Re: Why .avi format ?


Chris L Peterson wrote:
> On 28 Nov 2005 04:40:37 -0800, "atasselli@xxxxxxxxxxx"
> <atasselli@xxxxxxxxxxx> wrote:
>
....

> >>Readout noise makes for a stiff noise penalty...
> >
> >This depends on the read-out noise. The lower it is the less different
> >the 2 techniques are.
>
> Of course, but realistically all cameras now being used are high readout
> noise. Webcams are particularly poor in this respect.

True. But webcams are really the lower end of the video imaging
spectrum.

>
>
> >> Once you start imaging DSOs, you need long exposures- many minutes is
> >> usually required to maximize S/N.
> >
> >Not really. I've been imaging DSOs for the past 4 years without ever
> >taking exposures longer than 120s, 45s to 60s being the most common
> >durations. With low read-out noise camera there are benefits in using
> >short exposures.
>
> Unless you are imaging under horribly light polluted skies, it is likely
> that you are not optimizing your S/N with such short exposures. That
> isn't the same as saying that you aren't getting good results, however-
> just that they could be better.

I opine that while it is true that longer exposures will produce higher
S/N images (with the same length of time) it is not true that they are
going to be necessarily better. If the subject being imaged is bright
(relative to the scope/camera combination at hand) shorter exposures
(30s-60s) will go a great length in improving resolution, if you
haven't got a tip-tilt AO. There is also the problem with
oversaturation of bright stars in the field that can be worked around.
Of course, as you're going to discard all not-too-good frames, to build
up a an image with decent S/N might entail doubling the effective
exposure time.

BTW, my skies are pretty good (when is not cloudy, that is), at about
5.5 ZLM.

>
> There can be benefits to short exposures, particularly if your equipment
> isn't good enough to support accurate tracking for long periods. I have
> an experimental camera with essentially zero readout noise. You can do
> some very cool things with it, such as building the final image
> dynamically from subsecond exposures. The actual image is used for
> guiding, and the current sum image is always available, so the exposure
> can be run until the desired goal is visible. Focus can be monitored and
> controlled throughout the exposure, and bad frames can be rejected on
> the fly. Cool stuff, but these sensors don't seem to be under very
> aggressive development (although I guess they are being developed for
> military applications, since they perform better than the best
> intensifier-based night vision equipment).

As you said, cool stuff. Are they available commercially?

Best

Andrea T.

.

Relevant Pages

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