Astrophotography Exposure Calculator
The focal length of your lens or telescope in millimeters
Your lens f-number (required for NPF Rule)
Used to auto-derive pixel pitch if not entered directly
Pixel size in micrometers — auto-calculated from MP + sensor if left blank
Declination of your target (0° = celestial equator, 90° = Polaris)
Common Target Declinations
Orion Nebula (M42): -5°
Andromeda Galaxy (M31): +41°
Milky Way Core: -29°
Pleiades (M45): +24°
Polaris: +89°
Enter Your Camera Settings
Fill in your focal length, sensor size, and aperture to get the maximum safe exposure time and compare all calculation rules side by side.
How to Use This Calculator
Choose Your Mode
Select 'Star Trailing' if you are shooting handheld or on a fixed tripod and need the maximum shutter speed before stars trail. Select 'Sub-Exposure Planner' if you have a tracking equatorial mount and want to find the ideal per-frame exposure length for stacking deep-sky images.
Enter Camera and Lens Settings
Input your focal length in millimeters, choose your sensor size from the dropdown (this sets the crop factor automatically), and enter your aperture f-number. If you know your camera's pixel pitch in µm, enter it directly. Otherwise enter your megapixel count and the calculator will derive it automatically from your sensor dimensions.
Add Declination and Review the Chart
For the most accurate result, enter the declination of your target object in degrees (Orion ≈ -5°, Andromeda ≈ +41°, Milky Way core ≈ -29°). The comparison bar chart instantly shows all four rule results side by side — pick the most conservative value (NPF or Plate Scale) for the sharpest stars on a modern sensor.
Export and Plan Your Session
Click 'Export CSV' to save all inputs and results as a spreadsheet you can take to the field. For sub-exposure planning, set the Bortle scale to match your site and choose your camera type (Color, Mono, or Narrowband). The planner returns the recommended seconds per frame, helping you decide how many subs to collect for a useful integration time.
Frequently Asked Questions
What is the difference between the 500 Rule and the NPF Rule?
The 500 Rule is a quick rule of thumb: divide 500 by your effective focal length and you get a rough maximum exposure in seconds. It was calibrated for low-resolution film and early digital cameras (10–12 MP). The NPF Rule, developed by astrophotographer Frédéric Michaud, adds your lens aperture and pixel pitch to the formula, producing a result that is typically 30–60% more conservative on modern high-resolution cameras. For a Sony A7R IV (61 MP) at 24mm f/1.4, the 500 Rule gives about 14 seconds while the NPF Rule gives around 5–6 seconds — a dramatic difference. For best results on sensors above 20 MP, always prefer the NPF Rule over the classic 500 Rule.
How does declination affect the maximum exposure time?
Stars near the celestial equator (declination 0°) move at the full sidereal rate of 15 arcseconds per second relative to a fixed sensor. Stars near the celestial poles move much slower because they trace tighter circles. The correction factor is cos(declination): at 60° declination the apparent motion is only half as fast, doubling your allowable exposure. At Polaris (+89°) the correction factor is virtually zero, allowing very long exposures. For Orion (-5°) the correction is negligible. For Andromeda (+41°) you gain about 25% more exposure time. Entering your target declination into this calculator automatically applies this adjustment to the NPF and Plate Scale results.
What is pixel pitch and how do I find mine?
Pixel pitch is the physical size of each individual photosite on your camera's sensor, measured in micrometers (µm). It is the most important variable the 500 Rule ignores. A Sony A7 III has 5.93 µm pixels; a Sony A7R IV has only 3.76 µm pixels — meaning the A7R IV will show star trailing nearly 60% sooner at the same focal length. You can find your camera's pixel pitch on DxOMark, DigicamDB, or the manufacturer's spec sheet. Alternatively, enter your megapixel count and sensor format into this calculator and it will derive the pixel pitch automatically using the known sensor dimensions for each format.
What is the Bortle scale and why does it matter for sub-exposures?
The Bortle scale rates night sky darkness from 1 (pristine dark sky, no artificial light pollution) to 9 (inner city sky where only the brightest stars are visible). For tracked deep-sky imaging, the sky background is the main noise source competing with your target signal. In darker skies (Bortle 1–3), the sky is very faint, so you need longer sub-exposures to ensure sky noise exceeds read noise per frame. In bright suburban or city skies (Bortle 6–9), even short exposures are dominated by sky glow. The Robin Glover sub-exposure formula uses the Bortle-mapped light pollution value and your camera's read noise to compute the scientifically optimal sub-frame length, minimising the number of frames needed for a given final image quality.
When should I use a narrowband filter and how does it change my sub-exposure?
Narrowband filters (Ha, OIII, SII) transmit only a very narrow slice of light (3–10 nm bandwidth), blocking most sky glow from artificial light sources. This dramatically improves contrast on emission nebulae from light-polluted sites. However, because the filter blocks so much light, your sensor needs a much longer exposure to accumulate sufficient sky background photons for the Robin Glover threshold to be met. The narrowband multiplier in this calculator is 25× compared to monochrome. In a Bortle 5 suburban sky with 3 e⁻ read noise, you might need only 120 seconds per sub with a colour camera but 3,000 seconds per sub with a narrowband filter — essentially meaning you should use very long exposures (30–60 min subs) when narrowband imaging.
Does tracking completely eliminate star trailing?
A well-polar-aligned equatorial mount cancels the bulk of Earth's rotation, allowing exposures of minutes to hours without star trails from the sidereal rate. However, residual periodic error in the mount's worm gear, atmospheric refraction near the horizon, autoguider corrections, flexure in the optical train, and wind vibration can all cause minor trailing even with tracking engaged. For this reason, most deep-sky imagers still keep individual sub-exposures under 5–20 minutes and stack many frames instead of taking one very long exposure. The Sub-Exposure Planner in this calculator gives you the scientifically optimal frame length based on noise theory, not trailing concerns — combine both tabs to fully plan your session.