What is the NPF Rule in astrophotography?

The NPF Rule is a formula for calculating the maximum shutter speed before stars begin to trail in astrophotography. It's more accurate than the 500 Rule because it accounts for your camera's pixel pitch (sensor resolution) and lens aperture. The formula is: NPF = (35 × Aperture + 30 × Pixel Pitch) ÷ Focal Length.

How is NPF different from the 500 Rule?

The 500 Rule only considers focal length (500 ÷ focal length), while the NPF Rule also factors in your camera's sensor resolution and lens aperture. This makes NPF significantly more accurate, especially for modern high-megapixel cameras where the 500 Rule often results in star trails.

What is pixel pitch and why does it matter?

Pixel pitch is the distance between pixels on your camera sensor, measured in micrometers (μm). Cameras with smaller pixels (higher megapixels) have smaller pixel pitch and show star trailing more easily, requiring shorter shutter speeds. The NPF Rule accounts for this.

What shutter speed should I use for Milky Way photography?

Use the NPF Calculator to find your exact maximum shutter speed based on your camera and lens. Typically, for a 24mm lens at f/2.8 on a full-frame camera, you'll get around 10-15 seconds. Always use the 'Sharp' setting for critical work.

NPF CalculatorAstrophotography Tools

Coordinates Converter

Convert between equatorial and horizontal coordinates, look up celestial objects, and generate telescope GOTO commands for your astrophotography sessions.

Location & Time

Location Name

Latitude

Longitude

Date & Time

Object Lookup

1Input: Equatorial Coordinates

Right Ascension (RA)

Format: Xh Xm Xs or decimal hours

Declination (Dec)

Format: ±X° X' X" or decimal degrees

2Result: Horizontal Coordinates

Enter valid coordinates to see result

Quick Reference

Equatorial Coordinates (RA/Dec)

• Fixed to the celestial sphere
• Right Ascension: 0h - 24h (or 0° - 360°)
• Declination: -90° to +90°
• Used in star catalogs and telescope GOTO

Horizontal Coordinates (Alt/Az)

• Observer-centric, changes with time
• Altitude: -90° to +90° (horizon = 0°)
• Azimuth: 0° - 360° (N=0°, E=90°, S=180°, W=270°)
• Used for visual observation planning

Frequently Asked Questions

What is the difference between equatorial and horizontal coordinates?

Equatorial coordinates (Right Ascension and Declination) are fixed to the celestial sphere and remain constant for celestial objects. Horizontal coordinates (Altitude and Azimuth) are observer-centric and change as the Earth rotates. Altitude measures height above the horizon (0° to 90°), while Azimuth measures the compass direction (0° = North, 90° = East, etc.).

How do I convert RA/Dec to Alt/Az?

Enter your location (latitude and longitude) and the date/time of observation. Then input the Right Ascension (in hours, minutes, seconds format like "5h 30m 0s") and Declination (in degrees, arcminutes, arcseconds like "+45° 30' 0""). The converter will calculate the corresponding Altitude and Azimuth for your location at that time.

What format should I use for Right Ascension?

Right Ascension can be entered in sexagesimal format (e.g., "5h 55m 10s", "5:55:10", or "5 55 10") or as decimal hours (e.g., "5.92"). The converter accepts various formats and will convert between them.

What is Local Sidereal Time (LST)?

Local Sidereal Time is the right ascension of objects currently crossing your meridian (directly south in the Northern Hemisphere). It's useful for determining which objects are best positioned for observation. When an object's RA equals the LST, it's at its highest point in the sky.

How do I use the telescope GOTO command?

The GOTO command shown is compatible with Meade LX200 and similar telescope mounts. Copy the command and send it via serial connection to your mount. The command includes :Sr (set RA), :Sd (set Dec), and :MS (move to target) instructions.

Why does the altitude show negative values?

A negative altitude means the object is below the horizon and not visible from your location at the specified time. Wait for the object to rise, or check the visibility at a different time. Objects with altitude below 0° cannot be observed.

What objects are included in the lookup database?

The database includes all 110 Messier objects, popular NGC objects, named stars (like Sirius, Vega, Polaris), and common asterisms. You can search by catalog number (M31, NGC 7000) or common name (Andromeda Galaxy, Orion Nebula).

How accurate is the Alt/Az conversion?

The conversion uses standard astronomical algorithms and is accurate to within a few arcseconds for most purposes. It does not account for atmospheric refraction (which lifts objects near the horizon by about 0.5°) or proper motion of stars.

Understanding Celestial Coordinates

Equatorial Coordinate System

The equatorial coordinate system is the most commonly used system in astronomy. It's based on the celestial equator (the projection of Earth's equator onto the sky) and uses two coordinates:

Right Ascension (RA): Similar to longitude on Earth, measured in hours (0h to 24h) eastward from the vernal equinox
Declination (Dec): Similar to latitude, measured in degrees from -90° (south celestial pole) to +90° (north celestial pole)

Horizontal Coordinate System

The horizontal (or alt-azimuth) coordinate system is centered on the observer and uses:

Altitude (Alt): The angle above the horizon (0° at horizon, 90° at zenith)
Azimuth (Az): The compass direction, measured clockwise from north (N=0°, E=90°, S=180°, W=270°)

Why Convert Between Systems?

Star catalogs and planetarium software typically use equatorial coordinates because they remain constant. However, when planning observations or using an alt-azimuth mounted telescope, you need horizontal coordinates to know where to point. This converter bridges both systems, helping you plan sessions and operate your equipment.