Bias Frames

Bias frames are used to reduce noise in the final images. They are meant to capture the noise that is inherent in your camera and then subtract that from the final image.

These images can be taken in advance and reused, though you should retake them every few weeks.

Here are the settings I use:

  • Lens cap on - you want pure dark photos

  • Exposure time: Fastest possible

  • Quantity: 20-40

Light Frames

I won’t talk much about light frames here, other than saying this is the point when I usually take them.

Dark Frames

Like Bias frames, these are meant to reduce noise. This time it is the noise from the camera sensor as it heats up. When taking long exposures the sensor can heat up and cause artifacts in the images that we can remove by subtracting out the dark frames.

I take these after the light frames. The temperature of the sensor matters, which is why I do this right after the light frames when the outside temperature is the same. But see my note about using a cooled camera below, then the timing is not as important.

Here are the settings I use:

  • Lens cap on - you want pure dark photos

  • Exposure time: Same as lights

  • Quantity: 15-20

On my camera there is some serious Amp Glow in my dark frames.

Flat Frames

The last calibration frames I take are flat frames. These are used to remove gradients and even dust.

There are different ways to take these images. I use an illuminated tracing pad that I put over the lens. You could also try the “white t-shirt” technique.

Unlike the dark frames, temperature is not important with flat frames.

Here are the settings I use:

  • Flat white surface (tracing pad)

  • Keep the same focus as with the light frames

  • Exposure time: Adjust until the histogram is right in the middle

  • Quantity: 15-20

ASIAir and My Equipment

I use the AutoRun plans for all of the calibration frames when I’m using an ASIAir. For Bias and Flat frames, it automatically set the exposure time. It even calibrates the optimum exposure for the Flat frames.

Since my camera is cooled, I often take my dark frames before my light frames as I’m waiting for it to get dark. The camera sensor temperature should be the same no matter what. This also means I can reuse my dark frames if needed.

References

Here are some details from AstroBackyard: