[meteorite-list] Geminid pic / photographing meteors

[Chris Peterson]

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" <pict at pict.co.uk>
To: <Meteorite-list at meteoritecentral.com>
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