Introduction: Arduino Powered 'Scotch Mount' Star Tracker for Astrophotography
I learned about the Scotch Mount when I was younger and made one with my Dad when I was 16. It's a inexpensive, simple way to get started with Astrophotography, that covers the basics before you get in to the complicated telescope matters of prime focus, off axis tracking, etc. When I first made this mount it was back in the '90s so I had to use a film camera and get that film developed at the local camera shop, it was an expensive and long process (take the photos, use the whole roll, drop it off, few days later pick it up and see the results), it's so much faster, cheaper and easy to learn from trial and error now with digital cameras. You can see some old shots from 1997 on the last step.
The design I used back then, and today, came from this book Star Ware:
For this Instructable I also a Github repository for all the Arduino assets: Code, Schematic, and part list with URLs.
The Scotch mount works on a very simple principle of turning the clockwheel at certain times, but as I learned stability plays a huge role in how the photos come out. Turning the clockwheel on an unstable or flimsy design especially at high zooms introduces star trails and jittering into the photo. To overcome this and make the whole process easier and automated, I created a simple Arduino based motor drive based off a DC motor and some plastic gears (I pulled one of mine out of a broken toy helicopter).
There's other instructables out there for the Scotch Mount or Barndoor Tracker but for my design I wanted the mount small and portable so I can throw it in a backpack and take it to remote areas away from light pollution of Austin TX.
Step 1: 'I Was Told There Would Be No Math!'
The Earth spins roughly 360° in 24 hrs, if we break this down then it's 15° in an hour, or 5° in 20 mins.
Now the 1/4-20 screw is a common piece of hardware, it has 20 threads in an inch, so if it is turned at a rate of 1 revolution per minute then it will take 20 mins to travel that 1 inch.
Trigonometry gives us the magic number for our clockwheel hole which is 11.42 inches (or 29.0cm) from our pivot point at the center of the hinge.
Step 2: Materials
- Top Board, 3-inch-by-12-inch (3/4-inch)
- Bottom Board, 3-inch-by-12-inch (3/4-inch)
- Hinges, One long 3-inch hing is recommended, make sure it's a solid hinge with not a lot of "play", I used two simple hinges but there's a lot of wiggle and I may switch them out for a more solid hinge.
- Tangent screw, 1/4-20-by-4-inches-long round head screw
- 2 xTee nut, 1/4-20 internal thread
- Screw Eyes & Rubber band
- Tripod head (get a lightweight one but make sure it's solid, you don't want a cheap mount dropping an expensive camera, or the mount loosening and drooping during a shot).
- Clockwheel Gears (I used 3: a tiny for motor, the intermediate which has a tiny and large, and the large for the clockwheel itself).
- Plastic Standoffs for the motor stand. Started with 1" and cut them down to the size I needed once I had the right heights.
- Thin hobby plywood - for motor and gear mounts (I used a circuit board from Radioshack, thin, light and strong enough, use whatever works best).
- Assorted Springs (I used to assist the gears/screws and keep the gears inline). I got a couple from Lowes and pulled some others out of ballpoint pens and cut them down to the right sizes.
- Assorted Washers to keep the moving parts from grinding against wood.
- Simple bracket for the motor mount.
Arduino Motor Driver (specific parts are in the Github part list with URLs of where you can get them online):
- Motor Drive
- H-Bridge Motor Driver 1A (L293D)
- push button
- on/off toggle
Step 3: Measure & Cut the Top and Bottom Boards
Measure off 12" on each board, mark it, cut, and sand the edges.
Step 4: Drill Holes and Add Hardware
There are a bunch of holes to drill and due to the accurate measuring required I recommend you do the Clockwheel last (so you can measure the 29 cm exactly off the hinge)!
Tip: I recommend tapping the hole using a punch to help guide the hole in the right spot.
You are going to drill the following holes:
- Hinges - Don't just screw them in because the board may split, drill the holes on the edges of both boards, hole depends on hinge screw size, measure screw and use a slightly smaller drill bit.
- The Clockwheel - 29 cm from the center of the hinge pin, it will get a T-nut, the location of this hole is essential to getting the board and the sky turning at the same rate when the screw is turned at 1 rpm. The T-nut should be on the downward facing side of the board (towards the ground).
- Tripod Head - centered on Top board, Size depends on the Tripod head, I also used a washer on mine to hold it snug.
- Tripod Mount -Centered on bottom board, 5/16-inch and this hole will get a T-nut. The T-nut also should be on the downward facing side of the board (towards the ground).
