Last Updated on March 14, 2022 by Practical Astrophotography Staff

Many Budding Astrophotographers are often confused on exactly what Calibration frames are and if they even need to use them since they do take quite a bit of effort to acquire.

We’ve seen this come up numerous times in comments and descriptions of images, and in replies, so we wanted to put some info on here about what calibration frames are for, why we take them, and some suggestions for how to get the most out of them. We’re not an expert; We’re not going to get into the math. This is not intended towards experienced astrophotographers, this is meant for the new imagers to help steer them in the right direction…

Here is our Brief Guide to Calibration Frames: Bias, Dark, Flats, and Dark Flats

What kinds of calibration frames are there, and what is the purpose of each?

First things first- all calibration is about increasing the signal-to-noise ratio. Think of when you take an image with your cell phone in the house without a flash- the image looks grainy. If you do a video, you see the “noise“ flickering in the images as you play the video. When you do it in daylight the images look great because of the high signal-to-noise ratio. When dark, you have a low signal-to-noise ratio.

Calibration and stacking are all about increasing the signal-to-noise ratio. Note the light signal from what you are imaging is constant, the noise is NOT and varies randomly. Thus with math, we can take a lot of Light, Bias, Dark, Dark Flat frames and figure out what is noise and what is signal and make amazing pictures out of a noisy mess. Keep in mind if you have a camera with 12 million pixels it is impossible to have all the pixels act the same. Calibration allows us to mathematically calibrate each pixel to react the same way to each photon it sees.

• Bias Frames – Your Camera inherently has a base level of read-out noise as it reads the values of each pixel of the sensor, called bias. When averaged out, basically it’s an inherent gradient to the sensor. BIAS frames are meant to capture this so it can be removed.
• Dark Frames – When taking a long exposure, the chip will introduce “thermal” noise. Its level is magnified by three things – temperature, exposure time, and ISO. Dark frames are used to subtract this sensor noise from your image and mitigate “hot or cold” pixels. (Some modern sensors automatically calculate dark levels and don’t need dark frames). Dark Frames also will calibrate the chip so all pixels give the same value when not exposed to light.
• Flat Frames – I’ve seen people say flats help with light pollution. NOT TRUE AT ALL. Flat frames allow you to calculate the correction factor for each pixel so they all give the same value when exposed to the same quantity of light for a given optical path. Things like dust motes, lens vignetting consistently reduce the light to a given pixel, flat frames allow you to mathematically remove them to give a smooth evenly illuminated image.
• Dark Flat Frames– These are just like dark frames for your lights, but are for your flats. (May not be needed depending on what software & process you’re using)

How do I take each type of calibration frame?

• Bias – Make sure it’s dark (or put your camera in a box, etc), and put the lens cap on. Set your camera to the shortest exposure time possible. Take a bunch of pictures. Stack those.
• Dark – As above, darks are temperature and exposure time-dependent, as well as ISO. You want your camera in the dark, with the cap on. To match the temperature, it’s easiest to simply have your camera outside when the ambient temp is the same as when you image. (either does these after you’re done for the night, or on another night the temp is the same) Set the exposure time to match your lights. Take a bunch of images.
• Flat Frames – There are lots of ways to do flat frames. My favorite is currently to put a white t-shirt over the end of my scope and point it at a laptop screen. On a DSLR the exposure time should be so your histogram is between 1/2 and 2/3 on the back of the camera. Once you have it right, take a bunch. Since you’re trying to capture how the dust, etc. was, this should be done right after you’re done imaging for the night. You don’t want your focus, rotation, etc. To change before you get your flats since doing so will change your optical path to each pixel.
• Dark Flats – Since these will be short to match your flats, they’re quick. Just pop the lens cap on when you’re done with your flats, and take a bunch of darks at the same exposure time.

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How many of each calibration frame should I take?

• The general answer is “As many as possible, more is better”. Keep in mind that the reduction in noise goes by the inverse square law. Going from 1 exposure to 2 exposures will reduce the noise by a factor of ½, going from 2 frames to 4 frames will again reduce it by a factor of ½. Then you need to go to 8 frames, then 16, then 32, then 64 then 128, then 256. As you can see it gets out of hand quickly. More realistically; however, I go by the following.
• Bias – I take 50 – 100. They’re fast, and you can take them any time.
• Darks – Depending on your exposure time, these can be a chore. I go for at least 20, and up to 50. Though for really long exposures, I’ve used as few as 5.
• Flats – I have varied between 25 – 50, but really see no improvement over 25, so have stuck with that of late.
• Dark Flats – Match your number of flats
• Notice that these quantities don’t change at all based on the number of lights

General tips

• You Need to take Light, Dark, and Bias frames at the same temperature. .
• ISO Matters for all of these. You need a bias for each ISO you want. You need a dark that matches the ISO of your light. Your flat ISO must match your light ISO, and your dark flat must match your flat.
• Bias Frames last a long time. Take a bunch, use your stacking software (PixInsight or DSS) to make a master Bias, and just re-use that. We never saw a change in ours.
• Darks – Some modern sensors don’t need dark frames, they do dark subtraction via a function in the camera firmware. The best thing to do is test an image by processing it with and without darks and evaluate the difference they made.
• Darks – Build a dark library. This is made easier if you can settle on one (or two) standard exposure times & ISO Combinations (for me it was ISO 800 x 300s), and take darks at different temperature points. Create Master darks and label them. (i.e., Master_Dark_ISO800_300s_25c, Master_Dark_ISO8300_300s_20c) These masters will last as long as your Bias Masters.
• Take darks on cloudy nights! You can just put your camera outside and let them click away. Even if you haven’t imaged, if you don’t have darks at that temp, take some!
• Flats are a pain, but don’t skip them! Dust is very hard to remove later without them.
• For our DSLR images, We just stuck with ISO 800. This meant we only ever had to worry about calibration frames for a single ISO.

CONCLUSION
Calibration Frames are essential for eye-catching images, although it can be time-consuming to acquire them you will notice a difference in your images when you do take the time.

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