Darkframe Stacking Time Min

Total Dark-Frame Calibration Time

A dark frame is a calibration shot you take with the lens or telescope capped, so no light reaches the sensor. What it captures is the camera's own thermal signal and fixed-pattern noise, and you later subtract that from your light frames. To find how long your darks take in total, the math is total = (number of darks × exposure per dark) ÷ 60 in minutes. So 30 darks at 120 s each come out to (30 × 120) ÷ 60 = 60 minutes.

The subtraction only works if each dark matches its light frames in exposure time, in ISO or gain, and in sensor temperature. Thermal noise climbs as the exposure gets longer and as the sensor warms up, so a dark that was shot too warm or too short won't cancel the pattern you actually have. That's why most imagers take 20–50 darks and average them into a master dark, which keeps the read noise of the darks themselves from leaking into the result.

Applications

It's handy for budgeting how much calibration time you'll need at the end of a session. You can also use it when planning a reusable dark library, though that's really only practical with a cooled camera that holds a fixed, repeatable temperature.

FAQ

Must darks match my light frames exactly? Yes. Same exposure, same gain/ISO, same sensor temperature. Miss any of those and the thermal signal won't subtract cleanly.

How many darks should I take? Usually somewhere between 20 and 50. The more you take, the cleaner the master dark, and the less noise of their own they add to the stack.

Can I reuse darks from another night? You can, but only when the temperature, exposure and gain all line up. That's tough with an uncooled DSLR, which is why most people just shoot fresh darks every session.