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Dark, flat and bias frames
Calibration frames are extra pictures you take, with the same camera, to clean up the pictures of your target. They remove noise, hot pixels, dark corners and dust shadows. The idea is simple: your software uses them as a recipe, roughly (light − dark) ÷ flat.
| Frame | Fixes | How many | Cover on? |
|---|---|---|---|
| Darks | Hot pixels, heat noise, amp glow | 20 to 50 | Yes |
| Flats | Dark corners, dust shadows | 20 to 50 | No, even light |
| Dark flats | Noise in the flats | 20 to 50 | Yes |
| Bias | Fixed readout pattern | 50 to 100 | Yes |
Rule of thumb: keep the settings the same as your lights (exposure, ISO or gain, temperature) for darks, and keep the optics the same (focus, rotation, filter) for flats. Use dark flats or bias, not both.
Pick your camera for a short list.
The sensor warms up as the night goes on, so darks are best taken in the field at the same temperature as your lights.
- DarksYes20 to 50
Same exposure, ISO and temperature as the lights. Take them at the end of the session, lens cap on.
- FlatsYes20 to 50
Same focus, ISO and orientation as the lights. Take them before you touch anything.
- Dark flatsRecommended20 to 50
Darks at the exact exposure of your flats. Use these or bias, not both.
- BiasOptional50 to 100
Only if you are not using dark flats. Fastest shutter speed, cap on.
Dark frames
A picture of the sensor's own heat noise
- What it fixes
- Hot pixels, thermal noise and amp glow (the faint colored glow some sensors show at the edges of long exposures). These are the same in every frame, so a dark lets the stacking software subtract them.
- How many
- 20 to 50. Thirty is a common choice.
- Camera settings
- Same exposure length as your lights
- Same ISO (or gain and offset)
- Same sensor temperature (cooled cameras: the same set point)
- Same file format, raw
- Lens cap or scope cover on, and the viewfinder or eyepiece covered
- How to shoot them
- Cover the lens or telescope so no light gets in, then shoot a set with exactly the settings of your light frames. Nothing else about the camera is allowed to change.
- When
- Uncooled cameras: at the end of the session, before the camera cools down, so the temperature matches. Cooled cameras: any time, and you can reuse the set for months.
Flat frames
A picture of an evenly lit blank field
- What it fixes
- Vignetting (dark corners), dust shadows (the round dark donuts) and small differences in how sensitive each pixel is. Dividing your lights by the flat evens all of it out.
- How many
- 20 to 50 per filter.
- Camera settings
- Same focus position, focal length and aperture as the lights
- Same camera rotation and the same filter
- Same ISO (or gain and offset)
- Exposure so the histogram peak sits near the middle, about 40 to 50 percent of the way across
- Raw format, nothing clipped at the right edge
- How to shoot them
- Point the scope or lens at an even, dim light: an LED flat panel, a tablet showing a plain white screen with a sheet of paper over it, or a white T-shirt stretched over the front facing twilight sky or a lamp. Use auto-exposure (aperture priority) if your camera allows it, or adjust the shutter speed until the histogram is in the middle. Flat exposures are usually a fraction of a second.
- When
- Right after the session, without touching focus, the camera angle or the dust cap. Dawn is a good time. Redo them whenever you refocus, rotate the camera, change a filter or clean the sensor.
Bias (offset) frames
The shortest possible exposure
- What it fixes
- The fixed electronic pattern every frame carries, whatever the exposure time. It is the baseline the sensor reads out with no light and no time to build heat.
- How many
- 50 to 100. They are quick to shoot.
- Camera settings
- The shortest exposure the camera allows (1/4000 s, 1/8000 s or faster)
- Same ISO (or gain and offset) as the lights
- Raw format
- Lens cap or scope cover on
- How to shoot them
- Cap the camera and take a burst at the fastest shutter speed. Temperature barely matters, so you can make a set at your desk.
- When
- Any time. A set is reusable for months, as long as ISO or gain and offset do not change. Some CMOS astro cameras make odd bias frames at the very shortest exposure. If a stack looks wrong, use dark flats instead of bias.
Dark flats
Darks matched to your flats
- What it fixes
- The noise and baseline inside your flat frames. They are subtracted from the flats so that only the vignetting and dust pattern is left. They do the job bias frames do, and work better on many modern cameras.
