reply to post by zayonara
Glad to see you are taking an interest in astrophotography, something that I've been doing myself since 1997, using both film SLR cameras and digital
cameras, both wide field and through the telescope.
Here are some things to consider:
As you said, each frame is at 15 seconds of exposure. In the frames where we have motion (IE meteorite or satellite), we can see this motion against
the fixed back ground of stars as a streak.
Each anomaly that you have shown does not appear to have that streak, which would suggest there is no motion by it (unless it appears only in the very
last second of your exposure, and for that to happen every single frame, I would highly doubt).
So what could these things be?
Well, each one that you have shown, appears to be very dim (extremely low magnitude), which would make it visible to the camera during long exposures,
but not our eyes of course. It's possible that you may have captured very faint object that are not close to Earth (IE space junk, satellites, etc),
but are faint an far away (small asteroids or comets that are far from the sun, so no tail yet). The motions of these will not be very apparent as you
would need to photograph the same spot in the sky at the same time for several nights. If the objects are not on any star charts, and appear to move
against the background stars (over the course of several nights), that may have been what you are capturing on the camera.
The night's "seeing" for the air could be affecting things. "Seeing" is how much turbulence is in the upper atmosphere, and is what makes stars
"twinkle". Really good "seeing" makes the stars appear mostly steady (only a little bit of twinkle) and most seem to have a steady magnitude.
Really bad "seeing" can make even the planets seem to twinkle sometimes.
The point about this is, that some stars are so far and faint, that you can only see them with a camera, or by using a telescope with a large
reflecting mirror (helps concentrate the light for your eyes). If stars are far and faint enough, and the "seeing" that night is bad enough, the
stars magnitude can appear to be affected.
This means it would appear to wink on and off in your photo frame.
Brighter stars will not do that, but they will seem to 'twinkle" more.
You may be capturing very faint Variable Stars
who's magnitude (brightness) can fluctuate.
However, most of these stars have a cycle that is measured in days or years. It might be possible that you are catching them right when they enter a
brighter cycle (but that would be highly unlikely for it to happen that many times for you. Would be like winning the lottery every day).
Thermal noise on your CCD chip of your camera:
Thermal noise and cosmic rays may alter the pixels in the CCD array. To counter such effects, astronomers take several exposures with the CCD
shutter closed and opened. The average of images taken with the shutter closed is necessary to lower the random noise. Once developed, the dark frame
average image is then subtracted from the open-shutter image to remove the dark current and other systematic defects (dead pixels, hot pixels, etc.)
in the CCD.
CCD Chips and Astronomy
Just some ideas as to what could be in your photos.