How to take Flat Frames for Astrophotography
Learning how to take flat frames is an essential step for any deep sky astrophotography enthusiast. Once you know what to look for, taking successful “flats” is easy with both a DSLR or CCD camera. This is often a question I get from beginners that are just getting started in deep-sky astrophotography.
Early on, I was guilty of skipping calibration frames such as flats and bias frames. Now, I know that flat frames are as important as taking dark frames. The uneven field illumination and vignetting in my deep sky images was a real pain to correct in post-processing.
Like many aspects of deep sky astrophotography, your method of taking flat frames will vary depending on the equipment you use. The I have described for taking flats on this page is most useful for those shooting with a DSLR camera such as the Canon Rebel T3i I currently own.
This technique is affectionately known to the DSLR astrophotography community as, “the white t-shirt method”.
How to take Flat Frames for Astrophotography
This video tutorial will help you visualize the process of taking flat frames. I use what is known as the “White T-Shirt method” outlined by the creator of DeepSkyStacker in the FAQ section. The telescope uses the t-shirt as a filter when pointed at a bright source of light such as the dawn sky.
The telescope used in this video was an Explore Scientific ED102. The modest aperture of this apochromatic refractor makes capturing flat frames easier, as there is less area to cover. Larger telescope objectives, such as a Newtonian Reflector may find an artificial lighting solution more effective.
Flat frames are 1 of 3 important support files when calibrating your images. Each one is often necessary to produce the best possible image from your raw data. The image below features the frames captured using a DSLR camera with a William Optics Zenithstar 61 telescope on the Heart Nebula.
With a little practice and experimentation, taking flats will become a regular part of your astrophotography image capturing workflow. Capturing proper calibration frames makes a world of difference when you are ready to process your final image.
Once you find a method that works well for your camera and imaging location, you can stick with it for all future projects.
The basics of flat frames
shoot at same ISO/Gain as your light frames
keep the camera connected to scope/lens
maintain the same focus as light frames
shoot a minimum of 15-20 flat frames
temperature is not important (It is with a cooled CCD)
The flat frames are loaded into DeepSkyStacker to calibrate the final image. DSS creates a master flat from the set of flat frames and corrects the uneven field of your final image. Without adding flats, your stacked image may show noticeable vignetting and many dust spots over the image.
The White T-Shirt Method
My preferred method of capturing flats is to use the “white t-shirt” method. By stretching a plain white t-shirt over the telescope objective, you create an even flat field when pointed at a bright light source.
The intensity and consistency of the light source have a big impact on the quality of your flat frames. For example, shining a narrow beam of light from a flashlight at the telescope will not provide the even coverage of light needed for a flat frame.
A better option is to point the telescope at the early morning blue sky. This is much more evenly lit, with just the right level of brightness. The tough part is, you’ll need to leave your imaging gear out all night until morning. I realize that in many situations, this is just not possible.
Luckily for DSLR astrophotography shooters, there is a mode on the camera that is ideal for shooting flats. This mode is called “AV“, or Aperture Priority mode. This means that the camera will decide on the correct shutter speed to properly expose the image.
With the right level of brightness and evenly lit filter (like a T-shirt), AV mode will take an excellent flat frame with the press of a button. Depending on your camera and telescope configuration, you should see some slight vignetting and even dust particles in your flat frame.
When in doubt, have a look at the histogram. The flat frame below has the characteristics you are looking for. The histogram shows that the image is well exposed, and the dust particles from the camera sensor are evident.
Exposure Time of a Flat Frame
There is no set exposure time for a flat frame, although if you are using a DSLR, you may experience some familiar results using the white t-shirt method. For example, when shooting flats at ISO 800, the peak of the data hits the right side of the histogram at a lightning fast 1/4000 shutter speed.
If the data clips the right side of the histogram, that’s a dead giveaway that your light source is too bright. The solution here is to dim your light source by adding another layer of t-shirt or to point somewhere less intense.
Here is a great example of a how a well-exposed flat frame can completely remove the dust particles in your image from Al’s Astrophotography Blog.
How many flat frames do I need to take?
This depends on who you ask. Personally, I have found that 20 flat frames are enough to create a useful master flat file for DeepSkyStacker to calibrate your image. Since these are so quick to capture, there is no harm in collecting 20 or more frames for each session.
I have also heard that rotating the t-shirt between each flat can help create an even better master flat file. By evenly averaging out any of the subtle patterns picked up in the t-shirt fabric, you are covering all of your bases. I have never had to go to this length myself, but it is worth noting.
Taking Flats at Night
In these situations, you’ll need a reliable source of artificial lighting that evenly covers your entire telescope objective. A white t-shirt will still be used, to further dim the brightness of your light source. You can double-up on t-shirt layers if needed.
As seen in the photo below, pointing the telescope straight up will help you balance the lightbox and keep it flat against the objective.
It’s important that the light source lays completely flat up against the telescope objective. Here are some examples of artificial light sources you can use to shoot flat frames:
- laptop screen (all white, dimmed)
- tablet screen (all white, dimmed)
- dedicated astrophotography flat box
- artists tracer box (dimmable)
Taking Flat Frames with a CCD Camera
If you’re using a CCD or dedicated astronomy camera like the Altair Hypercam 183C, you won’t have the convenience of an aperture priority (AV) mode on the camera dial. In these situations, the camera control software will need to handle flat frames.
I prefer to use Astro Photography Tool for this process, which includes a handy feature called CCD Flats Aid. This function will let you take a series of exposure using variations lengths of exposures. The target ADU will need to be set, and then it is simply a matter of reaching the desired ADU for your flat frames.
Just like with the DSLR, the histogram will tell the story. When you discover the appropriate exposure length, you can set your target ADU. Then, you can save this result for your next imaging session, by choosing the “flats aid result” in the imaging sequence dialog box.
What if I didn’t take flats?
If it’s too late to capture proper flats to match your light frames, you’re not completely ruined. There are ways to get around using flat frames such as using this gradient removal technique that involves creating a synthetic flat frame in Photoshop.
This will work in a jam, and really helps fix the harsh gradients and vignetting. Unfortunately, the effect it has on dust and smudge removal is less effective.
Do yourself a favor, and make taking proper calibration frames a part of each and every imaging session.