Flat Fielding

Partial Success

On June 7th puffy clouds passed by on a windy day. There were large gaps between clouds, and the temperature was 80 degrees. Although solar features were not particularly spectacular this day, I was eager to try two new procedures to improve my solar imaging.

The first new procedure was a change to settings within the FireCapture software that controls my camera. I reduced the frame rate during video capture from 30 to 10 frames per second, and I increased the "save thread priority" in the performance settings. Both these changes were designed to increase the video capture rate and prevent capture from freezing while videos are recorded. The changes seemed to work! 400-frame videos were completed in 6.67 seconds! I was very pleased to have fast continuous video capture throughout my entire observing session.

The second new procedure was to obtain flat field images to remove annoying interference fringes from my pictures. A flat field is the image produced by a uniform light source. In a perfect world the flat field image would be featureless, with uniform brightness across the entire image. But if there are specks of dust on glass surfaces through which light passes, if the optical system causes the camera's field of view to be unevenly illuminated, or if there are interference fringes present caused by multiple reflections within the optics, then the image of a uniform light source will not be uniform. The flat field will record deviations from uniformity, and it can be used during image processing to remove the imperfections.

Expert solar observers have used plastic bags stretched over the telescope's objective lens to produce a uniform light input for flat fields. When the telescope is pointed at the center of the Sun's disc, the plastic bag, apparently, diffuses sunlight and smears surface features making light reaching the camera uniform except for optical imperfections. Some observers have used white plastic supermarket bags. I initially tried a white bag, but it blocked too much light. When I tried a clear plastic bag, it worked! The next image shows the plastic bag attached to the front of my telescope.

The next image shows the flat field image recorded with this setup when a 3X Barlow lens was used with my camera.

Notice the alternating dark/light vertical bands on the right side of the image. Also present are a set of very subtle oval rings, the dreaded "Newton's Rings". Next is an image made through the 3X Barlow lens without applying the flat field correction.

Notice the alternating dark/light vertical bands on the right side. Now look at the next image where the flat field correction has been applied.

Hooray! Flat fielding eliminated the vertical bands! Both of the previous two images have been identically processed except for the added flat field correction in the second image. In this example it's hard to see if flat fielding also eliminated the oval "Newton's Rings" because the rings were so subtle in the uncorrected image.

Removal of the vertical bands finally allowed construction of an unblemished mosaic with the 3X Barlow. I combined 9 individual images to produce the following picture of the eastern side of the Sun. (Click on the image for a larger view.)

After finishing with the 3X Barlow I installed a 5X Barlow and proceeded to record images for the same view presented above. The flat field recorded with the 5X Barlow is the following picture.

The 5X Barlow flat has the same vertical bands as the 3X Barlow flat, but the 5X flat also has much more pronounced "Newton's Rings".

Next is an image made with the 5X Barlow without applying the flat field correction.

Vertical bands are visible on the right side, and the oval pattern of "Newton's Rings" is also present with more intensity than the relatively subtle pattern seen in the uncorrected 3X Barlow image. The next image shows the result of applying the flat field correction.

Flat fielding eliminated the vertical bands, but the oval fringe pattern became worse! The previous two images were identically processed except for the application of the flat field to the second image. Geez! Why can't life be simple?

After considering this surprising effect for some time I finally realized the oval fringe pattern recorded in the flat had somehow picked up a 180 degree phase shift relative to the vertical bands recorded in the flat! I tested this hypothesis by applying an inverted flat to the original image. (In the inverted flat positive and negative are reversed.) The next picture is the result of this action.

Just as I suspected! Now the oval fringe pattern has been removed, but the vertical bands have been made worse! So, the process of creating the flat field for the 5X Barlow somehow created a 180 degree phase shift between the vertical fringes and the oval fringes. Light that produced a dark vertical fringe somehow produced a bright oval interference fringe instead of a dark oval fringe.

In spite of the failure to completely remove the interference patterns for the 5X Barlow I was able to salvage two decent images. The next picture is a 10-panel mosaic of the Sun's eastern side similar to the 3X Barlow mosaic image above. This 5X image is imperfect only in the upper left corner where some oval fringe pattern is most visible.

A cropped portion of the previous image shows good detail with no fringes. (Click on the images for a larger view.)

In the future I'll try to remove the troublesome "Newton's Rings". Perhaps a camera tilt in addition to the flat field will work.