Example Data Table
| Lens |
Exposure |
Radius |
Pitch |
Design |
Expected Use |
| 50 mm |
8 min |
200 mm |
1.25 mm |
Tangent |
Wide sky images |
| 135 mm |
10 min |
250 mm |
1.25 mm |
Curved arc |
Cleaner star fields |
| 200 mm |
6 min |
300 mm |
1.00 mm |
Isosceles |
Short telephoto tracking |
Formula Used
The sidereal tracking angle is calculated as:
Angle = 0.25068447 × tracking minutes
The screw travel depends on the chosen tracker style:
Tangent travel = radius × tan(angle)
Isosceles travel = 2 × radius × sin(angle ÷ 2)
Curved arc travel = radius × angle in radians
Screw turns are calculated as:
Turns = screw travel ÷ screw pitch
Motor rate is calculated as:
Microsteps per second = turns × steps per revolution × microsteps × gear ratio ÷ tracking seconds
Image scale is calculated as:
Arcseconds per pixel = 206.265 × pixel size ÷ focal length
Estimated pixel drift is:
Pixel drift = total angular drift ÷ image scale
How to Use This Calculator
Enter the camera focal length and pixel size first. Add the planned tracking time. Then enter the hinge radius and screw pitch from your tracker build. Choose the drive geometry that matches your design. Add motor steps, microsteps, and gearing if you use a motor. Enter polar alignment error and payload values for a fuller estimate. Press calculate. Review screw travel, speed, drift, and torque. Use CSV for spreadsheet records. Use PDF for field notes.
Why barn door tracking matters
A barn door tracker is a simple sky tracking mount. It turns a camera around a hinge. The goal is to match Earth rotation. Good values matter because small errors become long star trails. This calculator helps plan the hinge radius, screw motion, drive speed, and expected drift before a build starts.
Design choices
The hinge radius controls how much screw travel is needed. A longer radius usually gives smoother control. It also needs more travel. Screw pitch controls how many turns move the tracker. Fine pitch gives easier manual control. It also needs more turns. A motor drive needs the correct steps per second. A hand drive needs a clear turn interval.
Tracking accuracy
The tool estimates the sidereal angle for the selected exposure. It then converts that angle into screw travel for tangent, isosceles, or curved drive geometry. The curved option represents an ideal arc drive. Tangent and isosceles designs can work well, but their speed changes during the run. A constant speed drive can create geometric error. The calculator estimates that error across the exposure.
Imaging use
Focal length and pixel size convert angular drift into pixels. This makes the result practical for real images. A wide lens can hide more error. A long lens shows errors quickly. Polar alignment also matters. Even a well timed tracker will drift if the hinge is not aimed near the celestial pole.
Build planning
Use the results as planning numbers, not as a final machining standard. Hinges flex. Screws wobble. Wood moves with moisture. Loads can sag. Test the tracker under the real camera weight. Start with short exposures. Then increase time as alignment improves. Use the CSV export for build notes. Use the PDF export for a quick field sheet. Recheck speed after changing radius, pitch, or gearing. Small changes can shift the required drive rate.
Field habits
Field habits improve results. Level the base when possible. Tighten the hinge without binding it. Keep the screw tip seated. Mark the starting angle. Record the final angle after each run. If stars trail in the same direction every frame, adjust speed. If trails rotate, improve polar alignment first. Clean data saves time outside later during longer sessions.
FAQs
What is a barn door tracker?
It is a simple camera mount that rotates around a hinge. The movement follows Earth rotation. This helps stars stay sharper during long exposures.
Which drive geometry should I choose?
Choose tangent for a common screw build. Choose isosceles for a two-arm layout. Choose curved arc when your screw follows a curved path.
Why does screw pitch matter?
Screw pitch tells how far the screw moves per turn. A smaller pitch needs more turns. It can give finer control for manual tracking.
What does hinge radius change?
A larger hinge radius reduces sensitivity to small screw errors. It also requires more screw travel. The best size depends on your build space.
Why are start and end speeds different?
Some tracker shapes need changing screw speed. Tangent and isosceles drives do not stay perfectly linear. A curved arc drive is closer to constant motion.
How accurate is the polar drift estimate?
It is an approximate planning value. Real drift also depends on azimuth error, altitude error, target position, flexure, and tripod stability.
Can I use this for a manual tracker?
Yes. Use the average screw speed and quarter turn interval. Turn the screw smoothly. Check stars after a short test exposure.
Why include motor steps?
Motor steps help set a controller rate. Enter full steps, microsteps, and gearing. The calculator returns microsteps per second for the planned exposure.