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

FITS Viewer

View, analyze, and convert FITS astronomical images directly in your browser. No software installation required.

Supports grayscale and color FITS files. View header metadata, apply stretch algorithms, and export to standard formats.

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Supports .fits, .fit, .fts files

About FITS Files

What is FITS?

FITS (Flexible Image Transport System) is the standard data format used in astronomy. It stores image data along with metadata about the observation, equipment, and processing history.

Supported Features

8, 16, 32-bit integer images
32, 64-bit floating point images
Grayscale and RGB images
Full header metadata viewing
Multiple stretch algorithms
Export to PNG, JPEG, WebP

Frequently Asked Questions

What is a FITS file?

FITS (Flexible Image Transport System) is the standard file format used in professional and amateur astronomy. It stores image data along with extensive metadata (headers) about the observation, including exposure time, telescope, camera settings, coordinates, and more. Unlike JPEG or PNG, FITS preserves the full dynamic range and scientific data of astronomical images.

What are stretch algorithms?

Astronomical images have a very high dynamic range that monitors cannot display directly. Stretch algorithms compress this range to make faint details visible. Linear applies no compression,Sqrt (square root) gently compresses highlights, Log (logarithmic) provides stronger compression, and Asinh (inverse hyperbolic sine) is ideal for showing both faint nebulae and bright stars together.

Is my data secure?

Yes! All processing happens entirely in your browser. Your FITS files are never uploaded to any server. The file is read locally using JavaScript, processed in memory, and displayed on your screen. When you close the page, all data is gone. This makes the tool fast, private, and works even offline.

What bit depths are supported?

This viewer supports all common FITS bit depths: 8-bit unsigned integers, 16-bit signed integers (most common for CCD cameras), 32-bit signed integers, 32-bit floating point, and 64-bit floating point. Both grayscale and multi-channel (RGB) images are supported.

What software creates FITS files?

FITS files are created by astronomy camera software (NINA, SGP, APT, SharpCap, FireCapture), image stacking software (DeepSkyStacker, Siril, PixInsight, AstroPixelProcessor), and directly by scientific cameras and observatories. Many astrophotographers work with FITS throughout their workflow for maximum data quality.

Why Use This FITS Viewer?

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100% Private

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Instant Preview

No software to install. Just drag and drop your FITS file to view it immediately.

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Full Metadata

View all FITS header keywords including exposure, equipment, and coordinates.