AstroTracker - a Barn Door Star Tracker

Everyone can do astrophotography as long as you have a camera. Just pop it on a tripod, let the lens stay open for as long as possible and BAM! Beautiful stars, clusters and nebulas. But what is that? Are there streaks on the film instead of pinpoint stars? Fear not, for you need no longer reduce the exposure to only seconds. With the AstroTracker you can keep the lens open for minutes so you can collect all the light from all the fascinating, mysterious nebulas.

Please notice, that the pictures contain text in case you find yourself missing information.

This was a new hobby for me and I wasn't sure how much time I could spend on it so I decided to do it cheap. This is what I used:

• Two boards, I used a bed slat. Yep, I had a few too many in my bed that I added a decade ago.
• Hinge, the less play the better. Mine had a lot of play but it worked fine.
• Gears and motor. I salvaged mine from an old laser printer I had put away for harvesting later.
• PWM. This is the motor speed control unit. It can be bought for 1-2 dollars.
• Ball head mount. This will hold your 0.5 kg camera so if you're going to build it yourself you will have to make sure you do a good job! I bought mine for 2 dollars. If you're in doubt, I bought one exactly like this one.
• Tripod. I had an old one with the quick release plate missing so I installed the AstroTracker on it "permanently".
• A threaded rod. My advise is to use the smallest rod possible so you can run the motor at a higher speed so it runs more smoothly. I used a 30 cm long M4 sized rod that I also had to buy on eBay for some 4-5 dollars.
• Screws and bolts. Thought I bought these? Well, think again! I have a container full of screws that I hate for some reason, like slotted screws.
• A couple of cable strips for holding the motor.
• An old USB cable to install in your PWM if you intend to use a power bank.

You probably don't want to install the AstroTracker permanently on your tripod (except if it is junk like mine). Originally I had thought to use the bottom of this (I have one that is broken, I don't know why I kept it actually). You can install it with two screws and you should then be able to screw it on your tripod.

On to the next step!

First things first. How long should your base board be?

This we will calculate using this wonderful calculator that will spare us a lot of math (which you can read about under the calculator).

Simply choose the size of your threaded rod/screw under 'Preset' and write the desired RPM.

If you use an M4 rod and run the final gear at 2 RPM the length will be 320 mm. You will still need a PWM to perfectly fine-tune the speed in case the load of your different lenses is different and your motor needs more power to keep up at the right speed. Notice, that the rod won't rotate. The gear will have a nut in it and it will be doing the rotating and feeding the rod up through the hole.

We will be working on the baseboard before finally attaching the camera board to it. Also, as you can see, I added an extension to the board for my motor. That's because I built this on the go and had a different design at first.

The distance between the holes between the gears depend on the gears
you choose to use. Measure the distance from the center of one gear to the center of the articulating gear. This can be done in two steps: Distance from center to edge on gear 1 plus the same distance on gear 2. Remember the placement of the motor before drilling the holes! If you have one with a belt like mine then measure the distance from the motor to the first gear where the belt is tight but not too tight.

Alright, so 3 gears. The middle gear looks funny, right? That's because I joined two gears on top of each other with a screw because I needed more gear ratio. The screw also acts as a rod and goes through a hole in the board so the gear doesn't moves to the sides.

Please see the comments in the pictures for more information.

The gear with the nut (last picture): The nut is put above the hole inside a small "pit", so to speak. Lots of Super Glue. Then a plastic piece around it for support because the nut wasn't big enough to reach the walls. On top of it all ring extensions, but only because I needed some extra height so that the gear can rest on the plastic cap. Again, lots of Super Glue so it doesn't come off. If your gear doesn't have aforementioned "pit" just use plenty of glue, I'm sure it'll work. Give the glue one day to cure. I also suggest scratching the plastic a little so the glue has a rough surface to stick to.

After it dried I drew a red line so that I could see how fast it rotated. Also a red dot on the piece of wood next to it so I can see when the line passes the dot. It had to be 2 RPM as mentioned earlier.

At last I added a "platform" that I screwed in the scrap wood under the base board, please see picture. This was to support the last gear from beneath so that it doesn't fall and also, it keeps the gear from moving to the sides. This can be made from scrap wood if you don't have the fancy plastic I-don't-know-what-it-is.

This bad boy ran on 24V in its glorious days in the laser printer. Today it feeds on 5V from a power bank and makes it run slow enough for our purpose. It's connected with a belt, the whole set I found in the printer.

Make one hole for each cable strap in the vertical piece of wood. Put a folded piece of paper tissue/old cloth to take any vibrations the motor may cause and probably also noise. Then, after connecting the belt to the motor, put the cable strips around the wood and motor and through each its hole and tighten with a pair of pliers.

Put your motor and USB wires into the right terminals and test to see if the polarity of the motor is correct by checking if it's running in the correct direction. You can always change this, but it is crucial that you know which terminals are for the motor and which are for the power input! Then I fastened the PWM with cable strips over the heat sink. Much less work than using screws and it still looks good. The heat sink doesn't become remotely warm so you're not running a risk.

Suggestion: You can hammer a nail and hang a pouch on it for your power bank. I plan to do that for mine. Just don't let the pouch hang too low as the wind may make it move too much shaking the tracker. The shorter away from the nail the pouch hangs the better.

Put a raw plug on one end of the rod and make sure it sits tight. This will be used to put a rubber band over and put the band somewhere up near the gears. This will keep the rod from rotating and instead get fed up (hehe) through the hole.

This is the easier board. Only thing to be done here is installing the ball head mount. You want to install it

• At an angle to the board, as this will give you a wider range of motion. Remember that your tracker's hinge will be pointing at the polar star so if you install the mount directly on the board the camera will be pointing at the ground in neutral position.
• Near the hinge, so that the load will be on the hinge and not the rod.

As you can see I used an old window latch and installed the mount on top of it. You can also use a piece of scrap wood.

Now to install our advanced electro-mechanical astro-tracking device (patent not pending) onto our tripod!

I had this crooked plate on the tripod instead of the quick-release. I put two bolts through the groove in the middle and fastened it with bolts. The bolts under the board are only for support.

Time spent: A few hours.

Fun had: Oodles of fun!

Does it work? Yup. With the camera zoomed in I tried a perfect two minute exposure and then a 4 minute exposure, but that showed a tiny amount of trailing. Please notice, that I didn't take much time to align it. I simple looked over the hinge and aimed for Polaris and thought "yeah, this is probably good enough". Moreover, the more zoomed in you are the sooner you will see trailing.

You should really spend a few minutes aligning it properly, though, and perhaps get a laser for that purpose. I thought of gluing a piece of straw parallel to the hinge. Please refer to the internet as there are lots of suggestions for polar alignment, otherwise please feel free to ask.

I regret that I couldn't take pictures when working on the gears as they are incredibly greasy even after drying them off.

This was built mainly from junk but works surprisingly well! Also, I accidentally captured a globular star cluster as you can see which is always nice. No matter where you point your camera you will capture something interesting.

If I have used something that you don't have then I bet you can use something else instead as this is a really simple construction. If in doubt, ask!

Please leave your comments and "reach for the stars"!