Staring at the Sun, 12A :: home :: |
7/15/2010. Yesterday, I speculated about the contribution poor polar alignment was making to unsharpness in my solar photographs. Here's an overly dramatic look at the problem followed by the cold numbers:
That is not, obviously, a planet in transit or the end of the Earth in some distant epoch. It is the first and last frames of a typical series of solar exposures (eight, in this case) laid one atop the other and messed with in Photoshop until the two displaced rims are clearly visible. To measure the displacement, I drew a circle that spanned the misaligned portion of the solar disks and noted its diameter. The solar image is 3,520 pixels in diameter (call it 3600 pixels). If the Sun is about 30 arc minutes, then the image scale is 0.5 arc seconds per pixel. The displacement of the last frame relative to the first is 23 pxiels, or about 12 seconds of arc. The time between those two frames is 14 seconds. So my typical, casual, approximate polar alignment for short exposure daytime photos produces drift at a rate on the order of 12/14 seconds of arc per second of time. That is, just under a second per second. Does that hurt 1/2 second exposures in a system which can only resolve, at best, two arc seconds? In half a second, the displacement is about 0.4 arc seconds, or about a fifth of the theoretic resolution of the system. In a 1/4 second exposure, the displacement is half that. It probably doens't hurt, but it sure can't help. Also, I have not bothered to reset from sidereal to solar rate. Does that matter? That difference amounts to 360 degrees in 365 days. Call it 1 degree per day = 3600 seconds per day = 150 seconds per hour = 2.5 seconds per minute = way too little to care about.
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