Re: [APML] stacking & adding film images

["Steve Walters" <s-walters@comcast.net>]

Wei-Hao,

Then perhaps you can explain to me why I have found this to be true. I
routinely image at Cherry Springs PA which is the darkest site around the
east coast (I sure envy your sites in Hawaii). I have photographed for up to
3 hours there on E200 and recorded practically no sky fog at least at the
zenith. But in comparing images taken at 1 hour, 2 hours and 3 hours, I do
not find fainter objects recorded on the longer exposures. It is true that
objects that did record were more dense on the longer exposures. But fainter
objects did not appear.

Also, it is curious to me why you consider sky fog to be noise. There is
nothing random about it. It is uniformly distributed across the image and
has steady magnitude. In order for stacking to help, there must be some
randomness about the noise in order for the steady signal to gain SNR
through stacking. That's because the noise, being random, won't always be
present in an image while the signal will be. So stacking an image with sky
fog will not improve the SNR of a dim star because both the sky fog and the
star will accumulate at the same rate (stacking). If the SNR is  zero
because the star and the sky fog are at the same leve, it will still be zero
after dozens of images are stacked.

Steve...

----- Original Message ----- 
From: "Wei-Hao Wang" <wang@IfA.Hawaii.Edu>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Monday, March 29, 2004 9:49 AM
Subject: Re: [APML] stacking & adding film images


> Hi Steve,
>
> Thank you for pointing out this.  Basically, I have to disagree
> with you.
>
> Please don't forget we are under the atmosphere and we are not
> using  the HST.  There is always a sky background in a long
> exposure.  (Indeed, the background is nonzero even in the HST
> images.)
>
> As long as the background reaches the critical exposure and
> produces a little fog, both 15th mag and 16th mag stars will
> reach the critical exposure as well and will leave something
> on the sky fog.  By stacking images, we decrease the noise on
> the sky fog and the signal from the 16th mag star will gradually
> show up.
>
> Your arguement is only correct if we are talking about 1st
> or 2nd mag stars and exposures as short as 1 minute or so.
> In these cases, sky background hardly reaches the critical
> exposure and fain objects leaves nothing on the film.  But I
> don't think this is an usual case in today's astrophotography.
>
> Cheers,
>
> Wei-Hao
>
> ______________________________________________________________________
> Wei-Hao Wang  :)
>
> Institute for Astronomy at University of Hawaii
>
> Address:                       Phone: 808-956-9867
> 2680 Woodlawn Drive            Personal Website:
> Honolulu, HI 96822             http://www.ifa.hawaii.edu/~wang
> ______________________________________________________________________
>
>
>
> On Mon, 29 Mar 2004, Steve Walters wrote:
>
> > Wei-Hao,
> >
> > I think one of your points is incorrect. It's true that film is
non-linear
> > and some of your discussion is correct. But I think the real problem
with
> > your discussion is that there is a threshold in film that must be passed
in
> > order for anything to record at all. You ignore this in your second #
poing.
> > If, for some system, a 30 minute image records stars down to mag 15 but
does
> > not record mag 16, then you can stack as many 30 minute images as you
like
> > and you will not recover mag 16 objects.
> >
> > Stacking will improve the SNR of those mag 15 objects or whatever
dimmest
> > objects are recorded so it's worth doing.
> >
> > But it will not make a deeper image.
> >
> > Steve...
> >
>
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>


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