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.

Still Testing New Camera

Using a 3X Barlow Lens

May 23rd was the last mild observing day before scorching, humid, summer conditions began. It was 65 dry degrees with a light breeze. The Sun was very quiet with only a few small sunspots, but the excellent, comfortable conditions made getting familiar with my new camera enjoyable.

On this day I used a tilt adjuster between the camera and telescope to attempt removal of annoying dark interference fringes which appeared on previous images taken with a 3X Barlow lens. The very first video showed stray light leaking onto the camera chip from an uncovered gap in the tilt adjuster. I fixed this with black electrical tape. Since the interference fringes are not visible in the live laptop screen preview, I processed the first videos immediately to see if the fringes were removed by the tilt adjuster. I didn't see any fringes in the individual images, so I thought the tilt adjuster had worked. I then proceeded to record videos for a mosaic of the entire Sun. Eventually I constructed the 17-image imperfect mosaic shown in the first picture below.

The mosaic shows lots of prominences and filaments, but few sunspots. I like the resolution of details with the 3X Barlow. (Click on the image for a larger view.) Unfortunately, the tilt adjuster did not entirely remove the vertical interference fringes. They became more subtle, but they are still present as you can see on the image below. This is disappointing because they ruin an otherwise nice picture.

The inverted disc image looked fairly nice with the fringes barely apparent.

No fringes are visible in this image of the prominences alone:

Next is a 3-image mosaic of a particular region entirely within the solar disc. The picture has been processed to increase the visibility and contrast of features. (Click on the image for a larger view.) . The fringes are barely visible and detail is quite good.

Lots of trial and error remain before operation of my new camera becomes routine. Perhaps I can remove the interference fringes with a flat field. I'll try this next.

Testing New Camera

Interference Fringes

May 13th was a lovely day with 72 degree temperature, scattered clouds, light breeze, and, frequently, good seeing. It seemed like a good day to try different Barlow lenses with my new ZWO ASI174MM camera.

The first picture below, a 5-image mosaic, was made with a 3X Barlow lens. A large magnificent prominence adorns the northeastern limb and a string of sunspots are arranged on a diagonal line running down to the right from the prominence. (Click on the image for a larger view.)

The 3X Barlow seems to give better results than a 2X Barlow. I'm quite satisfied by the resolution of details in this image. Unfortunately, the picture is ruined by dark vertical bands. These are interference fringes caused by multiple reflections of monochromatic hydrogen alpha light within the optics of my system. I've encountered interference fringes before with my higher power Barlow lenses, but those fringes, often called Newton's Rings, were circular, indicating multiple reflections between a curved surface and a flat surface. The vertical bands above are almost perfectly straight, more like interference fringes caused by two nonparallel flat pieces of glass. These fringes were not apparent on preview images in the FireCapture camera operating software, so I wasn't aware of their presence until I later processed the images.

Sometimes interference fringes can be removed by slightly tilting the camera. This has worked for me in the past, but now the effect will not be immediately visible on my laptop. I'll have to process future images to see if tilting actually removes the fringes.

I calculated an image scale of 0.53 arc seconds per pixel for the 3X Barlow lens with my Lunt 100 mm telescope. The 3X angular field of view is 17.2 arc minutes by 10.8 arc minutes. The image above was reduced from full size to two thirds size because this seemed to give the most pleasing resolution.

Next, I tried a 5X Barlow lens. The following picture is a 15-image mosaic of the Sun's equatorial region reduced to half size from the original.

Once again, annoying vertical interference fringes ruin the picture, but the details and resolution are very nice. (Click on the image for a larger view.) 

I calculated an image scale of 0.32 arc seconds per pixel for the 5X Barlow lens. The 5X angular field of view is 10.4 arc minutes by 6.52 arc minutes.

Sunspot groups 2345 on the left and 2339 on the right show in good detail in the next 4-image mosaic made with the 5X Barlow. The image has been reduced to two thirds size. Click on it for best detail.

Finally, check out this detailed 5X Barlow single image of sunspot group 2339 which has been reduced to two thirds size. Interference fringes seem almost completely absent. White flares are present near center. (Click on the image for the best view.)

The previous image is among the best I've ever achieved for detail. I might use the 3X Barlow for my workhorse lens in the future if I can remove the interference fringes. 

Video file sizes from the ZWO camera are much bigger than files I get from my DMK41 camera. A 400-frame AVI ZWO video produces a 0.92GB file! A 1,000-frame AVI video produces a 2.3GB file! My image processing software, Registax6, would not handle the large 2.3GB file, so I will be limited to fewer video frames if I continue with Registax6. This really isn't much of a problem because 400-frame or 500-frame videos produce very nice images.