Thank you for your comments, I'll try to reply to the questions
query wrote:Why is the background black?
The background is black because of the very dark filters used to decrease the intensity of the light. If you look at the Sun with a solderer helmet, you'll see the same thing: the solar disk (color depending on the filter) surrounded by a black sky.
Hofi wrote:
How long do you usuallly exposure the sun?
for this picture, it's the shortest allowed by the camera: 1/8000s. This helps to decrease (a bit) the atmospheric turbulence effects and to freeze the movement of the space ships: even during such a short exposure time, they move by almost one pixel in the image!
DLH wrote:On the enlarged picture, the spacecraft are blurred. Is the blurring due to:
1. limited depth of field;
2. spacecraft motion; what is the exposure time; what is the exposure technique: moving shutter slit?
3. atmospheric turbulence?
1. no, at this distance the difference between focus at infinity and focus at ISS is infinitesimal
2. a little bit (see above)
3. yes. On last Saturday, I have taken a sharper image, thanks to better seeing turbulence conditions:
http://www.astrosurf.com/legault/iss_at ... _2010.html
The turbulence is not inside the telescope but in the atmosphere. During daylight, the Sun heats the ground and this causes convection movements with airmasses of different temperatures. On the image above, we see clearly the turbulence waves that blur some parts of the image.
