MarkBour wrote:I was wondering about the wording in the APOD caption that says: "In this combined evening skyscape ..."
whether or not that implies that separate images were combined.
As I read Chris' note above, it leaves me still wondering. Chris, it seems you figured out how large the various bodies should appear and then noted that they are considerably larger than that, right? You then mention level of exposure, but that would not change the pixel size of the bodies, correct? If you can "dumb it down" further, I may be able to follow what you're saying.
If you change the exposure time, you will change the apparent
size of the bodies. Nothing in the sky in this image is large enough to be resolved at all except for the Moon, and it's still quite small at this image scale. The Moon and Venus are so overexposed that all we're seeing is the glow of the sky around them and scatter inside the optics. The objects themselves are totally hidden by all this scattered light.
Here is an image I made of a moon pillar. In order to catch the pillar, I had to expose the image at ISO 800 for 4 seconds. The properly exposed Moon is a 1/30 second exposure at ISO 100. There's almost a factor of 1000 in total exposure between the two. In the pillar image, the Moon itself is completely lost in a much larger halo of saturation from the light scattering off the thin fog.
The APOD exposure was probably 30 seconds or longer. Had it been 1/100 of a second, we'd see a small crescent Moon, a small dot for Venus, a small dot for the ISS, and probably not Mars at all. Or, the aperture could have been stopped way down, the long exposure kept so that ISS showed as a streak, but again, all the dimmer stuff would be lost.
I don't know what is meant by "combined" in this caption. But it's not uncommon in such images for the foreground landscape and background starscape to be combined from separate images, given the often very different exposure requirements. It doesn't mean that they're at different image scales.
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