To track the sun by hand, aim the concentrator by watching the shadow of a pin or rod rather than looking at the sun, and when you re-aim, point it ahead of the sun by half the allowed error so the focus drifts across the target and out the other side. For a 5 cm spot at a 0.6 m focal length the allowed error is about 2.4° each way, a window of 4.8° or 19 minutes between adjustments; for a 2 cm spot it shrinks to about 7.6 minutes.
Part of Tracking Systems
Manual tracking is re-aiming a concentrator by hand at intervals, using a sighting aid so the operator never has to look at the sun.
Lead The Sun, Then Let It Pass
The sun moves 0.25° per minute (Tracking Systems). If the target tolerates a focus shift of s on either side of centre, and the focal length is f, the allowed pointing error is ε = atan(s ÷ f). Aim the dish ahead of the sun by ε; as the sun moves, the focus sweeps from one edge of the target to the other, spending a window of 2ε in tolerance before you must adjust again.
Inputs: f = 0.6 m.
- 5 cm spot (s = 2.5 cm): ε = atan(0.025 ÷ 0.6) = 2.4°. Window = 4.8°, and 4.8° ÷ 0.25°/min = 19.2 minutes, about 19.
- 2 cm spot (s = 1 cm): ε = atan(0.01 ÷ 0.6) = 0.95°. Window = 1.9°, and 1.9 ÷ 0.25 = 7.6 minutes.
The parent page’s interval of 15-30 minutes suits a target of about 5 cm or larger and fails for a small, hot spot. Leading the sun by half the window doubles the interval compared with re-aiming only when the focus has already left.
Aiming Without Looking At The Sun
Fix a thin rod or pin on the dish rim, parallel to the dish axis. When the dish points exactly at the sun, the rod’s shadow falls on its own base and has no length. To lead the sun, offset the aim until the shadow tip is a set distance (the rod length × tan ε) from the base, on the side toward which the sun is moving (the shadow falls away from the sun, and the dish is pointed ahead of it). For a rod 10 cm long and ε = 2.4°, that is 10 × tan 2.4° = 0.42 cm, a small offset, so mark it on a plate under the rod. Never look along the dish toward the sun, and do not look at the focus.
What You Need
- A mount with a smooth, lockable two-axis motion
- A shadow rod and plate with a mark for the lead
- A watch or other timekeeper
- A cover for the reflector
- Eye protection, and a second person if the work needs your full attention
Method: Track By Hand
Step 1: Before uncovering, set the dish to lead the sun by ε using the shadow plate.
Step 2: Uncover. Check the focus on the target with a stick (never your hand).
Step 3: At the end of each window, re-aim to lead the sun again.
Step 4: Write the intervals on a card. In the afternoon, set an alarm; people forget.
Step 5: Cover the dish at every break.
Common Mistakes
- Tracking by looking at the sun or at the reflection. Use the shadow.
- Waiting until the focus leaves the target. Lead it.
- Leaving the dish uncovered at a meal. The focus is still sweeping.
Safety
The operator is the hazard control
Tracking by hand puts a person next to a focus that ignites and a mirror that blinds. Never look at the focus, never aim the dish by sighting along its axis at the sun, keep your hands, face and clothes out of the cone between mirror and focus, and wear dark eye protection. Fatigue is a hazard: a missed adjustment leaves the focus sweeping across the work area, so cover the dish when you cannot give it your attention.
What’s Next
- Learn next: Gravity-Driven Clock Tracker — removing the operator from the loop
- Combine with: Shadow Stick — shadows as direction finders