Description
A dark frame mismatch occurs when the dark frames used for calibration do not match the conditions of the light frames: different temperature, exposure time, or gain/offset.
The subtraction of the thermal signal then fails. If the darks are too weak (cooler or shorter), bright hot pixels remain uncorrected. If they are too strong, the subtraction carves black holes at the locations of hot pixels, and the background may appear mottled.
Residual amp glow (a halo in a corner) is a frequent sign: the glow depends on temperature and exposure time, and a mismatched dark only removes it partially, leaving a trace or an over-subtracted region.
This is a calibration defect, unrelated to optics: it is fixed by rebuilding a properly matched dark library. Distinct from residual hot pixels caused by lack of dithering, and from raw amp glow that is not calibrated at all.
Visual signature
At 100%, one can spot isolated bright specks (uncorrected hot pixels) or, conversely, dark spots and black holes where the subtraction dug too deep.
A residual halo in a corner (poorly removed amp glow) or an area of background darker than the rest betrays a dark whose temperature or duration did not match the light frames.
The sky background may appear mottled or coarsely grainy in a fixed pattern, with the same motif at exactly the same position on every frame, the signature of a poorly subtracted thermal signal rather than random noise.
Without dithering, these residuals freeze in place and sometimes form streaks instead of being averaged out by stacking.
Differential diagnosis
Do not confuse with classic residual hot pixels: those persist mainly from lack of dithering and cosmetic correction, whereas a dark mismatch comes from incorrect temperature, duration, or gain of the darks.
Related to amp glow: amp glow is the electronic halo present on the raw frame; a well-matched dark removes it, a mismatched dark leaves a trace or creates an over-subtracted region in the affected corner.
Distinct from banding: banding draws regular lines, whereas a dark mismatch produces isolated spots or a non-linear mottled background.
Separate from random noise: dark residual is fixed (same position on every frame), while read noise changes with every image.
Probable causes
- Darks taken at a different temperature from the light frames
- Dark exposure time different from that of the lights
- Gain or offset of the darks not identical to those of the lights
- Dark library too old (new hot pixels have appeared since)
- Unregulated camera (DSLR) whose temperature fluctuates between lights and darks
- Absence of dithering, which locks residuals in place
Course of action
- Strictly match darks to lights: same temperature, exposure, gain, and offset
- For cooled cameras, build a dark library for each exposure/gain pair at regulated temperature
- For DSLRs, retake darks at the end of the session at the ambient temperature of the moment
- Regularly refresh the dark library (sensor aging)
- Enable dithering to neutralize remaining fixed residuals
- Supplement with CosmeticCorrection (PixInsight) on isolated hot and cold pixels
- Check consistency between master dark and master bias (dark optimization)
The Doc's advice
Darks are simple but unforgiving: they must match your lights on three parameters, temperature, exposure time, and gain/offset. A cooled camera at regulated temperature solves the problem for good: build a dark library at -10 degrees C (or your setpoint) for each exposure/gain combination, and you will not need to touch it for months. On an unregulated sensor (DSLR), forget old darks, retake them at the end of the session at the same temperature, otherwise you will carve black holes or leave hot pixels. And dither: even with perfect darks, it is your safety net against any fixed residual. Black holes mean you are over-subtracting; bright specks mean you are under-subtracting.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
How can I tell whether my darks are mismatched?
Two opposite signatures give the answer. If you see black spots (small dark holes) at the locations of hot pixels, your darks are over-subtracting: they are too strong (warmer or longer than the lights). If bright hot pixels remain, your darks are under-subtracting: they are too weak. A residual halo in a corner or a fixed mottled background also points to a mismatch. The reflex: compare temperature, exposure time, and gain/offset of the darks and lights; they must be identical.
Must darks be at the same temperature as the lights?
Yes, it is the most critical parameter. Dark current (and therefore hot pixels and amp glow) increases strongly with temperature, roughly doubling every 6 to 8 degrees C (varies by sensor). A difference of just a few degrees is enough to miscalibrate. This is why a cooled camera at regulated temperature is ideal: fix a setpoint (for example -10 degrees C) and the darks remain valid. On an unregulated body (DSLR), the darks must be retaken under temperature conditions as close as possible to those of the light frames.
Can a dark library be reused?
Yes, on a cooled camera at regulated temperature: a library of master darks for each exposure/gain pair remains valid for several months, saving precious time. It still needs to be refreshed occasionally as the sensor ages and new hot pixels appear. On an unregulated body (DSLR), reuse is risky: the temperature varies from session to session and old darks cause over- or under-subtraction. In that case, it is better to retake darks at each outing.
Why do I have black spots after calibration?
Black spots are the symptom of over-subtraction: the master dark removes more thermal signal than is actually present in the lights, digging a hole at the location of each hot pixel. Usual causes: darks taken at a higher temperature or for a longer duration than the lights, or a scaling problem between master dark and master bias (dark optimization incorrectly applied). The solution is to re-match the darks strictly to the lights, or to disable dark optimization if it is digging into the image. Dithering then helps dilute any remaining residual.