NavList:

Norm, you wrote:
"If the two stars are smeared because of camera motion, why is the ISS track so narrow? What am I missing? "

I didn't see this right away either, but once you DO see it, it's actually obvious. :)

At any instant of time, the ISS, Polaris, and the faint star with the "HIP" id are just points of light at this resolution. During the half-second exposure, the camera is moving towards the lower right at a rate of about 10 arcminutes per second. Meanwhile the ISS was moving across the sky towards the upper right at a rate of around 6° in ten seconds or 36 arcminutes per second. Much like a boat affected by a current, the motion of the ISS image across the photo-sensing array during the half-second exposure is the vector sum (or is it difference? ...we'll fix the signs in post) of those two motions. At every instant during the exposure, the camera is just seeing a dot of light, but its motion is "steeper" since the camera is moving. There are two special cases: the camera could track the stars perfectly, making their images perfect points and the ISS a trail, or alternatively, the camera could track the ISS perfecly, making the ISS image a nice dot while the star images would be trailed.

-FER

----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------