There are a lot of sites with videos, articles, and blogs that explain calibration frames in detail and provide instructions about “what goes where.” If you want to know the exact science behind calibration frames and fascinating in-depth discussions about electrons and well-depth, you should definitely not waste time reading this post. Those things are important, but many of you just want to take pictures and get results. I hope this gives you enough to understand what you need to get started without getting lost in the details.

If you’re more of a video person, check out Calibration explained in less than 4 minutes. It’s actually great if you have 4 minutes.

Let’s get right to it. Here’s how I take calibration frames. Trust me, it’s helpful to know when we go into the why. Last night, I took 30 1-minute pictures of the Orion constellation. This is what a single frame looks like:

Single frame

After I took the pictures, I came inside and pulled up an app on my phone that just makes the entire screen turn white. I dimmed my phone a little bit, then set it so my camera lens was pointed squarely at the phone. I kept the same ISO settings as my raw photos and took a few pictures with different exposures until I verified my histogram level was between 50% - 75%. It turned out to be 1/3rd of a second for this round. I then shot 20 pictures of just the phone screen. I took 20 more pictures with the lens cap on at the same ISO and exposure as the previous set. After that, I set my shutter speed to the fastest possible (1/4000th second for my camera) and, with the cap still on, took 60 more photos. Finally, I used a device called an intervalometer designed for my camera and took 30 more pictures at 60 seconds. Using the combination of the 30 photos and these “calibration frames”, here is what I was able to create.

Final image

Why you should take calibration frames

Simply put, cameras and their sensors are messy. You might pride yourself in keeping lens wipes handy, but what happens when you get dust inside your lens? The sensor itself is subject to random noise that creates fuzzy patterns and speckles of color that don’t belong. Calibration frames are special images you take to isolate things like dust, vignetting (the round “circle of light” or dark effect like what I showed in my original Orion image), smudges, and sensor noise. Image processing software understands how to use these frames to compensate and produce a less noisy, dust-free image. Some cameras are great and don’t require much in the way of calibration, but most will benefit to some extent. The trick is know which calibration to take and how to take them.

What calibration frames to take

There are four common types of calibration frames: bias, dark, flat, and dark flat. Let’s tackle these one-by-one.

Bias

Bias frames are taken with no light on the sensor using the fastest possible exposure time. The temperature and ISO or gain setting should match that of your main images. If you used multiple ISOs, you should take multiple bias frames. Every sensor has built-in defects that can be captured with bias frames. A perfect sensor would create a completely black image in the absence of light. But sensors aren’t perfect, so here’s what their bias looks like:

Bias

Dark

Dark frames are taken with no light on the sensor using the same exposure time, temperature, and ISO or gain that you used for your main images. The same way a long exposure collects more light to capture the faint details of nebulae, it also picks up noise from current in your sensor. In addition, some pixels are defective and can appear darker (“cold pixels”) or brighter (“hot pixels”) than neighboring pixels. Darks isolate these pixels so they can be brightened or darkened during processing to match the rest of the image. Finally, some cameras leak light and produce what is called “amp glow.” In the following dark, can you spot the three areas of amp glow and see just how many “hot” red, green, and blue pixels there are in the magnified inset?

Dark

Flat

Flat frames are taken with a diffuse, uninform (evenly illuminated) light source. A common practice is to point the scope skyward in the early morning and cover the lens with white paper or a t-shirt. The ISO will match your target image but the exposure time will vary. You need to set the exposure so that your histogram readout is between 50-75%. This ensures you pick up artifacts the lens like dust and smudges without oversaturating them. This image saves my pictures but also reminds me to clean my image train.

Flat

Dark flat

Dark flat frames are usually taken when you, for whatever reason, don’t have or take bias frames. They are darks taken specifically for flats. These are easy to do because you simply repeat the exact same sequence of photographs you took for flats, only this time with the lens cap on.

See for yourself

It’s all good to talk about calibration, but what does it look like? In this interactive photo, you can toggle flats and darks to see what the difference looks like.

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