Guide to Calibration Frames

Calibration frames serve a vital role in enhancing the quality of your astrophotography by minimizing noise and correcting various issues that can occur during long-exposure imaging. To maximize the potential of your astrophotography setup, it’s crucial to understand and utilize the following calibration frames:

Bias Frames: Capturing inherent sensor noise.

Bias frames help quantify and remove the noise present in your camera’s sensor. Due to slight variations in pixel characteristics, a gradient-like noise pattern can emerge. Bias frames, short exposures taken with the lens cap on, provide a baseline for noise removal.

Dark Frames: Correcting thermal noise in long-exposure images.

When taking extended exposures, your camera’s sensor can heat up, introducing thermal noise. The severity of this noise depends on factors such as temperature, exposure time, and ISO settings. Dark frames, captured with the lens cap on under conditions similar to your light frames, enable you to subtract this sensor noise and account for hot or cold pixels. Note that some modern sensors can internally perform dark subtraction, negating the need for separate dark frames.

Flat Frames: Ensuring uniform illumination across images.

Flat frames are essential for addressing optical imperfections that can result in uneven pixel sensitivity. Issues such as dust particles, lens vignetting, and other artifacts can cause variations in brightness across your images. To correct these problems, flat frames are captured by evenly illuminating your telescope or camera lens and adjusting the exposure time until a specific histogram level is achieved.

Dark Flat Frames: Addressing noise in flat frames.

Dark flat frames serve a similar purpose to dark frames but are tailored to the short exposure times typically used for flat frames. They help account for any noise specific to your flat frame exposures.

When determining the number of calibration frames to capture, keep in mind that more frames generally result in better noise reduction, though the improvement diminishes following the inverse square law. As a guideline:

How to Capture Each Frame


  • Setting: In complete darkness.
  • Method: Use your camera’s shortest possible exposure setting. Cover the lens or telescope with its cap to ensure no light seepage. Shoot multiple images in rapid succession.
  • Suggestions:
    • Capture Consistency: Bias frames should be taken quickly, one after the other, without much delay. This ensures the camera’s internal conditions remain consistent.
    • ISO Setting: Always use the same ISO setting for bias frames as you do for your light frames.


  • Setting: Also in complete darkness.
  • Method: Set the camera to the same exposure time you use for your main astro images. If you’re shooting the Milky Way at 30 seconds exposure, use that. The lens or telescope should remain covered. Ideally, try to match the ambient temperature of when you’ll be shooting the night sky. The closer the conditions, the more accurate the noise reduction.
  • Suggestions:
    • Exposure Matching: It’s essential that the exposure time of your dark frames matches that of your light frames. If your light frames are 5 minutes long, your dark frames should be too.
    • Temperature Matching: Dark noise is temperature-dependent. If possible, capture dark frames at the same temperature as your light frames. Some advanced cameras display the sensor temperature, which can be a handy reference.


  • Setting: In front of a uniformly lit surface.
  • Method: A common practice is to stretch a white t-shirt over the end of the telescope or lens, ensuring it’s smooth with no wrinkles. Then, aim the camera at a neutral light source, like a twilight sky or an evenly lit computer screen. Adjust exposure so the image is neither too bright nor too dark. It’s essential to maintain the same camera settings and orientation you use for your main images. This ensures that the correction perfectly aligns with any inconsistencies in the main shot.
  • Sugestions:
    • Uniform Light Source: Ensure the light source for flat frames is even and consistent. Some astrophotographers use a “lightbox” or the twilight sky to get a uniform illumination.
    • Maintain Optical Path: The position and orientation of the camera and any additional equipment (like filters) should remain unchanged between capturing light frames and flat frames. This ensures that any vignetting or dust shadows are correctly mapped.

Dark Flats

  • Setting: Complete darkness.
  • Method: After capturing flat frames, simply cover the lens or telescope. Keep the same exposure time as you used for the flat frames. Because these exposures are often shorter than the main images, the process is usually quicker.
  • Suggestions:
    • Exposure Time: As with dark frames, the exposure time of your dark flats should match that of your flat frames.

