[amsat-bb] Re: Dont loose the north
Tom Clark, K3IO
k3io at verizon.net
Mon Dec 31 12:12:02 PST 2007
Jim, ZL1TYF is correct in his interpretation. My QTH is about 35 km from
BWI (Baltimore-Washington Int'l) airport and is also on the
Boston/NYC/Philly/Washington/Atlanta/Miami east-coast corridor, so a lot
of high altitude planes pass overhead, These show up as a thin line
(cabin lights) and dots (wingtip beacons).
The bright curving traces are from planes landing or taking off from
BWI. The Aurora animated GIF, you will see one BWI plane whose track
split between two of the individual frames.
In these pictures, the shutter was held open for 88 seconds, and then 2
seconds of dead-time allowed for the JPEG frame to be written to the CF
card memory, so a new frame began every 90 seconds.
The two long exposure star trail photos were made by stacking a number
of these photos (140 for the non-aurora picture), each one on top of the
previous frame. The stacking algorithm involved a pixel-by-pixel search.
If the new pixel was brighter than the old one, then the new pixel was
saved. If not, the the older, fainter pixel was kept. This is to be
contrasted with the normal "open the shutter for an hour or two" scheme
because the black background (due to stray skylight and sensor noise)
doesn't "blow up". To do this processing, I used Max Lyon's Image
Stacker software (http://www.tawbaware.com/imgstack.htm).
If you view the 4-hour "140" picture at maximum resolution
(http://www.pbase.com/tomcat/image/5186538/original.jpg), you will see
that the star trail lines are dashed, due to the 88/2 second timing. The
earth did rotate under the stares during the 2 second gap! If you look
at this picture you will notice a gap about 2/3 of the way along each
arc. This is due to the Canon D60 I was using at the time running out of
battery power. It took me a few minutes to realize that the pictures had
stopped. The reason the 4-hour picture was restricted to 140 JPEG frames
is because, at the time, the biggest CF card I had was 256 MBytes in
size. Although the camera produced 6 MPixel images, the individual
frames were only ~1.5 MBytes because the JPEG compression (quite similar
to ZIP & RAR) squeezed a lot of black pixels down to only a few bytes!
(I really need to try a long overnight picture with my new 40D (thanks,
Santa) camera equipped with a huge 8 GByte CF card.)
More information about the AMSAT-BB