Spot Stars
Calculate max exposure to prevent star trailing (NPF Rule)
How to use Spot Stars
- 1Enter focal length, aperture and sensor pixel spacing.
- 2Read the maximum exposure of point stars (without dragtails).
- 3Sets the fast door below or below that value to keep the starpoint clear.
NPF Code (Little Star)
t = (35 x Aperture + 30 x Pixels Spacing [μm]) / Focal length [mm] (s)The NPF law adds the optic and sensor pixel dimensions to the 500 rule, which is more precise than the focus-only.
Usually shorter and safer exposure than the 500 law, especially on high pixel cameras.
When you shoot the stars, the Earth rotates and the stars move in the sky, and the fast door is too long towed (to become a orbit rather than a point). ** 500 Code** is a classic experience: maximum number of fast door seconds 500 ÷ focal length (full equivalent mm). 14mm full of about 35 seconds without visible drag, 50mm about 10 seconds. ** More stringent "300 Law"** (modern high pixels, because pixel density is higher and magnifying tows are more visible) replaces 500 with 300, which is more conservative. ** Equivalent coefficient to be calculated**: actual equivalent of APS-C (x1.5) of 21mm, at 500/21 ≈ 24 seconds, or directly by physical focal length set of 14 mm 500/14 divided by 1.5 ≈ 14 seconds. ** The high pixels body suggests a 300 rule and a later check with a "diversion tolerance". **
** Progress: complete resolution of the toweds with the Star Tracker** — it rotates around the Earth and can be exposed for minutes or even longer to the stars without a towed, low-noise sky. The disadvantage is the need for precision to the polar axis, weight and cost. ** Common filming recommendation: f/2.8 or greater + ISO 1600-6400 + Coded Fast Doors + Manual Focus to Infinity (In real time view to magnify bright stars)** and take a dark field/ Post-noise shot of the same parameter. ** Stars near the North Pole are almost intact, and the closer we get to the sky and to the equator, this is a pattern that can also be verified by the naked eye. **
| Focal length (full equivalent) | 500 Laws | Improvements 300 Code | APS-C（×1.5） | M43（×2） |
|---|---|---|---|---|
| 14 mm | About 35 seconds | About 21 seconds | About 14 seconds | About 10 seconds |
| 24 mm | About 20 seconds | About 12 seconds | About 8 seconds | About 6 seconds |
| 35 mm | About 14 seconds | About 8 seconds | About 5 seconds | About 4 seconds |
| 50 mm | About 10 seconds | About 6 seconds | About 4 seconds | About 3 seconds |
| 85 mm | About 6 seconds | About 3 seconds | About 2 seconds | About 1.5 seconds |
The law allows speed with the stars of different drawings. Door.
Frequently asked questions
NPF with 500 laws?
NPF is more accurate because it incorporates the optical circle and pixel spacing.
What's pixel spacing?
Physical width (mi) of individual pixels; the smaller the pixels, the shorter the exposure required.
Do you need a trace?
Deep-space photography is still needed; NPF only avoids visible tail drag and does not follow the sky.
Why would I use 500 rules to shoot or tow?
Three reasons: high pixel density (modern 40.0 million + airframe, 500 law is looser and 300 law is more stable); 2 problem of direction (the fastest drag near the Sun's equator, near the North Star, which you can see may come from low-altitude fast-literous stars);3 no equivalent of actual focal length (the APS-C body is also measured by full focus, with a result 1.5 times longer). **Practical approach: 100% check starpoints after scaling up, reducing speed doors by 30%-50% if towed or increasing ISO resonance. ** Very slight sharp star spot at late stage (but unable to restore the tow, only to reshoot).
How do we set the tracks?
Two routes: 1 ** single long exposure** (with an equatorial instrument or polar night, with a single exposure of 10-30 minutes, directly out of orbit, but with high noise and easy to be damaged by aircraft/twilight); 2 ** stacking method (recommended)**: with 500/300, dozens to hundreds of short exposures (15-30 seconds each, interval 1-2 seconds) and later with StarStaX or Sequator stacking into continuous orbits. ** Advantages of stacking method**: a single sheet of noiseable, deffixable frame of aircraft, high flexibility. Note: 1 tripod is absolutely stable and focused; 2 closes the tremors and uses electronic fast doors (avoids a quick door vibration); 3 Spacing with short-gate or airframe **-spacing mode**; 4 Northern hemisphere, with the Arctic star in centre and a concentric circle.