Sun Near Equinox

Learning Mosaic Construction

Exquisite, clear fall weather accompanied the autumnal equinox in my portion of Virginia. September 23rd was an ideal day to use my new telescope mount for its first full observing session. The new mount performed flawlessly. I set the tripod legs on marks made previously, leveled the base, turned the mount on, and commanded it to point to the Sun. The Sun appeared almost exactly centered on the eyepiece cross hairs! Only a very slight azimuth adjustment was needed to make centering more exact. All morning the near perfect tracking and precise mount movements made solar observing a real pleasure.

The morning of September 23rd was about 24 hours after the autumnal equinox, and the Sun was very nearly on the celestial equator. On display were three nice sunspots and several filaments. I captured some good individual images showing lots of detail. The most prominent sunspots, 1575 and 1577 are shown below.

Sunspots 1577(left) and 1575(right) (Click for full detail.)

Sunspot 1577 and some filaments (Click for full detail.)

Here's a nice two-image mosaic showing a wider view of the two sunspots.

Sunspots, filaments, and some prominences (Click for full detail.)

I took enough individual images, 22 in all, to cover the entire solar disk. The precise N-S and E-W motions of my new mount made this task relatively easy. Unfortunately, it was not so easy to construct a high-quality whole-disk mosaic. When I attempted to combine all 22 images I got the following initial result, reduced here to one quarter of its original size.

Two things are immediately apparent. First, there are three notches in the Sun's circular rim because I got sloppy with my planned sequence of disk-covering movements. Second, the individual images vary in brightness and do not invisibly blend as desired. My version of Photoshop Elements, apparently, is incapable of successfully blending the images. Maybe the latest version would do the job. I don't know.

I spent many hours manually adjusting levels, but I could never produce a whole-disk mosaic without some of the dreaded patchwork appearance. It was possible to produce three, 4-image mosaics showing strips across the solar disk. Here they are in sequence from north to south across the Sun's equatorial region. (Click on each to see larger detailed views.)

It may look like these three strips could be combined, but the result shows unwanted visible borders where the images join. For example, here's the result of combining the top two strips. (Again, click for more detail. These images are about 4,000 pixels wide, but the blog format will not display them to full size.)

Notice the visible borders present in spite of my best efforts to eliminate them. Part of the problem is variation in image quality from one individual image to another. Sometimes the atmosphere is very steady for one image but not for the next one. The variation in image sharpness from one image to the next shows up as a visible boundary of changing sharpness. I'm beginning to think it may be impossible to produce a seamless whole disk mosaic at this image scale because there are too many changes happening while the images are recorded. I need to capture the entire disk with fewer images. This could be done with a bigger video camera chip, or by using less magnification. My plan is to try a 1.5X barlow lens instead of the 2X lens I have been using. Images will be smaller, but maybe I can capture the whole disk quickly enough with fewer images.