What Is A Barn Door Tracker?
A barn door tracker (also known as Haig or Scotch mount) is a camera mount used to take long exposure photographs of the night sky without the star trails caused by the rotation of the earth. It consists of two boards connected by a hinge, with a bolt which is turned to move the two boards apart or towards each other to counter the effects of the rotation of the earth.
There are many types of barn door tracker. This guide is for a manual single arm version which consists of a single "arm" (board) and is operated manually by the user. More information on other types of barn door tracker can be found at Starnamers Blog and a motorised version is detailed on this aticle on PetaPixel.
This guide details how I went about building my own tracker, and some of the useful information I discovered whilst doing so which will hopefully save you time when you build yours.
How It Works
As mentioned above, the barn door tracker works by joining together two boards and then moving them apart at a specific rate. The hinge joining both boards is pointed at the celestial pole and a drive bolt is turned to move the top board which counters the rotation of the earth and prevents star trails. The diagram below gives a basic indication of the finished structure.
The most important part of the barn door tracker is the radius of the curvature - the distance between the drive bolt and hinge. This is determined by the thread size of your drive bolt and will control the rate at which the top board raises for every turn of the drive bolt. The formula to calculate this distance is below (where RPM is the revolutions per minute of the drive bolt and TPI is the screw threads per inch):
Distance (inches) = RPM / ((2π / 1436) * TPI).
I have created a tool which will automatically calculate the distance needed in either inches, cm or mm and is also preset for the standard metric screw sizes such as M6 and M8 where the thread size is not obvious.
Building is relatively straight forward. I have listed all the parts I used to build mine at the bottom of the page but it is essentially a couple of solid boards that will not bend, a hinge and a few screws and nuts.
- Drill Holes
Firstly you want to drill the holes in the boards. The bottom board will have two holes, one for for connecting your tripod to the mount and another one for the drive bolt to go through. The drive bolt position needs to be calculated (as described above) but the tripod hole can go anywhere, although near the hinge is a good idea to give the drive bolt plenty of clearance.
The top board will have a hole for the tripod head to be secured through. This can also go anywhere but closer to the hinge make the unit more stable.
- Tripod Hole
Take the bottom board and drill the hole that will be used for the tripod to attach itselt to the mount. This can go anywhere on the board but I would recommend putting it fairly close to where the hinge will go. Once drilled you will want to insert the ¼" BSW (Whitworth) t-nut into the under-side of it and make sure it is flush to the board as the tripod will be screwing into this and you want it as secure as possible. Remember the tripod will be tilting (sometimes at very steep angles) and the entire mount and camera will be supported through this connection so you want it to be rock solid. A small screw coming in from the top to tighten the t-nut against the bottom of the board help with this.
- Drive Bolt Hole
Next step is to make the hole for the drive bolt in the bottom board. As described above, the hole needs to be a certain distance from the hinge and this distance depends on the type of drive bolt you are using. Once the hole has been made, insert the drive bolt t-nut into the hole and make sure it is secure.
- Tripod Head Hole
Make a hole in the top board for the tripod head to sit on. The hole will need to be big enough to fit a ⅜" screw through it.
- Join Hinges
Once all the holes have been made it is time to join the boards together with a hinge. I used a long piano hinge and cut it to size. You can use multiple smaller hinges but the one single long hinge will be more secure and you won't have to worry about exactly lining up all the smaller ones.
- All that is left to do it attach the mount head to the top board and insert your drive bolt and you're done with the bare essentials. You now have a working barn door tracker which you can go out and use right away.
- Additional Extras
The following enhancements will make using your barn door tracker easier:
- Attach a small tube along the outside of the hinge (I used an empty biro). You can use this to look through when doing polar alignment which will make things more accurate.
- In addition to the tube, holding a laser pen against the hinge will point a straight beam in the same direction of the hinge which can make polar aligning easier.
- Find an old CD, DVD or anything circular and mark on it every 30°. Attach this to the bottom of the drive bolt and also attach something to the bottom board which points to the disc (I used a straw). Every mark on the disc represents 5 seconds of movement which will make time keeping whilst turning the drive bolt easier.
A Tale of Two Screws
You will need specific sized screws and nuts for attaching the mount head and tripod to you barn door tracker. Thankfully standards apply so most tripods and cameras will have the same sizes. However, if you are not in the USA, these pieces are can be hard to come by as they are in the old inch format and not the metric formats that most non-US countries have now adopted so don't expect to walk into any hardware store in the UK and pick these up. I found the best source to be either eBay or ModelFixings.co.uk.
Attaching the Tripod to the Tracker
To attach your tripod to the tracker you will need a ¼" BSW (Whitworth) T-nut. The bolt on the top of the tripod will screw and secure into this as it would when attaching your camera to the tripod. UNC thread could also work here at a pinch but they have a 60° thread instead of the 55° thread on the Whitworth so could possibly damage the thread on you tripod.
