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Why .avi format ?



 
 
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  #11  
Old November 28th 05, 04:42 PM posted to sci.astro.amateur
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Default Why .avi format ?

Michael McCulloch wrote:
On Sun, 27 Nov 2005 18:28:20 -0500, Tom Rauschenbach
wrote:

From reading this newsgroup I understand that .avu format seems to
be the conventional file format for digital imaging. I usually
associate that format with motion pictures. Why is that the format
used for
time exposures through a telescope ?


The AVI format is only used for solar system imaging where the object
is not dim (e.g. the planets and the moon),


This is not strictly correct. Check the Yahoo group QCUIAG for the popular
modified webcams which are quite capable of long exposures to image also
deep sky objects. Or see my web page :-)

the exposure is 1/10 sec
or so, and the goal is to get numerous images that 'freeze' moments of
good seeing from which to pick the best for stacking to increase
contrast and decrease image noise.


True for planetary/solar/lunar imaging. The exposure is usually much shorter
than 1/10 sec though.

Time exposure astro imagers do not use AVI.


This is not true. I have several long exposure modified webcams and they all
produce AVI files. I also have a higher end camera producing FITS.

Clear skies
Carsten A. Arnholm
http://arnholm.org/
N59.776 E10.457


  #12  
Old November 28th 05, 10:38 PM posted to sci.astro.amateur
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Default Why .avi format ?

On 28 Nov 2005 04:40:37 -0800, "
wrote:

This is functionally equivalent to high order adaptive
optics.


Not really. High order adaptive optics allow to correct the exit pupil
phase differences while select-and-stack simply rejects those that do
not qualify.


Which, in terms of the final image, is substantially similar. Sure,
discarding bad images is equivalent to AO with very poor QE, but all the
same, I consider such processing to effectively be post-processed AO.


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.


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.

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).

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #13  
Old November 29th 05, 01:10 AM posted to sci.astro.amateur
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Default Why .avi format ?

For even Jupiter (Saturn normally does not present any cloud features that show
up in amateur imaging) a few minutes of video can acquire a thousand images or
more, and a few minutes is not long enough to show motion of the cloud features,
at least at image sizes where Jupiter is perhaps a quarter to a third the width
of the video, so I don't see this as a major reason to not go video as I think
you're implying.

It's true that adaptive optics can produce more consistent, sharper images, but
for those that don't have the equipment to do it, video is a good way to go.

As for the readout noise, are you sure you're not referring to sensor or thermal
noise? This is a big difference between CCDs and CMOS imagers, and it's the
main reason one should use CCDs, in order to get the noise low enough.
Technically, readout noise is the noise induced by the circuitry used to read
the CCD/CMOS data out, and is different from the noise the sensor itself
produces.

--- Dave
--
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Let inspired figures of light pass by
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Challenges infinity, and is soon gone




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Chris L Peterson wrote:
On Sun, 27 Nov 2005 20:58:48 -0500, Tom Rauschenbach
wrote:

snip

There is only one fundamental benefit of short exposures- the ability to
capture images during brief moments of atmospheric stability. A
collection of many images can be graded for quality and the bad ones
discarded. This is functionally equivalent to high order adaptive
optics.


Not really. High order adaptive optics allow to correct the exit pupil
phase differences while select-and-stack simply rejects those that do
not qualify. If time is important (and it IS important in planetary
imaging as the planets rotate) and the resolution is high, video
imaging does not even come close to the perfomance (theoretically)
achieveable with adaptive optics. The only real good thing of video
imaging is that it can operate in the visibile without any restriction
(except for coherence angle, of course). And, yes, it is immensely


Readout noise makes for a stiff noise penalty, but with hundreds
or thousands of images the noise is substantially reduced. But the
technique is only useful for very bright objects- the Sun, Moon, and a
few planets.


This depends on the read-out noise. The lower it is the less different
the 2 techniques are.


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.

Regards

Andrea T.



  #14  
Old November 29th 05, 09:38 AM posted to sci.astro.amateur
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Default Why .avi format ?


Chris L Peterson wrote:
On 28 Nov 2005 04:40:37 -0800, "
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.

  #15  
Old November 29th 05, 10:02 AM posted to sci.astro.amateur
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Default Why .avi format ?


David Nakamoto wrote:
For even Jupiter (Saturn normally does not present any cloud features that show
up in amateur imaging) a few minutes of video can acquire a thousand images or
more, and a few minutes is not long enough to show motion of the cloud features,
at least at image sizes where Jupiter is perhaps a quarter to a third the width
of the video, so I don't see this as a major reason to not go video as I think
you're implying.


