300 dpi may be arbitrary, but it is the often quoted standard for print resolution, for images that are intended to be viewed at very close range. This comes from the fact that the average "normal" human vision yields a resolution of about 1 arcminute, which at a 12 inch viewing distance (very close inspection) equates to 0.00349 inches. 300 dpi equates to a diameter of 0.00333 inches per dot, so this makes it just under 1 arcminute of angle at 12 inches. Therefore, people with normal vision should not be able to detect any pixelation in a 300dpi image.
Images far below 300 dpi can be printed without pixelation, although the image scale does also affect the ability to perceive sharpness. Images are always perceived as being razor sharp when the details that can no longer be resolved by the image (due to the optics and the quality of the image) are also too small to be perceived by the human eye in the print. But keep in mind that a photo of the Moon that you hang on your wall becomes like any other fine art or photograph. Most people enjoy these images from several feet away, and don't scour the image for detail. Because this forum caters to the pixel peeping crowd, it is easy to forget that people don't generally inspect wall art that way!
The other trap to be aware of is the 100% scale trap. This will be different from monitor to monitor. On my MacBook Pro, which has the Retina display, the screen resolution is 220ppi. For all practical purposes, if an image looks sharp at 100% scale on this monitor in Photoshop, then nobody would detect any flaws at 300 dpi print size, even if inspected at 12 inches. But I have a monitor at work that is much larger in size, but only 150ppi, and on this monitor, if I display an image at 100% scale and inspect close up, I can easily perceive the reduced sharpness compared to my 220ppi monitor. When evaluating a potential image print size, be sure you are displaying the correct image size on your screen, and viewing from an appropriate distance.
A 20 inch print of the Moon is a very attainable goal, even with a DSLR and a modest scope. You can still benefit from image stacking, using individual raw files, if you convert your raw files to tif format and then use Autostakkert.
Edited by Tom Glenn, 25 August 2019 - 12:46 AM.