When capturing each type of frame, it’s important to maintain stability. Using a tripod or a steady surface is crucial. Also, using a camera’s built-in timer or a remote trigger can prevent any shake or vibration from pressing the camera’s shutter button.

How many should I take?

  • Bias Frames: Capture between 50 to 100 frames. These are quick to acquire and can be used across multiple imaging sessions.
  • Dark Frames: Aim for a minimum of 20 frames, potentially increasing up to 50 frames depending on your exposure time. In some cases, as few as 5 dark frames may suffice for very long exposures.
  • Flat Frames: A range of 25 to 50 flat frames is typically sufficient. Extensive testing has shown that exceeding 25 frames may not significantly improve results.
  • Dark Flat Frames: Match the quantity to your flat frames.

Consistency is paramount when capturing calibration frames. Ensure that all frames are taken at the same temperature, and match ISO settings for calibration frames with those used for your light frames.

In conclusion, calibration frames are indispensable tools for astrophotographers seeking to produce high-quality images of the cosmos. By understanding the purpose and significance of each calibration frame type and tailoring their quantity to your specific setup and conditions, you can elevate your astrophotography to new heights of clarity and detail.

General Tips

  • Consistent Temperature: Ensure Light, Dark, and Bias frames are taken at the same temperature. Temperature variations can influence the noise in the image, so consistency helps in getting more accurate corrections.

  • ISO Consistency: The ISO setting is crucial for calibration frames. It needs to be consistent across all frames. For instance, if your main image uses ISO 1600, all calibration frames should use the same setting.

  • Longevity of Bias Frames: Once captured, bias frames can often be reused over extended periods. These frames tend to remain consistent unless there are significant changes to the camera or its sensor.

  • Evaluating Dark Frames: Not all modern sensors require dark frames. To determine if they’re beneficial for your setup, process images both with and without dark frames and compare the results.

  • Building a Dark Library: Instead of capturing new dark frames every time, consider creating a library of dark frames for various conditions (exposure times, temperatures, and ISO settings). Label them clearly, so you can quickly find and use the appropriate frame when needed.

  • Cloudy Nights: Cloudy nights can be disappointing for capturing celestial objects, but they’re perfect for capturing darks. Use these nights to build and expand your dark library.

  • Flats Are Essential: While they might seem tedious, never skip flat frames. Dust, smudges, or optical issues can be challenging to address in post-processing without them.

  • Limit ISO Variability: Especially for beginners, consider sticking to one or two ISO settings. This reduces the variety of calibration frames you’ll need and simplifies the process.

  • Remote Triggers: Using a remote trigger can reduce camera shake when taking calibration shots, ensuring more consistent and clearer results.

  • Regular Updates: Even if you’ve built libraries, periodically update your calibration frames, especially if there have been changes to your equipment or if you notice shifts in image quality.

  • Meticulous Labeling: Always label and organize your calibration frames properly. Given that you might be capturing many frames over time, good organization will make it easier to locate and use the right calibration frames when processing.

  • Reusability: While bias and dark frames can often be reused over several sessions (unless there are changes in equipment or conditions), flat frames should be taken every imaging session due to the potential for dust movement and other changes in the optical path.

  • Stacking Calibration Frames: Before applying to your light frames, stack multiple of the same type of calibration frames to create a “master” calibration frame. This averages out random noise and gives a cleaner correction.

Cheat Sheet

There are four types:

  • Bias: Capture inherent sensor noise.
  • Dark: Counteract thermal noise in long exposures.
  • Flat: Correct for variations in light sensitivity.
  • Dark Flat: Match darks for flat frames.

How to capture them:

  • Bias: Short, dark exposures with the lens cap on.
  • Dark: In the dark, matching exposure times and temperature.
  • Flat: Use a consistent light source like a laptop screen.
  • Dark Flat: Short exposures, similar to flat frames.

Recommended quantities:

  • Bias: 50-100
  • Darks: At least 20, up to 50
  • Flats: 25-50
  • Dark Flats: Match your flat frames

Calibration frames are not a burden but a path to realizing the full potential of your equipment. The meticulous preparation and capture of these frames will yield images that truly showcase the awe-inspiring beauty of the night sky.

This article was updated on October 1, 2023