First, amateurs have taken images showing small transient features
(w.o. but not only) on Saturn since few years ago, even with "small"
apertures (I did it with a 8" MN/MCT, see
http://www.geocities.com/andreatax/2...opposition.htm or
http://www.geocities.com/andreatax/2...opposition.htm).

Secondly, the intrinsic limit of the length of a video on a planet like
Jupiter is much less then you might think of, 120s-160s is about as
good as it gets at its maximum angular diameter with a 10". I'm
assuming a sampling of 4 pixels per airy disc diameter. Saturn can even
be worse than that as it is far dimmer than Jupiter, although the
length of the video recording can be 2.5x that of jupiter. To build a
high resolution, good quality image of saturn you really need thousands
of good images and that only happens when the seeing is extremely good.


It's true that adaptive optics can produce more consistent, sharper images, but
for those that don't have the equipment to do it, video is a good way to go.

As for the readout noise, are you sure you're not referring to sensor or thermal
noise?


Thermal noise is irrelevant. We're talking about readout noise.

Andrea T.

  #16  
Old November 29th 05, 04:04 PM posted to sci.astro.amateur
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Default Why .avi format ?

On 29 Nov 2005 00:38:24 -0800, "
wrote:

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


Yes, but even most high quality video cameras have high readout noise.
It is intrinsic to the CCD sensors available, and cheap electronics just
adds to it.


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.


Interesting. Different people report different results here, so I think
it has a lot to do with the specific details of your local seeing. I've
made thousands of seeing measurements, looking at the frequency spectrum
of the shift, and for me, the short exposure advantage ends at about one
second. That is, I'll see reduced star sizes up to about one second, but
no difference between star sizes for longer exposures.


There is also the problem with
oversaturation of bright stars in the field that can be worked around.


This is the usual reason I will choose to shoot short exposures. It
costs me in noise, but keeps all my data linear. Of course, a decision
like this depends also on the ultimate intent in collecting the data.


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


Not really. A few very specialized (non astronomical) cameras using the
sensors are available, but are extremely expensive. The sensors
themselves are available (or were a few years ago) as engineering
samples, but you need to make your own camera around them. The one I
have has lots of problems- dead columns, hot pixels, and the like, so I
don't use it for more than testing. If they ever get the bugs worked out
of the technology, and these sensors become common, it will
revolutionize astroimaging.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #17  
Old November 30th 05, 03:37 PM posted to sci.astro.amateur
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Default Why .avi format ?


Chris L Peterson wrote:
On 29 Nov 2005 00:38:24 -0800, "
wrote:

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


Yes, but even most high quality video cameras have high readout noise.
It is intrinsic to the CCD sensors available, and cheap electronics just
adds to it.


You'll be amazed on how low it is for very high end video cameras. They
use mainstream CCD that are used also by cooled CCD cameras
manufacturers. And if you think what the dedicated CCD they use for
"fast" AO can do the best has yet to come...

Reg's

Andrea T.

  #18  
Old November 30th 05, 03:54 PM posted to sci.astro.amateur
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Default Why .avi format ?

On 30 Nov 2005 06:37:22 -0800, "
wrote:

You'll be amazed on how low it is for very high end video cameras. They
use mainstream CCD that are used also by cooled CCD cameras
manufacturers.


All of which have high intrinsic readout noise (typically 8-15
electrons). And this readout noise is worse when the chip is clocked
fast- pretty much a requirement for video. Especially at high speeds, it
is extremely difficult to design a CDS amplifier that doesn't add yet
more noise. Since this is the only thing under the control of the camera
designer, I guess it's where most of the effort goes with very high end
video. But still, they can't get better than the sensor specs, which is
why all video cameras- regardless of price- are readout noise limited
when used on astronomical targets.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #19  
Old November 30th 05, 06:12 PM posted to sci.astro.amateur
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Default Why .avi format ?

Well, I wouldn't call 8 e- "high", expecially in planetary imaging.
Besides I was think to something in the range of 2-3 e-.

Andrea T.

  #20  
Old November 30th 05, 07:48 PM posted to sci.astro.amateur
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Default Why .avi format ?

David Nakamoto wrote:

AVI is a videofile format.


Rather, AVI is a container format. Inside an AVI file, the movie can be
coded with any of a variety of codecs, typically DivX, XviD or some of
Intel's old Indeo codecs. Recently, however I have seen a lot of AVI
file that are actually MPEG4 or even h.264 (HD-video), which both run
natively in QuickTime 7.

Webcam movies are most likely Indeo 5.0 or DivX.

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