(Bias, Darks, Flats — and when not to use them)
Astrophotography sensors do not record “pure signal”. Every exposure contains fixed electronic offsets, thermal noise, and optical artefacts.
Calibration frames exist to remove predictable errors, so stacking can focus on real sky signal.
However, not all calibration frames are always helpful, especially for DSLRs and uncooled cameras.
What they are
Bias frames are zero-length exposures taken with the lens cap on.
They capture the camera’s electronic readout offset and read-noise pattern.
What they correct
Fixed electronic offset
Part of the read-noise structure
Required for proper flat calibration
When to use them
✔️ Always useful for DSLRs
✔️ Often useful for OSC workflows that rely on bias-based flat calibration
Bias frames are fast, stable, and do not depend on temperature.
What they are
Dark frames are exposures taken with the lens cap on, using the same exposure length, gain/ISO, offset, and ideally temperature as your lights.
They capture:
Dark current (thermal electrons)
Hot pixels
Amp glow (if present)
Fixed-pattern noise tied to exposure length
The problem with uncooled cameras
For DSLRs and uncooled OSC cameras, sensor temperature can drift by 5–10 °C or more during a single night.
That means:
Dark current no longer matches
Amp glow strength changes
Subtraction becomes imperfect
Noise can be added, not removed
Residual banding or glow can remain
In many real-world cases, poorly matched darks do more harm than good (so just avoid and dither).
What they are
Flats are evenly illuminated images taken to map:
Vignetting
Dust shadows
Optical unevenness
What they require
Same optical train (focus, filters, orientation)
Proper offset correction (bias)
Flats are essential — they correct errors that stacking and dithering cannot.
How to do them... Put your telescope gently against a white monitor, or place a white LCD panel onto it. Keep the same gain. Change the exposure so the histogram is 50%.
Take 20-30 images.
These are suggestions, not dogma.
✅ Bias + Flats
✅ Dithering, always if possible
✅ Strong rejection during stacking
⚠️ Skip darks by default
✅ Flats
✅ Dither + strong rejection