Last night I was trying to create a time-lapse of the Orion constellation. Setting all the camera settings to manual (max ISO my mobile camera allows 1600 , shutter speed 1 sec), I was trying to capture the images of Orion constellation. For testing the image quality I first set the interval to be 1 second and captured 10 images from the camera. But something which I didn't understand was why the images differ much even when the physical conditions were same. I could capture the two bright stars of the Orion in only first image and later I couldn't. Is this because of change in some physical properties of the camera (sensors and the components) after taking images before ? What's the reason ?
Stars are point sources of light. This makes it especially difficult for a Bayer masked sensor to generate consistent results as the sky moves slightly between each exposure. The raw data obtained on a Bayer masked sensor must be interpolated by comparing and averaging the amount of light striking adjacent pixels as well as nearby pixels sensitive to the same color. But when a point source of light, such as that from a distant star, strikes the sensor it may fall on only one or two pixel wells. The demosaicing algorithms used by many camera makers will often see these extremely small and isolated light sources as shot noise and remove them in an attempt to denoise the image.
I could capture the two bright stars of the Orion in only first image and later I couldn't. Is this because of change in some physical properties of the camera (sensors and the components) after taking images before?
One reason may be the normal twinkling of the stars. When you look at the stars, you're obviously looking up through an atmosphere made up of many layers of air that are constantly changing slightly in density. Changes in density affect the way the air refracts light, so that at one moment light from a given star hits your eye (or your camera) directly, and the next moment the light from that same star bends away enough that the star disappears or just gets dimmer.
You can reduce the problem by taking a long exposure, or by taking multiple shorter exposures and using software to stack them up and create a single image.