[Leica] I got published in Sky & Telescope magazine, but...

[Howard Ritter]

Mark—

Actually, blur due to the Earth’s rotation is not really evident in this image. In 2 seconds, an object on or near the celestial equator (like the Moon) will move through 30 seconds of arc, which is 1/60 of the Moon’s angular diameter. In the small image, this is a fraction of a millimeter.  I cannot detect this in my image even when viewed large. The image of the Moon is rather poor, since it was taken with a relatively short FL (for an astronomical object) of 400 mm, and focus was questionable, since modern lenses seem not to need infinity stops any longer (except when they do). If you look at the stars in the field, you’ll see that there is no “trailing” due to the Earth’s rotation, which would affect them the same as the Moon. The stars do not quite appear as points, probably again due to imperfect focus, but they are round, indicating that the exposure was short enough that the tiny amount of trailing was small compared to image imperfections. And in any case, I was not going for a high-definition image of the Moon’s face, but of the eclipsed Moon in a starry sky. You’re certainly correct that a good image of the Moon itself, filling the frame or even bigger, would have to be made with a shorter exposure. Such images are usually made with telescopes on motor-driven mounts that track celestial objects. But since the Moon is a landscape in full sunlight when it’s not eclipsed, the f/16 @ 1/ISO rule of thumb works. A camera @ ISO 400 on an f/8 telescope would need a shutter speed of about 1/800 sec for the un-eclipsed Moon, and the blur due to the Earth’s rotation without the motor mount would be then about 1/50 of a second of arc, equivalent to about 100 feet of distance on the surface of the Moon and therefore totally invisible when viewed at any scale.

I needed 2 sec @ f/8 and ISO 6400 because the darkest part of the fully eclipsed Moon is dramatically darker than the un-eclipsed Moon, on the order of 1/10,000th as bright, a fact that is not obvious to the visual observer.

As for the orbital motion of the Moon, it’s in the opposite direction to the Moon’s apparent motion in the sky due to the Earth’s rotation, but it's negligible in any case.

—howard


> On Mar 17, 2015, at 10:36 AM, Mark Rabiner <mark at rabinergroup.com> wrote:
> 
> 2 seconds an amazingly long exposure for the fast moving moon.
> The blur is  plainly evident in the image even quite small.
> 
> http://forums.popphoto.com/showthread.php?338537-Moon-minimum-shutter-speed
> 
> 
> On 3/16/15 5:13 AM, "Peter Dzwig" <pdzwig at summaventures.com> wrote: