/maxexposure
/maxexposure [focal_length] [aperture] [scope] [camera] [pixel_pitch] [object_name] [declination]
Calculates the maximum exposure time before star trailing becomes visible on an UNTRACKED shot (a fixed tripod, no mount tracking Earth’s rotation) — using the Full NPF Rule, a real physics-based formula, not a rough rule of thumb like the “500 Rule.” Accounts for your optics (focal length, aperture), camera sensor (pixel pitch), and the declination of whatever you’re shooting — stars near the celestial poles trail much slower than stars near the celestial equator, so the same gear can tolerate meaningfully longer exposures depending on where you’re pointed.
Usage: /maxexposure scope: [a saved profile] camera: [a saved profile] once you’ve set both up with /scope and
/camera — or provide focal_length/aperture and/or
pixel_pitch directly instead of either. A directly-given
focal_length or aperture always overrides a saved scope’s
value for that one field, which is handy for a quick one-off
change (a focal reducer added tonight, say) without re-saving
the whole profile. Add object_name: M31 to auto-fill
declination from this bot’s own catalog, or declination: 45
to set it manually. Leaving both out defaults to 0° (the
celestial equator), the worst-case assumption — safe, but
possibly more conservative than necessary if you’re actually
shooting somewhere else in the sky.
This is a real formula (Frédéric Michaud’s Full NPF Rule), not a guess — but it’s still a guideline: your own tolerance for trailing, focus precision, and atmospheric seeing all affect how a given exposure actually looks.
Parameters
| Name | Required | Description |
|---|---|---|
focal_length | No | Lens/scope focal length in mm, if not using a saved scope (overrides scope's value if both given) |
aperture | No | f-number, e.g. 2.8, if not using a saved scope (overrides scope's value if both given) |
scope | No | One of your saved telescope profiles (see /scope) -- supplies focal_length and aperture |
camera | No | One of your saved camera profiles (see /camera) |
pixel_pitch | No | Exact pixel pitch in microns, if not using a saved camera |
object_name | No | A deep-sky object (M31, NGC 6960, etc.) to auto-fill declination |
declination | No | Manual declination in degrees, if not using object_name (defaults to 0°, the worst case) |