Astrophotography Exposure and Field-of-View Planner
Plan framing, sampling, NPF exposure limits, star trailing, sub-exposures, and integration time for your camera and optics.
Last reviewed: June 2026Camera
Optics
Target & Conditions
Optical Parameters
Exposure Limits
Integration Planning
Exposure Comparison
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What this planner is actually doing
This page turns a camera, lens or telescope, target declination, tracking mode, and sky brightness into practical planning numbers. It estimates effective focal length, pixel scale, field of view, maximum untracked exposure by the NPF rule, the older 500-rule exposure, star-trail size at the NPF limit, a starting sub-exposure, total integration time, and the number of subframes needed.
The calculator is most useful before a session: compare whether a target fits your sensor, whether your sampling is reasonable, and whether a planned exposure length is plausible. It is not a replacement for test frames. Real results depend on focus, seeing, transparency, tracking error, guiding, filter bandpass, camera gain, read noise, sky background, and how aggressively you reject poor frames while stacking.
Camera and optics inputs
Sensor size and resolution define pixel pitch. The planner auto-calculates pixel size from sensor width and pixel width, but you can override it when your camera specification lists a more accurate value. Focal length and a Barlow or reducer factor produce effective focal length, which drives both field of view and image scale.
Pixel scale is computed as 206.265 x pixel size in micrometers divided by effective focal length in millimeters. A smaller value gives more pixels across a target, but it is not always better. If seeing and mount performance blur the image to several arcseconds, extremely fine sampling mostly spreads the same signal across more pixels.
Exposure-limit outputs
The NPF result uses the simplified nightscape formula from aperture, pixel pitch, focal length, and declination. It is stricter than the 500 rule for many modern high-resolution cameras because it accounts for pixel size and where the target sits in the sky. The 500 rule is kept as a familiar reference, not as the final recommendation.
The star-trail output converts the NPF exposure into angular motion and pixels. This helps you decide whether a small amount of elongation will matter at your final display size. For tracked or guided imaging, the planner switches from untracked exposure limits to heuristic sub-exposure suggestions based on tracking mode and Bortle class.
Worked default example
With the default full-frame 36 x 24 mm sensor, 6000 x 4000 resolution, 6.00 um pixels, 200 mm focal length, f/2.8 aperture, no Barlow or reducer, declination 0 degrees, tracking mount, and Bortle 5 sky, the planner reports a 200 mm effective focal length and a pixel scale of about 6.19 arcsec per pixel.
The field of view is about 10.31 x 6.88 degrees, so it is a wide-field Milky Way or constellation setup rather than a small-galaxy setup. The simplified NPF limit is about 1.4 seconds, while the 500 rule gives about 2.5 seconds. Because tracking is enabled and the sky is Bortle 5, the planner suggests 2-minute sub-exposures, about 10 total hours, and roughly 300 kept subframes.
How to use the results in the field
- Check framing first: if the field of view is too narrow, use a shorter focal length, a reducer, a larger sensor, or a mosaic plan.
- Use NPF for untracked stars: start shorter than the displayed limit, then inspect the corners at the magnification you actually care about.
- Treat tracked sub-exposure as a starting point: shorten it if stars bloat, the background clips, or guiding is unstable; lengthen it only when the histogram and star shape support it.
- Prioritize total integration: faint nebulae and galaxies usually improve more from more clean subframes than from forcing a single sub-exposure longer.
Common mistakes
- Using ISO as a light-gathering control: ISO changes camera amplification and file values; aperture, exposure time, sky brightness, and total integration govern collected photons.
- Ignoring declination: fields near the celestial equator trail faster than fields near the poles.
- Comparing NPF and 500 as equal rules: the 500 rule can be a rough legacy estimate, while NPF is usually the better starting point for modern sensors.
- Overreading the sampling badge: pixel scale is only one part of image quality; seeing, focus, tracking, optics, and processing matter too.
Sources
This page is aligned with the Societe Astronomique du Havre NPF rule reference, NASA's astrophotography guide, the AAVSO CCD/CMOS photometry guide, and STScI guidance on exposure-time calculations.