The sensitivity is very much related to exposure time. The longer the
shutter is open, the more the sky background (and its associated noise)
fills each pixel. This rapidly washes out fainter meteors. The actual
exposure time for a meteor is the amount of time its image dwells on a
single pixel. All the rest of the time, that pixel is accumulating signal
and noise unrelated to the meteor. It turns out that for typical meteor
speeds and typical focal lengths, video rates are just about optimal for
sensitivity. Of course, if you're not using video, you do need to leave the
shutter open long enough to catch most meteors in their entirety.
Remember, there is no such thing as noise cancellation. Some camera play
tricks to hide noise, but they do so at the expense of signal. You cannot
eliminate or even reduce noise- if you could, it wouldn't be noise, but
something systematic.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
----- Original Message -----
From: "John Hendry" <[email protected]>
To: <[email protected]>
Sent: Thursday, December 16, 2010 1:51 PM
Subject: Re: [meteorite-list] Geminid pic / photographing meteors
Chris,
To be clear about how I personally was looking at this; the length of time
the shutter is open has no bearing on the sensitivity to the meteor
exposure. That I thought was entirely controlled by aperture and ISO
sensitivity (i.e. "film speed"), along with the velocity, brightness and
trail persistence of the meteor. Camera field of view might also have a
bearing as the meteor image will spend a longer time over a particular
pixel sensor with a shorter focal length (i.e. wider field of view) and
thus be brighter in the image (though smaller). When you say "the longer
your exposure, the less sensitive you will be to meteors" then I can see
this from the point of view that the meteor exposure can be progressively
obscured by scattered light in the sky (from the
sun/moon/streetlights/background starlight) and from sensor noise in the
case of digital cameras. With sensor noise cancellation and a pitch black
sky, I would expect exactly the same meteor image from a 5 second exposure
versus a 30 minute exposure at the same f-stop and ISO, though the lower
magnitude stars (specifically those that haven't fully reached the cameras
upper exposure limit with the shorter shutter) will appear brighter as the
shutter is kept open longer. Is this about right or am I missing
something? I'm just not clear why I would lose fainter events with longer
shutter speeds other than for the reasons I outlined above.
I like your video idea... you could edit out all the dead action and make
something that looked like a much more exciting bombardment... though
jumping stars would probably give the game away unless you're using a
tracking mount. Plenty of scope for fun. Love your telescope images. M51
is just fantastic.
Cheers,
John
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