Re: [APML] Equivalent ISO of an ST10E CCD

[lkt <tan@cox.net>]

Jerry,

	The difference between the film and CCD example is in one case film
speed is defined by ISO. As you have said, you are familiar with
determining the exposure time for this case. The speed of the CCD is
defined by Quantum Efficiency (QE), or what percentage of photons get
converted to electrons. So if you can determine per unit time, how many
photons are hitting the CCD detector, coupled with the f-ratio one can
figure out when the CCD will reach half well capacity. So, yes, in
theory you can figure this out. Maybe we would need a method of
measuring the photon flux of the object of interest. I am sure there are
methods of doing this but I am not familiar with it. In practice, I do
not belive this would be too important but of course it will be more
information at your disposal.

Loke



Jerry Lodriguss wrote:
> 
> >Well, however long it takes is how long it will take <g>. OK I do not
> >know this because this depends on the brightness of the object, the
> >f-ratio of the scope and the QE of the CCD. This is akin to asking how
> >long an exposure on film which will produce the hump in the middle. At
> >any rate, the hump in the middle is not the best. Presuming noise
> >remians sonewhat controlled, the hump to the right has better SNR
> >provided you are not saturating (clipping).
> 
> Hi Loke.
> 
> I'm sorry, but the question I asked was straightforward enough. For film,
> the answer is not "however long it takes is how long it will take".
> 
> If I put a gray card in the sun and I know the film speed and f/ratio, I
> can tell you exactly how long the exposure would be to put the spike in the
> histogram at level 128.
> 
> The brightness of the object is defined as a gray card in the sun, the
> f/ratio of the scope is whatever you want it to be, just pick an f/ratio,
> and the QE of the CCD is for the ST10E. Isn't that known?
> 
> The hump in the middle IS the best because it is the DEFINITION of what I
> am looking for: the exposure that will put the hump in the middle.
> 
> Jeez. I'm getting frustrated.<G>
> 
> Jerry
> Photoshop for Astrophotography Book:
> http://www.astropix.com/APBOOK/0_PROMO/PROMO.HTM
> 
> Astrophotography, Tips and Techniques
> for Digital Enhancement in Photoshop:
> http://www.astropix.com
> 
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