When adding the T-nuts I recommend you put some glue down before you hammer it in, and be gentle hammering. I started a split on my bottom board (see photo) that I had to repair.
When you mount it on a tripod, the Tripod mount hole and t-nut gets the most stress (torqued back and forth from the weight of the camera when on angles) so that T-nut is likely to loosen or come out entirely so make sure you adequately glue it and try to keep the weight centered when using the mount. A good stable mount is crucial for photos without star trails/jiggles.
Step 5: Motor Mount and Gears
First glue a standard 1/4-20 nut to one of the gears, this will be the main clock-drive gear, I used a generous amount of Gorilla Glue for this (you can see in the photo).
Second glue a tiny gear to the other large gear, this is our intermediate gear, I used a simple cut down wood nail as the axel.
Mount the motor to a bracket (I zip tied and then later glued when I had the alignment right).
The setup is that the motor turns the large gear at a relatively fast rate (1 rev / 5 seconds or so), this is connected to the tiny gear, which travels at the same rate. The tiny gear aligns to the main clock-drive gear but since the circumferences are different the clock-wheel gear turns at a much slower rate. We are aiming for a speed of 1 rev/min and the motor travels a little too fast for that. So by using a off and on in the Arduino code I managed to slow the gear down. This setup is called a Gear Train and you can learn a bit more about it here (http://science.howstuffworks.com/transport/engines-equipment/gear-ratio3.htm) You'll have to experiment with what values work for on and off time to make the gear spin at the correct rate for your motor and gears.
You need a good housing to keep everything lined up and spinning smoothly. Take care to line up your holes and use springs and washers to keep the gears traveling on smooth surfaces and not grinding against either board. This probably took me the most time out of the project.
Step 6: Motor Circuitry
Circuitry is pretty simple, with the majority of the connections going to the H-Bridge Motor Driver, use the attached image or a Fritzing project file is included in the Github package as well.
A push button was added to reverse the direction (or you can "rewind" the clock-wheel by hand as well).
On/Off switch just made it easier to turn on and off the drive when not in use/development, you can also just pull the power to the Arduino as well.
Motor direction depends on how it was wired up, if you're spinning the wrong direction, just reverse the polarity.
Step 7: End Result, Tips and Tricks
And use! Align the tripod, sight the North star down the hinge, with the hinge being on the left side of the setup (otherwise you will track in the opposite direction).
Try to keep the entire setup balanced and stable. Don't touch it during shots, or pull on the cables (use a remote trigger for your camera), and try to use techniques like Mirror Lockup (if your camera supports it) to get clear shake-free shots. There are plenty of tutorials available about astrophotography and you'll learn quickly from experience.
The images show two shots I did using the whole setup, this was in the light polluted suburbs of Austin TX on not the clearest night but they came out nice. Orion was about 2.5 mins long and the larger sky shot was 5 mins (but was too long due to the amount of light pollution and had to be scaled back in Lightroom). There's also 3 images of Comet Hale-Bopp from 1997, this was with a hand-turned mount as well as a traditional film camera. You can see what vibrations or an incorrect alignment can do to the shot.
Final Tips and Thoughts:
- Cameras and Glass in lenses are HEAVY, I had to use springs to try and take the weight off the clock-gear and to assist the gears. The motor I used did not have crazy amounts of torque/power so if there was too much weight or the gears were flush on the boards then it had a hard time turning the gear or would straight up lock up. A stronger motor will help, but this is just what I had available.
- Polar aligning is key. The setup will track wrong if it's not aligned properly. You need a sturdy tripod balanced and centered (one with a bubble level helps)!
- There is an inherent error to the tangent mount that shows up on longer exposures, you can use a corrective cam to adjust for it, found here:http://www.astrosurf.com/fred76/planche-tan-corrigee-en.html. I'm not worried about it because I'm using a very wide angle lens (20mm compared to 50mm) and durations of around 5 mins tops.
- Astrophotography is inherently hard and frustrating. Don't go out expecting awesome photos the first time, there is a learning curve, sure more expensive and precise equipment can help, but not if you don't know or appreciate how they work. But start small, master the basics, then you will know how to use the expensive equipment and will be able to use it well. You can still get great shots with simple setups. The old shots from 1997 were "the best" out of about 100 shots, so it was a learning process. With Digital you can take photo after photo and learn from your mistakes and victories to refine your skill.
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