best astrophotography software
I don't understand this. All astronomical objects are photographed with the lens focused at infinite, so why would you need to go to 200% live view at night to focus on a star? Makes no sense.
Modern auto-focus lenses and telescopes will focus well beyond "infinity." In fact, in modern camera lenses, there is no actual "infinity" focal point.
In auto-focus lenses, there is focal space beyond whatever infinity point is currently accurate because the auto focus finds the best focal point by cycling on both sides and finally settling on the right point. If there was a stop at a factory set point, the autofocus gear chain and motor would hit that point regularly and be damaged.
More importantly, on, for example, my 900mm focal length telescope I have marks on the Crawford focuser to get me in the vicinity of focus for different sets of equipment. If I am using a Barlow lens between the camera and the focuser, then the focal point is very different than with just the camera.
But wait! It gets better! As the night progresses and the temperature drops, the telescope tube shrinks and, as a result, the "infinity" focus point moves! If one has the most sophisticated focal equipment on a telescope like mine, the computer will actually adjust the focus of the telescope as the night progresses and the temperature drops. At the magnifications normally used in astrophotography, even the droop of the tube and focuser makes a difference. That droop is different when the telescope is pointed very nearly straight up versus, for example, 40% above horizontal. Yet another factor comes into play as the atmosphere changes. The earth's atmosphere is, in effect, part of the lens through which one images. If one is imaging at, for example, 40% above the horizon, there is a lot of "air mass" between the imager and the target. The result is that the curved mass of the atmosphere changes the best focal point when compared with a shot at 80%. Then there is the issue that the earth's atmosphere refracts light differently as it cools (it becomes denser).
To get the focus at the best point, both software programs and the human eye take the image to 200%. With a very high quality monitor, like a Mac Thunderbolt, the 200% setting actually presents something like a 1:4 pixel image. It is possible, using some software and certain cameras, to reach a resolution that is actually 1:1, but that is commonly achieved at 400%. At that level a star jumps around very noticeably because of atmosphere movement. The star also displays a color difference when the focuser has traveled beyond the best focus.
Depending on your monitor, you probably will be able to see the star jumping around and changing shape at even 100%. Interestingly, if the star is out of focus, it will not appear to move around but details in the photo will be lost. Getting the best possible focus and keeping it is one of the more significant challenges in astrophotography.
There's a lot of good information you've provided to help understanding focus. Of course with all the jumping around and blurring caused by less than ideal seeing, best focus can be difficult to judge.
As you mention, I've also found it very helpful to have a focuser with repeatable focus settings. The JMI Event Horizon crayford focuser is quite nice in this respect. It has dual speed focus knobs and a numeric scale. For instance I'll try 9.1 - looks OK; 9.2 - better; 9.3 maybe OK; 9.4 getting soft again; back to 9.25 for best focus. It has a motorized hand paddle which I rarely use, because the G-11 mount is rock solid with little vibration from touching the focuser knobs.
As others have mentioned, perhaps the best way to get accurate focus is with a Bahtinov Mask. Kendrick has them in sizes from camera lenses up to large telescopes. The following graphic copied from the Kendrick website (http://www.kendrickastro.com/astro/kwikfocus.html) shows an eyepiece or camera view with the mask in use.
Since the mask blocks a lot of the light coming in, I usually crank up the ISO on a bright star to be able to see the diffraction pattern as shown above.
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