Flaws

Camera Struggles Continue

Beautiful observing conditions were in place on April 3rd - clear skies, light breeze, and temperature in the mid 60's. I began solar imaging with my new ZWO ASI 1600 camera and a 5X Barlow lens. Annoying circular interference fringes appeared with the 5X Barlow and stubbornly resisted all attempts at removal this day. Perhaps you can notice interference fringe traces in the first picture below. It shows active area 2737, the only significant feature on the solar disc this day.

Two prominences were present. With a 3X Barlow both were visible on the laptop preview screen only if I lowered the gamma value drastically. This caused flaws in the background. In other words, increasing the visibility of dim features also increased the visibility of flaws in the camera's chip performance. The next image shows the problem.

Prominences should appear against a uniform darker background. Instead, the background is neither uniform nor particularly dark. (Click on the image to see the flaws in more detail.) Any attempt to darken the background also removed the prominences. Also, notice concentric curved defects in haze above spicules along the upper left rim.

To understand the nonuniform background problem I examined several dark frames. These are images made when no light is falling on the camera chip.  Ideally, a dark frame should be uniformly dark. It should record zero brightness levels in every pixel on the camera chip. In the non-ideal real world the camera records small varying pixel brightness levels caused by thermal effects. There shouldn't be drastic variations between different areas on the chip. Unfortunately, drastic variations did seem to be happening on my camera's chip. For example, look at the image below which was constructed by averaging 10 video frames while the lens cap blocked light from entering the camera.

As you can see above, the dark frame isn't uniformly dark across the chip. A pixel on the right side is about 1.9 times brighter than a pixel on the left. (I have enhanced the difference slightly so it will be visible here, but the factor of 1.9 comes from the original unenhanced image. This dark frame was made with exposure time 29.7 ms, gain 123, and gamma of 24.) Apparently, the nonuniform response is inherent in the chip itself. It is not coming from the Sun. In the future I need to correct for this flaw in order to get better images. 

I captured more than enough overlapping images to construct a disc mosaic, but Photoshop Elements would not create a flawless mosaic. Close examination reveals blurred portions of the following 4-panel mosaic as well as slight discontinuities along the rim in a couple places. The flaws are more visible if you click to produce a larger image.

Once again, the background is not uniformly dark. One of the prominences on the right side doesn't even show up.

Apparently, I need to experiment more with exposure time, gain setting, tilt adjuster angle, and camera temperature to see if I can improve results. Although drastic changes to gamma made prominences visible, they also significantly increased background levels. The camera's wide field of view is definitely wonderful, but I have to eliminate flaws to take full advantage of it. Results so far have been very discouraging.