NPF Rule Approximate Shutter Pixels
Named simplified NPF(35 N+30 p)/(fcosδ) heuristic, bounded|δ|≤80°; no polar infinity, seeing/demosaicing/sharpness guarantee. Model only, not an ephemeris, measured stellar identity or observational guarantee.
Description
NPF-style exposure estimates use optics, pixel pitch and declination.
NPF-style exposure estimates use optics, pixel pitch and declination.1
The chosen approximation is a shutter heuristic rather than a measured trail tolerance. Use NPF Rule Approximate Shutter Pixels to evaluate the supplied inputs under the model explained below.
When to use NPF Rule Approximate Shutter Pixels
Use NPF Rule Approximate Shutter Pixels to evaluate the supplied inputs under the model explained below. Establish the units, measurement scope and model assumptions before entering values.
How to use NPF Rule Approximate Shutter Pixels and understand its results
- Establish the units, measurement scope and model assumptions before entering values.
- Enter F number, Pixel pitch micrometers, Focal mm, Declination degrees using the field table and validation rules below, then run the tool.
- Read Approximate npf seconds; interpret the units, signs, ordering and unknown-value placeholders described below.
- Correct reported input errors and rerun. Apply successful results according to their specific meanings and check the assumptions before using them in a decision.
| Input field | Meaning | Value type | Requirement |
|---|---|---|---|
| fNumber | F number | number | Required; constraints below |
| pixelPitchMicrometers | Pixel pitch micrometers | number | Required; constraints below |
| focalMm | Focal mm | number | Required; constraints below |
| declinationDegrees | Declination degrees | number | Required; constraints below |
Enter positive f-number, pixel pitch in μm and focal length in mm, and finite declination with absolute value≤80°. Return (35×fNumber+30×pixelPitch)/(focalMm×cos(declination)) in s. No tracking, actual pixel-trail constraint or near-pole extrapolation is added.
approximateNpfSeconds=(35×fNumber+30×pixelPitchMicrometers)/(focalMm×cos(declination)) Use the field order, units and branch conditions described above.
| Output field | Meaning | Value type |
|---|---|---|
| approximateNpfSeconds | Approximate npf seconds | number |
{
"input": {
"fNumber": 2,
"pixelPitchMicrometers": 4,
"focalMm": 50,
"declinationDegrees": 0
},
"output": {
"approximateNpfSeconds": 3.8
}
}Limitations and assumptions
The chosen approximation is a shutter heuristic rather than a measured trail tolerance. Apply the exact scope and assumptions explained above.
All listed fields require explicit values. Unexpected fields, incorrect types, nonfinite numbers and sparse arrays fail validation. Domain-specific bounds, alignments and supported tokens are explained above; extremely large inputs or outputs can also be rejected. Input errors differ from successful false results, empty match lists and unknown-evidence placeholders.
Alternative or Complementary methods
Use Rule of 500 Astrophotography Shutter for a related task: Astrophotography shutter heuristics relate focal length to a tolerance for apparent star motion.
Use Deep-Sky Sub Length Light Pollution Check for a related task: Read noise and sky-background variance affect useful subexposure duration.
References
-
NPF Rule Approximate Shutter Pixels — tool.io
Similar or alternative tools
- Rule of 500 Astrophotography Shutter
Astrophotography shutter heuristics relate focal length to a tolerance for apparent star motion.
- Deep-Sky Sub Length Light Pollution Check
Read noise and sky-background variance affect useful subexposure duration.