Calibration Frames Demystified, Part 1 – Darks and Flat Darks
Presented By Dan Higgins
When I was starting with astrophotography many years ago, I started with film cameras. I had a Minolta Maxxum that my father purchased for me. I was always attempting to focus through this little viewfinder to no avail. Finally, I achieved focus and there was extreme vignetting and speckled dots all over the place.
As technology advanced, calibration frames were invented to alleviate the issues we may have when it comes to vignetting and noise. Unfortunately, I originally thought that flats wouldn’t make that big of a deal. For years, I only took darks and cropped out the vignetting. That was a mistake.
In this article, we will discuss Dark Frames and Flat Darks and how you can use them to improve astro image quality.
To take the best quality astrophotos, you really do need to take calibration frames. These calibration frames are imperative to making your time-intensive photos from looking alright to looking spectacular!
There are 4 basic types of calibration frames:
Flat Darks (or Dark Flats depending on who you are talking to)
Let us take these one at a time and explain why they are important.
What is a Dark Frame?
A dark frame is exactly what it sounds like! An exposure with no light! While you do an exposure, the CCD or CMOS chip tends to heat up. This Dark Frame records the thermal noise that is generated during your exposure. It reveals Hot and Cold Pixels and a lot of fuzziness called noise. However, a couple of rules go with a dark frame.
Must be the same temperature as your light frames
Must be of an exposure length at least that of your light frame (It can be longer, that will be explained below)
What does a Dark Frame look like?
A Dark Frame isn’t what many people think. Many people think that a dark frame is completely black…WRONG!
The dark frame reveals things like thermal related defects such as hot/cold pixels, bad columns, and fixed pattern noise.
Let’s look at a couple of dark frames.
Below I have taken some dark frames with my QHY 16200-A camera. The exposure times are 1 minute, 2 minutes, 3 minutes, 5 minutes, and 10 minutes respectively. You can see how time can affect your dark frames!
As the exposure time gets longer you can see the bad pixels multiply and the photo gets much noisier (Speckles in the background)
Temperature works the same way…the warmer the sensor is, the more thermal noise you will see in your picture. That is why a cooled camera is very beneficial when taking astro photos.
How do I expose a Dark Frame?
To expose a dark frame, you must ensure no light gets into your optical train. There are a couple of ways to attain this.
Put the Lens Cap on your scope
If you have a camera with a mechanical shutter, it will remain closed (Your image acquisition program will ascertain it is a dark frame and the shutter will remain closed)
I’m a little crazy so I drape a blanket over my telescope (But do not cover the fans of your camera!) just in case there are any light leaks in the image train. An image train can develop light leaks (an opening somewhere in your image train that allows unwanted light in). In my experience, that usually occurs from loose accessories in your image train. Ensure they are all tight and flush! If there are still light leaks you may need to seek out other means via social media or your telescope manufacturer.
As mentioned before the Dark Frame has to be at the same temperature and the same or greater exposure time. I like to take dark frames of the same length as my light frames. However, if you wish you can take long exposure darks and use them for your shorter lights when you use dark optimization. For example, you can take a 10-minute dark and use that for all lights that are 10 minutes or less in exposure time. I have done this with mixed results, but it can be an option.
How many Dark Frames do I need to take and why so many?
This is a question we get all the time at AstroWorld. I take 50.
The reason you need to take multiple darks is so when you make a master dark (a stacked picture of all your dark frames), it will be an accurate average of your sensor’s thermal noise.
The more statistical data you have, the more accurate the calibration frame will be! The master frames are needed for all types of calibration frames (Bias, Dark Flats, Darks, and Flats), but your stacking program can either make these calibration masters for you or you can do them manually.
When taking dark frames, cooled cameras have a distinct advantage when it comes to thermal noise.
Cooled cameras have a built-in cooling system. The cooling system can regulate the temperature of the chip so you can always be sure that your sensor is at the desired temperature.
When you can regulate the temperature of your sensor, you do not have to take the dark frames on the same night as your light frames because you can regulate the temperature of the camera.
However, uncooled cameras (including DSLRs) do not have this capability. It will be necessary to take the dark frames either on the same night as your imaging night(recommended) or on another night of the same temperature (not recommended).
Unfortunately, if you take darks on your imaging night, this can severely cut down on your imaging time.
Check out my video on cooled and uncooled cameras at AstroWorld TV on YouTube below.
You may have heard other astrophotographers talk about dark libraries.
If you have a cooled camera, you can make what’s called a dark library. Since you can regulate the temperature on a cooled camera, you can wait until a cloudy night and take your darks whenever you want! You don’t have to take away precious imaging time by taking darks!
Set up your camera and let it go to work!
I take 50 of each: 1 minute, 2 minutes, 3 minutes, 5 minutes, 10 minutes, and 20 minutes (I do narrowband imaging, so I do very long exposure times to capture data). I like my darks to be the same exposure as my light frames. I feel that I get a slightly better-calibrated image. However, there is nothing that’s stopping anyone from taking 50 ten-minute darks and using them to calibrate their images that are 10 minutes or less when using dark optimization (If your software supports that)
What is Dark Optimization?
Dark optimization is a process used in some programs (like Pixinsight) where the process can extrapolate what your thermal current is by using the dark current value of the current image.
For example, if you have 10-minute darks and 1-minute lights the program will take the value of the dark current of the 10-minute image and use 1/10th the value.
This is a very rudimentary explanation, but I think it gets the point across.
If you would like a more in-depth explanation, check out the book Inside Pixinsight by Warren A. Keller on page 21 under Image Calibration of Flat Frames. Warren gives a great explanation of optimizing darks.
Flat Darks have almost the same requirements as Dark Frames.
CMOS users use Flat Darks to calibrate defects out of their flat frames. Defects like read noise and amp glow can carry through and affect flat calibration so flats are calibrated using a matching master dark to remove these at an early stage.
Read noise is inherently different than thermal noise. Read noise is the noise created by the electronics in the camera.
The basic process is: light hits the sensor and is then converted by an analog to digital converter (ADC). The converted image is then amplified to create an image on your screen. This conversion creates electronic read noise and is needed to calibrate the read noise out of your flats.
It’s worth talking about amp glow at this point. This manifests itself by bright areas in the corner of many CMOS cameras. This is caused by IR radiation from amplifier electronics around the sensor hence the name.
Not all sensors suffer and it mainly affects older generations of cameras.
By calibrating your flats using matching darks these effects can be removed to create clean calibration files for integration into a master flat.
Many CMOS users have stopped using a bias in place of Dark Flats because the pixels tend to match more accurately because they are the same exposure time as your flat frames.
Since I am a still CCD user, I continue to shoot bias frames(See Part 2 of Calibration Frames Demystified)
Dark Frames are an essential part of your astrophotography arsenal. The sooner you start taking them, the happier you will be with your astrophotos.
Keep Imaging, Keep Educating and Clear Skies!
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