Arduino controlled all metal barn door tracker

[rocky_mountain_tinkerer]

Hi all,

After being away from astronomy for 30 years or so I am finally getting back in to it, and I am specifically interested in imaging. I'm a good year away from being able to afford a good quality eq mount (let alone a telescope), so I thought I would build a barn-door tracker for use with my DSLR. I have seen lots of designs made of wood using piano hinges, etc., but I wanted to see if I could push the envelope a little by building out of metal and being careful with the tolerances. I am fortunate enough to have a nice little workshop with both a metal lathe and a small milling machine so that was a definite benefit.

I had seen several designs using a curved screw to avoid tangent error, but I chose to use a straight screw driven by a stepper motor and compensate for the error programatically by slowly increasing the delay between steps as the angle of the tracking arm increased. Most of the parts are machined aluminum or brass bar stock, about fifteen dollars worth, the stepper motor I got from Amazon - it's a 28BYJ-48 (64 steps per revolution using half-step driving, and it's geared 64:1, so a total of 4096 steps per revolution) with a ULN2003 driver board, both for about five bucks, and the motor is controlled by an Arduino Uno - about twenty bucks, so my total cost is around forty dollars (and a lot of time ).

I'm still working on the Arduino sketch - I had originally planned on using a function to automatically calculate the delay between steps, but the floating point math slowed it waaaay down. So now I'm just using a list of constants, changing the delay every degree based on a spreadsheet of calculated values. I plan on doing some actual measurements with a dial indicator this weekend to see how close I am, and, to see if I need to increase the resolution of the changes to maybe two or three updates per degree. This weekend will be a good time to spend in the shop, since we're snowed in with complete cloud cover for the next couple of days. Anyway, the motor starts stepping as soon as power is supplied, and there is a button on top of the enclosure to reset or rewind it to the zero position. There is also a limit switch that stops it when it reaches zero, resets the step count, and begins stepping again.

One other thing I wanted to point out - an update, sort of. I had originally machined a 3/16" hole in the tracking arm parallel to the hinge axis and pressed a piece of brass tubing in to use as a "sight" for polar alignment, but my first time out experimenting it became immediately apparent that my eyesight is not what it was when I was younger. As a result I ended up ordering a Celestron polar finder scope and building a mount for it, and it works very well. Of course it also just about doubled the price of my mount (anothe $40), but it was well worth it.

Here are some pictures of the (nearly) finished mount, along with one of my first images, a stacked set of 10 fifteen second exposures of ISON taken Tuesday morning with a 500 mm lens. I'm definitely doing much better than without the mount, since in tests the longest exposure I could get with the 500 mm lens and not have very noticible trailing was around two seconds. I got out later than I wanted Tuesday morning so I didn't get a chance to take more pictures, and now, as I said, the skies are completely clouded over so if ISON survives it's close passage around the sun I'm hoping to get more and better images the first part of December. I'm also interested to see what I can do with some of the brighter galaxies, and, if it works that well witht the 500 mm lens I should be able to get exposures of at least a couple of minutes with a shorter lens, say 50mm or so.

Possible improvements:

1) A better drive screw - I'm currently just using 1/4 20 all thread.
2) IR shutter control (using possibly another Arduino board so as not to mess with the timing of the stepper controller) so that I can start it and then sit in my Jeep and drink coffee with the heater running while my camera takes pictures
3) A screw adjustment for polar alignment (especially the latitude adjustment - right now I just have to loosen the clamp screw and rotate the whole mount, then tighten it back up... not very elegant).

Anyway, thoughts, comments or suggestions are more than welcome!


Images:

Full sized image of the mount

http://www.peasley.com/barn_door/Image1.jpg


A close up

http://www.peasley.com/barn_door/Image2.jpg


And another closeup

http://www.peasley.com/barn_door/Image3.jpg




 

[baggywrinkle]

Very nice indeed..good project and it looks pro.

[Viewmaster]

Great stuff you have there.  We trackers must keep trecking.    "To barn trek where no one has trecked before."

Actually there have been a number of all metal trackers built.  For example, here is piccy of my own all metal construction with crystal controlled tracking motor. 

It has a removable EQ mount with Wixey digital encoders for reading the RA and DE.  So I can set these in the warm, indoors

Second shot shows encoders in place.

Still being adjusted but so far the tracking error is 2 seconds in 25 minutes.

It's a permanent outdoor installation so once PA was set it never needs adjusting again.

 

[rocky_mountain_tinkerer]

Very nice! Quite a ways beyond what I've built, but definitely something to aspire to. Once I get my tracker working well, I would like to start planning something a little more advanced. Thanks very much for the post and the ideas!