I have received dozens of emails from sharp-eyed observers of my latest comet movie commenting on the new, mysterious “asteroid” that appears in my movie:
http://home.att.net/~dpersyk/new.htm
The “asteroid” – actually there are a couple of them – is in fact an errant warm pixel. The movement in the movie is a bit perplexing, so I thought I’d explain it to the group.
My movie spans a time period of 1 hour and 28 minutes, compressed into a mere 7 seconds and 23 frames. During that time, the star field has drifted in declination a few arc minutes due to imprecise polar alignment -- this is simple field rotation. And this is important! Because now the warm pixel appears to be moving with respect to the star field. This motion is apparent when I run the video sequence. The solution is to apply a 3x3 median filter to each frame before assembling them into an animation.
I first encountered this wandering warm pixel phenomenon in hour-long stacks of H-a subframes. The sky background as imaged via a 6 nm FWHM filter is essentially pure black, and the warm pixels leave their little footprints across the stacked image due to field rotation. The 3x3 median filter fixes the problem.
Many thanks to the visitors to my site for pointing out the “asteroid”. Alas, I didn’t discover anything new.
Clear skies,
Dennis Persyk
Igloo Observatory Home Page http://dpersyk.home.att.net
Hampshire, IL
New Images http://home.att.net/~dpersyk/new.htm
http://home.att.net/~dpersyk/new.htm
The “asteroid” – actually there are a couple of them – is in fact an errant warm pixel. The movement in the movie is a bit perplexing, so I thought I’d explain it to the group.
My movie spans a time period of 1 hour and 28 minutes, compressed into a mere 7 seconds and 23 frames. During that time, the star field has drifted in declination a few arc minutes due to imprecise polar alignment -- this is simple field rotation. And this is important! Because now the warm pixel appears to be moving with respect to the star field. This motion is apparent when I run the video sequence. The solution is to apply a 3x3 median filter to each frame before assembling them into an animation.
I first encountered this wandering warm pixel phenomenon in hour-long stacks of H-a subframes. The sky background as imaged via a 6 nm FWHM filter is essentially pure black, and the warm pixels leave their little footprints across the stacked image due to field rotation. The 3x3 median filter fixes the problem.
Many thanks to the visitors to my site for pointing out the “asteroid”. Alas, I didn’t discover anything new.
Clear skies,
Dennis Persyk
Igloo Observatory Home Page http://dpersyk.home.att.net
Hampshire, IL
New Images http://home.att.net/~dpersyk/new.htm
