What are Flat Frames and why do we need them?
Flat frames are calibration images taken of a uniformly illuminated surface (such as a flat box or twilight sky). They correct unwanted image artifacts such as vignetting, dust shadows on the sensor, or uneven illumination caused by the optical system. For flats to work effectively, they must be captured under exactly the same camera conditions as the actual light frames.
What is ADU?
ADU stands for Analog-to-Digital Unit – the digital unit your camera uses to measure brightness. Each pixel outputs an ADU value ranging from 0 (completely dark) to the maximum value of the bit depth. The goal for flat frames is to achieve a medium exposure level: not too dark (poor signal-to-noise ratio), and not too bright (risk of saturation and non-linearity).
Which parameters must be identical between Flats and Lights?
For calibration to work correctly, the following settings must be identical for both flat and light frames
| Parameter | Why it matters |
| Gain | Determines amplification and therefore the ADU level |
| Offset | Shifts the black point – affects the entire ADU range |
| Camera Temperature | Influences dark current noise |
| Binning | Changes effective pixel size and bit output |
| Filter | Flats must be taken separately for each filter |
| HCG/LCG Mode | Fundamentally changes the gain characteristics |
Camera Bit Depths – The Correct Basis
The maximum ADU value depends directly on the sensor’s bit depth. Here are the official specifications for both cameras:
| SkyEye 62AM | Sony IMX455 (Mono) | 16-bit | 65,535 |
| SkyEye 24AC | Sony IMX410 (Color) | 14-bit | 16,383 |
The Formula: Calculating ADU Target Value with Offset
The offset shifts the entire ADU baseline upward. This means the usable dynamic range does not start at 0, but at the offset value. The correct formula is:
ADU_Target = Offset + (Max_ADU - Offset) × PercentageA target range of 40-60% of the usable range is recommended. 50% is a well-established starting point.
Calculation Example: Offset = 125, Target = 50%
SkyEye 62AM (16-bit, Max. ADU = 65,535)
ADU_Target = 125 + (65,535 - 125) × 0.50
= 125 + 65,410 × 0.50
= 125 + 32,705
= 32,830 ADU→ Target value: ~32,800 ADU
SkyEye 24AC (14-bit, Max. ADU = 16,383)
ADU_Target = 125 + (16,383 - 125) × 0.50
= 125 + 16,258 × 0.50
= 125 + 8,129
= 8,254 ADU→ Target value: ~8,250 ADU
Summary of Results
| Camera | Bit Depth | Max. ADU | Offset | ADU Target (50%) |
| SkyEye 62AM | 16-bit | 65,535 | 125 | ~32,800 |
| SkyEye 24AC | 14-bit | 16,383 | 125 | ~8,250 |
Adapting the Formula for Other Cameras
You can apply this formula to any camera. You will need:
- The bit depth of your sensor (from the official datasheet!)
- The offset value you use for your light frames
- Your desired target percentage (recommended: 50%)
General formula for any camera:
Max_ADU = 2^Bit_Depth - 1
ADU_Target = Offset + (Max_ADU - Offset) × 0.50Examples for common bit depths:
| Bit Depth | Max. ADU | Target at Offset 0, 50% | Target at Offset 125, 50% |
| 12-bit | 4,095 | ~2,048 | ~2,110 |
| 14-bit | 16,383 | ~8,192 | ~8,254 |
| 16-bit | 65,535 | ~32,768 | ~32,830 |
Practical Tips
- Always use the same gain, offset, HCG/LCG mode, and binning as your light frames
- With narrowband filters (Ha, OIII, SII), EKOS may need significantly longer exposures to reach the target value – this is normal
- After the first test flat, check the histogram to verify the peak is around ~50%
- For color cameras (like the 24AC), the ADU value in EKOS refers to the green channel (highest sensitivity)
- Always capture flats per filter set – one flat set for LRGB, another for narrowband
Clear skies and happy imaging!
