How does temperature affect long-exposure noise?

Dark current — the thermal signal that builds up even with no light — roughly doubles for every 6 to 8 °C rise in sensor temperature. A 30 °C summer night can produce several times more hot pixels than a 5 °C winter night for the same exposure, which is why cooled astro cameras exist.

What is the relative thermal-noise index?

It is a single number normalised so that a 1-second exposure at ISO 100 and 20 °C equals 1×. A reading of 10× means roughly ten times the thermal-noise standard deviation of that reference, useful for comparing scenarios rather than predicting an exact pixel value.

What is LENR and when should I use it?

Long Exposure Noise Reduction takes a second, equal-length dark frame with the shutter closed immediately after your shot and subtracts its hot pixels and fixed pattern. Use it whenever the tool reports Moderate severity or higher, accepting that it doubles total capture time.

Does lowering ISO reduce thermal noise?

Yes. ISO gain amplifies the accumulated dark signal linearly, so halving ISO roughly halves the thermal-noise index in this model. For very long exposures, a lower ISO combined with a longer time often produces a cleaner file than high ISO with a shorter time.

Why stack short exposures instead of one long one?

Several shorter sub-exposures accumulate less dark current each, and averaging them in software reduces random noise by the square root of the number of frames. This is the standard astrophotography approach and often beats a single very long exposure for shadow cleanliness.

Long Exposure Noise & Hot Pixel Estimator

Long exposures are where digital sensors get noisy in a way that has nothing to do with the scene: heat. This tool estimates how much thermal noise — hot pixels and fixed-pattern noise — a planned exposure will build up, so you can decide whether to enable Long Exposure Noise Reduction or change your settings before committing minutes of capture time.

How it works

Three physical effects combine to set the thermal-noise level.

Dark current and temperature. Even with the shutter closed, thermal energy frees electrons that the sensor counts as signal. This dark current roughly doubles for every 6.5 °C rise in sensor temperature:

dark current multiplier = 2 ^ ((temperature − 20) / 6.5)

Accumulation over time. Dark signal builds up linearly with exposure time, and the associated shot noise grows with its square root:

thermal noise ∝ sqrt(dark current × exposure time)

ISO amplification. ISO gain multiplies the accumulated dark signal linearly, so the final relative index is the square-root term multiplied by ISO / 100. The tool normalises everything to a reference of one second at ISO 100 and 20 °C, which equals an index of 1.

How to read the result

The index is mapped to four bands. Below 2× thermal noise is negligible. Between 2× and 8× you will start to see hot pixels in the shadows and LENR is worthwhile. Above 8× the noise is significant, and above 25× it is severe — at which point dark-frame subtraction, a lower ISO and a cooler sensor all help.

Tips and notes

Sensor temperature usually runs a few degrees above ambient air, and rises further during a burst of long exposures as the electronics warm up. Pause between frames on hot nights.
LENR doubles total capture time because it shoots a matching dark frame. For star trails built from many sub-exposures, it is often better to shoot a few dedicated dark frames and subtract them in software instead.
This is a planning estimate. Real noise depends on the specific sensor’s dark-current spec and on amp glow, which this simplified model does not include.