The average value of a pixel that was pointing at a star will tend to be slightly brighter than a pixel that was pointing at dark sky, but this difference might be smaller than the random variations on any single photo. Taking lots of photos is the equivalent of rolling a dice a lot of times- you are taking a lot of samples of each individual piece of sky and taking the average brightness to eliminate random variations. This noise obscures the actual subject of your photo and means you can't tell what is a speckle and what is a dim star. This is because of the random signals, or noise, the sensor picks up. If you take a single photo in the dark, it will look speckly. There's no way you could know that from a single roll, but by taking the average of lots of rolls you can see the pattern of behaviour. If the results average to a different value, you can be fairly sure your dice is loaded.
Roll it a thousand times, and you can be pretty sure the results will average out to about 3.5. Roll a dice once and you might get any result from 1 to 6.
#Photo stacking software free free#
This step is more about the theory of how stacking works- if you don't care about random distributions and would rather get going, feel free to skip it. The image above shows a comparison of just Cassiopeia between one of my raw photos and the finished stack. My examples will be taken from a set of photos I took of Cassiopeia and the surrounding sky.
Fortunately, stacking images can get rid of all of these problems and increase the amount of detail visible in return for a little effort. An SLR with bulb mode will happily leave the shutter open for as long as you want, but this is a guide that will work with any camera. Thirdly, most cheap digital cameras will only take single exposures for a maximum of 15 or 30 seconds. The second problem is that digital cameras pick up stray radio signals, cosmic rays, thermal vibrations and all sorts of other things which aren't starlight. This is a cool effect if it's what you're after, but if you want a picture of the sky as it appears to your eyes it's not helpful. Ideally, you want to leave your shutter open for as long as possible, but there are problems with this.įirstly, the stars move around the sky, so on any exposure above about 15 seconds the stars will stretch out into curved streaks. Very little light reaches the earth from a dim star, so just pointing your camera at the sky and clicking the shutter will only capture a few of the brightest stars. The main problem with stars and things you want to photograph is that they're so dim.
0 kommentar(er)
