Source: https://guthlabs.ai/docs/instruments/star-trail-astro-exposure-500-rule-npf-rule/

instrument / verified-clean

# Star Trail / Astro Exposure (500 Rule / NPF Rule)

The current ledger marks this instrument verified clean within the source-bound verification corpus.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Inspectable records

[01**Overview**Purpose and lifecycle](<https://guthlabs.ai/docs/instruments/star-trail-astro-exposure-500-rule-npf-rule/>)[02**Mechanism**Method and assumptions](<https://guthlabs.ai/docs/instruments/star-trail-astro-exposure-500-rule-npf-rule/mechanism/>)[03**Implementation**Binding and validation](<https://guthlabs.ai/docs/instruments/star-trail-astro-exposure-500-rule-npf-rule/implementation/>)[04**Qualification**Evidence and errata](<https://guthlabs.ai/docs/instruments/star-trail-astro-exposure-500-rule-npf-rule/qualification/>)

## Ledger state

The current ledger marks this instrument verified clean within the source-bound verification corpus.

**Area**: Audio, Music, Photography & Color

**Tranche**: tranche-4

**Evidence ID**: t4-241

**Ledger rows**: 1

No ledger note is attached.

## Review findings

The arithmetic formulas are acceptable under the stated unit assumptions, but the specification must explicitly define focal length in mm, pixel pitch in micrometers, aperture as f-number, and positive-value constraints. Because those units and constraints are not currently stated in the input definition, the instrument needs review before being marked PASS.

### Formula and scope findings

- Both formulas are standard rule-of-thumb estimates, not physically exact visibility thresholds; the stated honest label correctly identifies this limitation.

- The NPF approximation does not explicitly account for viewing magnification, output size, star declination, tracking, or the user's acceptable trailing criterion.

### Unit assumptions

- For the 500 rule, focal length is in millimeters, crop factor is dimensionless, and the result is in seconds.

- For the NPF rule, focal length is in millimeters, aperture N is the dimensionless f-number, and pixel pitch p is in micrometers.

- The NPF formula t=(35*N+30*p)/f is used without an additional crop-factor term; the sensor's pixel pitch must correspond to the selected camera and sensor mode.

- The exact units are not specified in the instrument input description; omitting mm and micrometers creates a material ambiguity, especially for pixel pitch.

## Boundary hardening

**BREAK_CLAIMED** · SELF_FLAGGED

A zero focal length produces division by zero in both formulas and must be rejected. In addition, applying crop factor to the NPF formula versus only to the 500 rule changes the result from 5.4285714286 s to 2.7142857143 s, exposing a convention-dependent output.

## Errata and corrections

No applied erratum is attached to this unique-name record.

## Inspectable example

4 review vectors are recorded. One source-bound sample is shown below.

````
{
  "inputs": {
    "focal_length_mm": 20,
    "crop_factor": 1,
    "aperture_f_number": 2.8,
    "pixel_pitch_um": 4.14
  },
  "expected_outputs": {
    "500_rule_seconds": 25,
    "npf_rule_seconds": 11.11
  },
  "working": "500 rule: t = 500 / (20 * 1) = 500 / 20 = 25 s. NPF: 35*N = 35*2.8 = 98; 30*p = 30*4.14 = 124.2; numerator = 98 + 124.2 = 222.2; t = 222.2 / 20 = 11.11 s."
}
````

## Related workflows

[Telescope Night Kitfocal,pixel → max exposure](<https://guthlabs.ai/docs/instrument-workflows/telescope-night/>)[Photographer's Exposure Benchastro](<https://guthlabs.ai/docs/instrument-workflows/exposure-bench/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json · VERIFIED_CLEAN`

- `evidence-tranche4/tranche4-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`