Attaching the Mount Head to the Tracker
To attach your mount head to the tracker you will need a ⅜" UNC screw. You will want the length to be long enough to go through the top board of the tracker and into the mount head, but not so long that it will reach past the end of the screw socket. I found that 1½ inches worked for me but it depends on the depth of the screw socket on your mount head and the thickness of the board you have used.
How To Use Your Barn Door Tracker
In order to use the tracker, it will need to be polar aligned. Polar alignment is the process of aligning the hinge of the tracker with the north or south celstial pole (depending on your location). This will counter the effects of the earth's rotation as you move the tracker. Polar alignment is much easier in the northern hemisphere due to the pole star, Polaris, being easily visible to the naked eye and only ¾° from the north celestial pole. Southern hemisphere users have a harder job as there is no easily visible star to use as a reference - Sigma Octantis is the closest at about 1° distance.
The following links can provide more information on polar alignment:
- Wikipedia article on polar alignment
- Northern hemisphere guide by Asto Baby
- Southern hemispeher guide by Celestron
- Google search on polar alignment
The following camera settings are recommended to get the most out of the barn door tracker:
- Shutter Speed: As long as possible - or use Bulb mode if you have a remote shutter control. After all, this is why you want to use a barn door tracker in the first place!
- Aperture: Open all the way - select the lowest F-Stop number to maximise the amount of light captured by the camera.
- ISO: Set this to the highest value your camera can handle without generating extra noise. 800 is a good place to start. The higher the ISO, the more sensitive the sensor will be to light.
- Focus: Enable manual focus and set to infinity.
- Mirror Lock: Enable mirror lock to reduce the vibrations caused by the camera mirror moving into position. If your camera does not support this, placing a dark cloth over the lens just before starting the exposure will also acheive the same effect.
- Picture Format Shoot in RAW format if you are planning on doing some post processing. This format is not processed by the camera so retains the full data for each shot but will also take up a lot more space on your memory card.
Depending on the focal length you are using, you don't need to continuously move the drive bolt. The following is a rough guide to how often you need to operate the tracker and how long it will be effective for:
- Wide angle lens (30mm or less): ½ turn every 30 seconds - effective for about 15 minutes
- Normal lens (35-65mm): ¼ turn every 15 seconds - effective for about 10 minutes
- Zoom lens (70mm or more): 1/12 turn every 5 seconds - effective for about 5 minutes
These times may not seem very long compared to a professional tracking system but they can be very effective, especially when multiple exposures are taken and then "stacked" using software such as Photoshop or automated tools like Registax or DeepSkyStacker.
Barn Door Tracker
Here are some photos of my completed barn door tracker. The larger versions of each photo are annotated with the part numbers in the Parts section below.
Here is a photo taken with the barn door tracker. It is a composite of 23 exposures 60 seconds each and then stacked together. More photos will be added once the weather improves!
These are the parts I used to build my barn door tracker along with sources and prices. The total cost came to just under £35; I also used a saw, phillips screwdriver and a drill during the construction process.
|1||2 x Wood boards|
I used 300x200mm pieces but any size can work
(providing there is enough length for the drive bolt and they don't bend!)
|Old coffee table||-|
|3||8 x screws for piano hinge||Toolbox||-|
|4||Ball & socket tripod head||Amazon||£27.95|
|5||Tripod screw - ⅜" x 1½" UNC||modelfixings.co.uk||£0.56|
(Pack of 4 for £1.50 + £0.75 postage)
|6||Tripod mount T-nut - ¼"-20 UNC (3 prong)||eBay||£0.58|
(Pack of 5 for £1.50 + £1.40 postage)
|7||Tripod mount t-nut securing screw - ¼" BSW x ⅜"|
Securing the tripod connecting t-nut from the other size mades it more secure
(Pack of 10 for £2.10 + £0.60 postage)
|8||Empty biro tube|
Used for polar alignment
|9||Drive bolt - 100mm M8 coach bolt||eBay||£0.73|
(Pack of 4 for £2.90)
|10||Drive bolt T-nut - M8 (4 prong)||eBay||£0.22|
(Pack of 10 for £0.99 + £1.25 postage)
|11||Drive bolt Cap Nut - M8||eBay||£0.10|
(Pack of 10 for £1.03)
Used to make turning the drive bolt easier
Used to point to the turning DVD
|Total Cost: £30.41|
Other Barn Door Tracker Guides
- Constructing a Barn-Door Tracker
- Building a DIY Barn Door Tracking Mount for Long-Exposure Astrophotography
- Starnamer's Single-Arm Manually-Driven Barn Door Star Tracker
- Gary Seronik's Motorised Guide with Curved Drive Bolt
- A Tracker Built using a metal hinge
- Phil Harrington's Guide
- Astronomy Boy's Barn Door Tracker
- Stargazers Lounge DIY Astronomy Forum