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How to Fix Uneven Illumination with Solar Flat Fields

Solar flat fielding

Save your solar images with flat fields!

If you have ever done any solar imaging, you may have come across uneven illumination that can ruin your single images and make your mosaics a nightmare to process. Luckily, you can create solar flat fields (even after you’ve finished imaging) that easily correct this and save your final image!

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There are two ways to fix uneven illumination with solar flats:

  • The best way: shooting flat fields during your imaging session
  • Or, creating an artificial flat field in your image processing application

We can make this…

solar image before fixing illumination
BEFORE: solar image with uneven illumination

…look like this!

solar image with fixed a solar flat field
AFTER: the same solar image, fixed with a flat field

For this tutorial I will assume you are using a monochrome planetary-type camera

Method 1: shooting flat fields with your telescope

solar flat field telescope prep
Place a clear plastic bag over the objective end of the telescope

The first and best method is to shoot a flat field during your imaging session.

Note, this only works if you can get the full field of view of your imaging system illuminated by the sun, typically this would be when using a Barlow lens.

The process:

  1. Point your scope somewhere near the centre of the sun, in a region that is evenly illuminated, with no sunspots, no filaments, etc.
  2. Place a clear plastic bag, not too thick, tightly over the front of your scope, and adjust the exposure so that you have the same level histogram as your light frames.
  3. Capture a video with the same number of frames as your lights.
  4. In your favourite stacking program, there should be a way to create the master flat using this video, and apply it to all your lights during stacking — so simply do that!
    Note: In AutoStakkert!2, it’s under image calibration / create master frame,  then load master flat to apply it to all light frames as they are stacked.

Done! Your final stack should now be more evenly illuminated than if you didn’t apply the flat field.

Method 2: creating an artificial flat field

The second method, the artificial flat, can be used for mosaics and full disk images. It’s also a great one to use on older data you may have that you want to re-process or haven’t processed yet!

My description following for creating an artificial flat field is be optimised for Photoshop, but it can be adapted easily for other image processing programs.

The process:

  1. Duplicate the current layer.
  2. Select the new layer and apply a median filter (found in filters noise), with a pixel radius of 50.
  3. Open curves, and apply the default preset darken. Repeat this 3 times.
  4. Change the blending method to difference.
  5. The flat is now applied. You can adjust the opacity of the layer to reduce the effect of the flat, where most of the time I find 50% works best.
  6. Now flatten the two images into one. It will look dark so use levels to brighten it up again.

Done! Your image should now look more evenly illuminated, and mosaics will merge into better looking final compilations!

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About the author

Paul Stewart

Living partly in his home and the backyard observatory he built himself in Timaru, South Island New Zealand, Paul prefers to image the Sun and other dynamic objects like the ISS, asteroids and comets — not the boring stuff that has been the same for millions of years.

An accomplished and published solar (and even deep-sky) astrophotographer, he was short listed for ROG Astronomy Photographer of the Year in 2015, and published in the Astronomy Photographer of the Year collection book.

Learn more about Paul on his website.

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