drift over time by providing an absolute frame of reference." But the data sheet implies that the gyro can only give the rate of rotation, not the absolute orientation. The magnetometer accuracy may not be a big deal as it is really only needed in the initial orientation of the scope whereas the gyro and accelerometer can be used to determine (theoretically) the orientation of the scope when it is in use.

Link to comment
Share on other sites

The  GPS  and IMU arrived. Wired up each one separately and they worked just as expected. The GPS got a fix and from the IMU I could get roll, pitch and yaw readings which were fairly stable. Hit a bit of a hitch when I wired up the two simultaneously. The program does strange things which seem to be related to a conflict when reading from the two devices. Once I sort that out (any help is welcome) I will look into some calibration code I found that helps with accuracy and also hooking up the LCD screen to display results. A minor hitch there in that I bought the display as a shield and it occupies all the headers without using them all. So I need a work around to access the relevant pins for the GPS and IMU. 

Link to comment
Share on other sites

Sorted out the timing conflict and can now get readings from both GPS and IMU. Not as stable as I'd like and it's still early days yet. Next step is to hook up the LCD screen. To do that I need to get an Arduino prototyping board which will let me get at the pinouts while the display is plugged in. I'm also ripping out unwanted code as I'm getting perilously close to the 32k limit. So far I can see that I could at least get a handy set of digital setting circles. I was going to post a pic but it came out fuzzier than any of my astrophotography attempts. I'll try again in daylight

Link to comment
Share on other sites

Latest activity was to test the noise levels on the various sensors and the likely real resolution. To do this I captured the readings in a file which I then loaded into a spreadsheet to do the calculations.

Results are as follows:

Accelerometer: Without any filtering (other than on the chip) I can get resolution to about 13 arc minutes. If I apply a  moving average filter over 16 samples (about 1 seconds worth) this comes down to about 3 arc minutes

Magnetometer: Without filtering I get about 13 arc minutes on this one also. With the same moving average filter I get about 4 arc minutes.

Gyro: Unfiltered the noise level is down around 0.75 arc seconds per second but I need to adjust for bias. Not bad seeing as the rotational speed for tracking the sky is 15 arc seconds per second so well above the noise level. This suggests a use for the gyro in correcting for noise in the other sensors.

Link to comment
Share on other sites

  • 8 years later...

Hi @kens, wondering if you had any updates on your project? I'm hoping to get into some landscape astrophotography with my Nikon D7200. I built myself a dodgy-DIY barn door tracker using a stepper motor + Arduino Nano. I tried polar aligning by attaching my phone to the tracker and using the PhotoPills app to point the tracker towards the celestial pole - the accuracy wasn't too bad, but I'm hoping to find a reasonably quick, low-budget method to improve on this and your project looks very interesting!

Link to comment
Share on other sites

Hi Greg,  I think that using a magnetometer is likely to give problems as anything steel will distort the earths magnetic field. I made a digital setting circle for my Dob with an arduino which became unusable when I changed the alt of the scope. 

I have also made a barndoor tracker for my camera using an arduino and stepper motor. I use a compass to find aprox North. Then a wixey digital inclinometer to set the alt at my latitude. I have attached a 10mm dia tube 200mm length along my tracker hinge and use this to sight on to Polaris and fine tune alignment. 

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.