I have encountered some setup considerations while performing various pre-trip tests in preparation for viewing and recording the solar eclipse. In my 8/3/2017 blog I described my first concern regarding camera selection, focus and image field of view. My next consideration was my mount. Do I use my SkyProdigy Alt-Az mount or my CG-5 Equatorial mount?
I began with the SkyProdigy mount for ease of set up and use.
I began with the SkyProdigy mount for ease of set up and use.
As I looked at my image captures I was reminded how Alt-Az mounts are subject to field rotation over a period of time. Since the partial total eclipse time frame is about 3 hours, field rotation will occur. Here are two images about an hour apart illustrating the effect of field rotation (note the location of the prominences along the edge of the Sun’s image)
Of course you can always rotate the images to the same orientation later, but it does require post processing to do so. I also found that my SkyProdigy tracking varies, especially over long periods of time. Thus I would need to periodically manually re-center the Sun due to any tracking issues.
Then I decided to try my CG-5 Equatorial Mount.
Then I decided to try my CG-5 Equatorial Mount.
It Equatorial mount tracks much better and is not subject to field rotation. However, there is an issue called Meridian Flip that occurs as you track past the Meridian, which you will certainly do at high noon (or 1pm on Daylight Saving time) during the eclipse. You typically can track a little past the Meridian before the mount stops, but you should change its orientation before it stops if you plan to continue tracking for a period of time. To make matters worse, I would need to do this at about 1pm where I will be (Columbia, MO), and the total eclipse begins there at 1:12. It’s not that difficult to do. You manually move your telescope to point in the opposite direction and then re-select the Sun to let it slew to the new telescope orientation. Then you finish up by re-centering your target (i.e. The Sun). But you need to do it at the right time and get it done so you are not messing with it when the total eclipse starts. Here is my CG-5 telescope orientation at 1pm and at 1:05pm after the Meridian flip. Another consideration is images captured after the flip will have to be rotated 180 degrees to match the images before the flip.
At 1 pm before the flip:
At 1 pm before the flip:
At about 1:05 pm after the flip:
I also found that even though my equatorial mount tracks much better, I still needed to re-center the Sun periodically since I had only done a rough polar alignment. So I think I’ll avoid the Equatorial mount Meridian Flip issue since it is so close to the time of totality, and stick with the SkyProdigy Mount for this event. I can make sure each image I capture is well centered and later rotate to align the images.
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