Flat frame calibration is an essential step in astrophotography, as it removes vignetting and dust shadows to ensure your final image is clean and uniform. However, deciding whether to use dark flats or bias frames for calibrating flat frames can sometimes be confusing. This guide will help you calculate when you need to use dark flats instead of bias frames, ensuring optimal results for your astrophotography.
The Basics: Bias Frames vs Dark Flats
What Are Bias Frames?
Bias frames capture the read noise and offset inherent in your camera sensor at the shortest possible exposure time. They are simple to create and useful for calibrating flat frames with very short exposure times, where dark current is negligible.
What Are Dark Flats?
Dark flats are similar to dark frames but match the exposure time, gain/ISO, and temperature of your flat frames. They include both bias signal and dark current noise, making them ideal for calibrating flat frames with longer exposures.
Why Does Exposure Time Matter?
The key factor in deciding between bias frames and dark flats is dark current — the thermal noise that accumulates over time in your camera sensor. For short flat frame exposures, dark current is negligible, and bias frames suffice. But as flat frame exposures lengthen (e.g., when using narrowband filters or dim light panels), dark current becomes significant, requiring dark flats for accurate calibration.
Noise Considerations: A Trade-Off
While dark flats ensure better calibration accuracy, they can introduce calibration noise if not enough dark flats are averaged. Bias frames, on the other hand, introduce less noise but leave residual dark current uncorrected in longer flat exposures.
Total Calibration Noise Breakdown:
- Bias Calibration Noise:
- Low noise due to short exposures.
- Fails to correct for dark current in long flat exposures, leaving residual error.
- Dark Flat Calibration Noise:
- Includes both read noise and dark current noise.
- Reduced by averaging multiple dark flats.
How to Calculate the Crossover Point
The crossover point is where the total noise from using bias frames equals the total noise from using dark flats. To calculate this:
- Estimate Dark Current Noise:
- Dark Current (μ) = Exposure Time (s) × Dark Current Rate (e-/pixel/s).
- Noise = √(Dark Current).
- Consider Read Noise and Averaging:
- Dark Flat Noise = √(Read Noise^2 + Dark Current Noise^2) / √(Number of Dark Flats).
- Bias Noise = Read Noise / √(Number of Bias Frames).
- Compare Residual Error:
- Bias Calibration Total Noise = √(Bias Noise^2 + Residual Dark Current Noise^2).
- Graph or Calculate the Crossover:
- Plot total noise for both methods against flat frame exposure time.
For most modern cameras, the crossover point typically occurs around 1-3 seconds of flat frame exposure. Below this, bias frames are sufficient. Beyond this, dark flats are necessary for accurate calibration.
Example: ZWO 6200MM Pro at -10°C
For a ZWO 6200MM Pro camera cooled to -10°C, with a dark current rate of 0.0025 e-/pixel/s:
- For flat frames under 2 seconds: Bias frames are adequate, as dark current is minimal.
- For flat frames longer than 2 seconds: Dark flats are necessary to account for the growing dark current.
In the graph below, there are two markers to note. left vertical line indicates the point at which current starts to accumulate, affecting the accuracy of your images. The rightmost darker dotted line represents the point at which bias calibration frames alone will exceed the noise of dark calibrated frames.
The “million-dollar question” is how much after 2 seconds up to ~28 seconds impacts the quality of your images. Normally, I don’t see many people do flat calibration of flat frames above 28 seconds, so the obvious point at which you would need flat darks is anything nearing 28 seconds as your noise would increase relative to the noise from dark calibration which will remain flat beyond this point.
Example ASI 1600mm
In comparison to the more modern 6200, the 1600 has a much higher dark current and a higher read noise. On the 1600 the accuracy starts to be questionable around five seconds. If you have flats of 5 seconds or less, then bias calibration is enough but beyond 5 seconds, you should consider flat dark calibration.
Sensor Observations
In plotting out the differences between the older 1600mm and the newer 6200mm it’s interesting to see the difference in read noise and dark current as it pertains to flat calibration.
- The ASI 1600MM Pro has a higher dark current rate than the ASI 6200MM Pro, making residual dark current errors grow faster with exposure time.
- The accuracy transition point occurs later (~5 seconds), but the noise crossover point (~20 seconds) happens earlier than for the 6200MM, reflecting older sensor technology.
- This highlights how modern sensors (like the 6200MM Pro) handle dark current more effectively, making bias frames usable for longer flat exposures.
Practical Tips for Astrophotographers
- Short Flat Exposures (<2 seconds): Use bias frames for simplicity and lower noise.
- Long Flat Exposures (>2 seconds): Use dark flats to ensure accurate calibration, even if they add slight noise.
- Number of Calibration Frames: Average at least 20 dark flats or 50 bias frames to minimize noise.
- Match Conditions: Ensure dark flats match the exposure time, gain/ISO, and temperature of your flat frames.
Conclusion
Choosing between bias frames and dark flats depends on your flat frame exposure time and the level of precision you need. By understanding the trade-offs and using the calculations above, you can make informed decisions to achieve cleaner, more accurate astrophotography images.
Start testing with your own equipment to determine the crossover point for your specific setup and enhance your calibration workflow!
