Shoot short-period variable stars like the Unicorn V474 (the light-changing period is only about 3 hours). It is necessary to take a set of photos per minute for the overlay, and shoot 180 sets of photos in one cycle, but recently found that after shooting for a while, It may be due to the vibration of the mirror, which causes the focus ring to rotate, resulting in out of focus. The out-of-focus photo will not overlap in the DSS.
以下就是剛開始拍攝的準焦, 以及拍攝一段時間後失焦的對照圖(使用Astroart 放大 300% 檢視)。右圖準焦的拍攝時間是：2019/1/18 22:45:48, 左圖失焦的拍攝時間是：2019/1/18 23:03:18
顯示開始拍攝後的 18 分鐘後星點就明顯放大失焦了！
The following is the right focus that was just started, and the comparison of the out-of-focus shots after a while (using Astroart to zoom in 300%). The shooting time of the right picture right focus is: 2019/1/18 22:45:48, the shooting time of the left picture out-of-focus is: 2019/1/18 23:03:18
The star point is obviously enlarged and out of focus after 18 minutes after the start of shooting.
這影響非常嚴重, 辛苦拍攝 1 個多小時的相片, 竟然只有 18 分鐘可使用！
This effect is very serious. I have been working hard for more than 1 hour and it is only 18 minutes!
搜尋網路上的資料發現有一種裝置可固定對焦環, 搭配 Sky-Watcher 的攝星儀(Star Adventurer)使用(請參考下方網址影片18:12處), 但這又是另外一筆花費：
Searching for information on the Internet found a device that can hold the focus ring and use it with Sky-Watcher's Star Adventurer (please refer to the video at 18:12 below), but this is another cost:
https://www.youtube.com/watch?v=lFpMLTGlJes
另外有一種防止垂頭的鏡頭帶(如下), 之前有用過不好用也不予考慮：
There is also a lens strip that prevents the head from falling (below), which has not been considered because it is not good useful before:
https://www.xfastest.com/thread-67252-1-1.html
後來研究下我的 Tamron 90mm 鏡頭, 發現只要將對焦環拉動出現 AF 字樣, 則對焦環就會空轉而不會變焦, 但『拉動』這個動作會不會導致原本已準焦的對焦環失焦只能實際測看看才知道。
Later, I studied my Tamron 90mm lens and found that as long as the focus ring is pulled and the AF word appears, the focus ring will idle without zooming, but the "pull" action maybe cause the focus ring that was already in focus to be out of focus. Can only be actually try it to know.