- How many
- 20 to 50.
- Camera settings
- Exactly the same exposure length as your flats
- Same ISO (or gain and offset)
- Raw format
- Cover on, no light
- How to shoot them
- After the flats, leave every setting alone, cap the camera and shoot a set. If you used auto-exposure for the flats, note the exposure it settled on and set it manually.
- When
- Right after the flats. If your flat exposure never changes, one set is reusable.
More helps, but with diminishing returns
Calibration frames are averaged into one master frame. The random noise in that master falls with the square root of the number of frames. Going from 10 to 40 halves it. Going from 40 to 160 halves it again, for four times the effort.
Why the numbers differ
Bias frames take a tiny fraction of a second each, so a hundred of them cost almost nothing. Darks take as long as your lights, so most people stop near thirty.
Your lights come first
Calibration frames matter, but a clean dark site, good focus and more light frames help more. If time is short, shoot the flats, then the darks, and skip the rest.
For a DSLR or mirrorless camera. Cooled astro cameras set these in their capture software, and add gain, offset and the cooling set point.
- File format
- Raw (never JPEG)
- Mode
- Manual (flats can use aperture priority)
- Long exposure noise reduction
- Off
- High ISO noise reduction
- Off
- Image stabilization
- Off
- Mirror lock-up or electronic shutter
- On for flats and bias, to avoid vibration
- White balance
- Anything. It does not affect raw
- Focus
- Manual, and do not move it between lights and flats
- Before dark: set focus, note ISO or gain and your exposure length. Write them down.
- Shoot your lights through the night.
- Straight after the last light, and before touching anything, shoot the flats.
- Cap the camera and shoot the dark flats, then the darks (uncooled cameras: now, while the sensor is still warm).
- Bias frames, if you are using them, can wait until later.
- Keep each set in its own folder, named with the settings, such as darks_120s_ISO800.
Changing something between the lights and the flats
A new focus position, a rotated camera, a refitted filter or even a bumped dust cap makes the flat the wrong shape for your lights. Take flats before you pack anything away.
Mismatched ISO, gain or exposure
A dark taken at ISO 800 does nothing for lights taken at ISO 1600. Write the settings down at the start of the night.
Using bias and dark flats together
They solve the same problem. Pick one. The stacking software should be told which one you used.
Flats that are too bright or too dark
A flat that clips on the right, or sits too far to the left, gives a wrong correction. Keep the peak near the middle.
Leaving in-camera noise reduction on
Turn off long exposure noise reduction and high ISO noise reduction. Long exposure noise reduction doubles every exposure by taking its own dark, and it is applied before your stacking software can do a better job.
Shooting darks in a warm room
An uncooled camera's dark has to match the temperature of the lights. Do it outside, right after the session.
- Do I need all four types?
- No. A good minimum for a DSLR or mirrorless camera is darks and flats. Add dark flats or bias if you want to go further. A cooled astro camera benefits from all of them, and a smart telescope needs none.
- What is a light frame?
- A light frame is an ordinary picture of your target. Calibration frames are the extra pictures you take so the software can clean the lights.
- Can I use dithering instead of darks?
- Dithering moves the pointing slightly between frames, so hot pixels fall in different places and are rejected when you stack. It works very well on low-noise sensors, and many people use dithering with only flats. Darks still help, particularly on uncooled cameras and sensors with amp glow.
- Can I reuse calibration frames from another night?
- Darks, if the exposure, ISO or gain, and temperature all match (easy on a cooled camera, hard on an uncooled one). Bias frames, yes. Flats, only if nothing in the optical path has changed, which is rare.
- Does the filter matter for flats?
- Yes. Each filter has its own dust shadows, so shoot a set for each one you use.
- What does the stacking software do with them?
- It averages each set into a master, then for every light it subtracts the master dark and divides by the master flat. Free programs such as DeepSkyStacker, Siril and Sequator handle this, as do PixInsight and Astro Pixel Processor.
- Is a T-shirt flat really good enough?
- For most cameras, yes. The goal is only a smooth, even light. A white T-shirt over the lens, pointed at the twilight sky, works well. If your flats show a gradient, a cheap LED panel is the usual upgrade.
Next: check your exposures with the histogram guide, or read about filters.