---
title: Exposure Calculator
description: "The Exposure Calculator answers the oldest question in astrophotography – *how long should my subs be?* – with a full camera-and-sky noise model instead of folklore. It has two modes: **SNR Efficiency** for deep-sky work, and **Exoplanet Star** for point-source…"
url: "https://astro-pm.com/docs/exposure-calculator/"
updated: "2026-07-12"
category: "Astro PM App Guide / Analysis Tools & Calculators"
---

# Exposure Calculator

The Exposure Calculator answers the oldest question in astrophotography – *how long should my subs be?* – with a full camera-and-sky noise model instead of folklore. It has two modes: **SNR Efficiency** for deep-sky work, and **Exoplanet Star** for point-source photometry where saturation control is everything.

### Equipment & Camera Model

Pick a telescope and camera from your Equipment library; the calculator pulls focal length, aperture, and reducer from the scope and pixel size, read noise, full well, and QE from the camera, displaying the derived image scale (206.265 × pixel size / focal length). If your camera has defined **gain modes**, choose one and the read noise / full well / gain fields update to that mode’s values – all three are overridable for what-if analysis.

### Sky, Target & Exposure Inputs

- **Bortle Class (1-9) and SQM** – linked in both directions; the sky flux in the model comes from the SQM value, your aperture, pixel scale, and QE.
- **FWHM (arcsec)** – seeing; spreads target light over more or fewer pixels.
- **Target Type** – twelve presets with realistic surface-brightness flux values, from Bright Emission Nebula (2.0 e-/px/s) down to IFN / Galactic Cirrus (0.02), plus Custom for your own value.
- **Filter** – twelve options from Luminance through 3nm narrowband to dual-band and IR, each carrying measured sky- and signal-transmission factors (a 3nm Ha filter passes ~1% of skyglow – this is why narrowband laughs at light pollution).
- **Sub length, binning (1×1-4×4), bias, sensor temperature, dark current** – the housekeeping terms; dark current follows the doubling-per-6°C rule from your entered temperature.

[![](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131949.png)](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-131949.png)

### SNR Efficiency Mode: the Results

The model computes single-sub SNR, then expresses it as **efficiency** – your SNR versus a hypothetical read-noise-free camera. That framing makes the answer actionable:

| Result | Meaning |
| --- | --- |
| SNR (single sub) | signal × t / √(total noise) for one frame |
| SNR Efficiency % | how close you are to ideal – green ≥90%, yellow 70-90%, red <70% |
| Optimal Sub (95%) | exposure where read noise costs you only 5% – the sweet spot |
| Optimal Sub (90%) | the practical minimum worth using |
| Max Before Clip | where the brightest pixel hits full well |

The key insight the chart makes visceral: past the 95% point, longer subs gain almost nothing while costing you more ruined frames per guiding hiccup or cloud. The curve plots efficiency against sub length (1-3600s) with your current sub marked, so you can see exactly where you sit.

![](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-132136.png)
Reflection Nebula in Bortle 5 with 14" RC and Red fitler, 300s exposure

### The Well-Fill Visualization

The stacked bar shows where your electrons actually come from at the current settings – **bias, dark, sky, and signal** stacked on a log scale up to full well. One glance tells the story of your site: under Bortle 8 skies with a luminance filter, the sky bar dwarfs the signal bar – and the calculator is quietly explaining why sub length barely matters there and why the narrowband filters transform the picture.

### Exoplanet Star Mode

Transit photometry inverts the problem: one bright star, and the goal is keeping its **peak pixel** at a healthy well fraction (50% is the classic sweet spot) – saturated means ruined, too shallow wastes precision. Inputs: star V-magnitude, target well %, optical throughput, airmass and extinction, and **spectral type** (O through M) – the star’s color changes how its V magnitude translates into your imaging band.

Results: the **Recommended Exposure** headline, star flux and in-band magnitude, peak pixel rate, per-frame **aperture SNR** and **photometric precision in millimagnitudes** (a typical transit is 5-20 mmag deep – the number tells you if you can see it), and the **system zeropoint**.

   **Tip:** The zeropoint is theoretical until you calibrate: the [Image Annotator](/docs/image-annotator/) can measure real star fluxes in your frames and calibrate the filter – the recommendation turns green once it rests on measured data instead of the ~40×-optimistic theoretical model.