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Why .avi format ?



 
 
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  #1  
Old November 27th 05, 11:28 PM posted to sci.astro.amateur
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Default Why .avi format ?



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 ? I undertstand that .jpg wouldn't
give one access to the data on a per pixel basis, but wouldn't a .bmp file
work ? Is the .avi format chosen only by Toucam users ?

I'm also surprised by the heavy prevelance of Windows software for this.
Are FreeBSD, OSX, Linux, Solaris really as rare as they seem to be within
the Astro imaging crowd ?
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  #2  
Old November 27th 05, 11:43 PM posted to sci.astro.amateur
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Default Why .avi format ?

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), 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.

Time exposure astro imagers do not use AVI.

---
Michael McCulloch
  #3  
Old November 28th 05, 12:43 AM posted to sci.astro.amateur
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Default Why .avi format ?

Tom Rauschenbach:
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.


That's exactly it. People make motion pictures of solar-system objects,
then they use software such as Astro IIDC, Keith's Image Stacker,
Registax, etc. to select the best frames, which are then combined by
the software to produce a better image than could be obtained with a
single exposure.

Why is that the format used for
time exposures through a telescope ? I undertstand that .jpg wouldn't
give one access to the data on a per pixel basis, but wouldn't a .bmp file
work? Is the .avi format chosen only by Toucam users ?


Users of the ToUCam and similar cameras who use Macs use the QuickTime
format. Windows users use AVI.

I'm also surprised by the heavy prevelance of Windows software for this.


No prevalence at all for me :)
http://www.davidillig.com/astromac.shtml

Are FreeBSD, OSX, Linux, Solaris really as rare as they seem to be within
the Astro imaging crowd ?


All of those together don't approach the Windows share in amateur
astronomy. In /professional/ astronomy, a 2005 survey published by the
Illinois Institute of Technology showed the following OS prevalence.
Numbers are percentages.

OS 2003 2004 2005
Linux 37.5 42.2 40.8
Mac OS X 08.5 13.3 33.4
Sun OS 33.4 26.7 14.2
Windows 13.3 14.7 11.6

The reason Windows isn't as popular in professional astronomy is that
it can't run Unix apps natively; the other OS's listed are all Unix
based. The growth in OS X is attributed to the fact that it can run
both Unix and mainstream apps such as MS Office, Photoshop, et cetera,
simultaneously, out of the box -- no HD partitioning or dual booting
required.

Davoud

--
usenet *at* davidillig dawt com
  #4  
Old November 28th 05, 01:58 AM posted to sci.astro.amateur
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Default Why .avi format ?

On Sun, 27 Nov 2005 19:43:41 -0500, Davoud wrote:

Tom Rauschenbach:
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.


That's exactly it. People make motion pictures of solar-system objects,
then they use software such as Astro IIDC, Keith's Image Stacker,
Registax, etc. to select the best frames, which are then combined by
the software to produce a better image than could be obtained with a
single exposure.


That makes a great deal of sense to me. Especially in light of statements
that seem to imply that long exposures aren't really needed for CCD data
collection. Although I have to wonder why there aren't discussions
about readout noise/image. What are the costs and benefits of many short
exposure images over fewer long exposure images ? Of course tracking
issues are an obvious one.


  #5  
Old November 28th 05, 02:46 AM posted to sci.astro.amateur
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Default Why .avi format ?

On 2005-11-27, 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 ? I undertstand that .jpg wouldn't
give one access to the data on a per pixel basis, but wouldn't a .bmp file
work ? Is the .avi format chosen only by Toucam users ?


The software supplied with webcams will produce .avi files. It is only
natural to use them. They aren't used for long exposures, but instead
for collecting a series of images to be stacked and processed. "Raw"
files would give the most control over image processing. The popular
Registax program will stack and process images broken out from .avi
files.

As far as windows is concerned it is a question of having device drivers
for the webcam. If you had linux drivers you could use sane or xsane
to collect the images and the GIMP, IRAF, etc. to process them.

I think that among dedicated astro cameras Finger Lakes advertises
linux drivers. I'm not sure about others.

--
The night is just the shadow of the Earth.
  #6  
Old November 28th 05, 03:14 AM posted to sci.astro.amateur
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Default Why .avi format ?

Tom Rauschenbach:
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.


Davoud:
That's exactly it. People make motion pictures of solar-system objects,
then they use software such as Astro IIDC, Keith's Image Stacker,
Registax, etc. to select the best frames, which are then combined by
the software to produce a better image than could be obtained with a
single exposure.


Tom Rauschenbach:
That makes a great deal of sense to me. Especially in light of statements
that seem to imply that long exposures aren't really needed for CCD data
collection. Although I have to wonder why there aren't discussions
about readout noise/image. What are the costs and benefits of many short
exposure images over fewer long exposure images ? Of course tracking
issues are an obvious one.


Remember that when we are discussing webcams we are talking about
photographing bright solar-system objects only, not deep-sky objects.
Readout noise is negligible; the exposures are short. Webcam lunar and
planetary photography has little in common with deep-sky photography
using a dedicated CCD camera such as an SBIG or Starlight Express.

By the same token, long exposures don't enter into the equation, as the
objects we are photographing are bright. This technique -- combining
video frames -- is used because atmospheric turbulence is a severe
hindrance to planetary photography. Take a lot of pictures, though,
(4,000 maybe) and a certain number of them will exhibit much less
turbulence than the majority. These are the ones that are aligned and
combined to produce images such as these
http://www.buytelescopes.com/gallery/gallery.asp?c=17165 by Alan
Friedman. Mr. Friedman uses FireWire webcams and Astro IIDC running
under Mac OS X. (Note the exception to what I wrote above; the
referenced page includes a pretty remarkable photo of the M57 -- the
Ring Nebula -- made with a FireWire webcam.)

Davoud

--
usenet *at* davidillig dawt com
  #7  
Old November 28th 05, 04:58 AM posted to sci.astro.amateur
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Default Why .avi format ?

What are the costs and benefits of many short
exposure images over fewer long exposure images ? Of course tracking
issues are an obvious one.


A long exposure will be blurred because of seeing conditions.
Very short exposures freeze the "ripples" from the atmosphere.
If you get enough and stack them you can eliminate most of the
problems caused by viewing through the atmosphere. This is why
amateurs today produce better moon and planetary photos than
the pros did a few decades ago.

Clear Skies

Chuck Taylor
Do you observe the moon? If so, try
http://groups.yahoo.com/group/lunar-observing/

If you enjoy optics, try
http://groups.yahoo.com/group/ATM_Optics_Software/
*********************************************

  #8  
Old November 28th 05, 06:09 AM posted to sci.astro.amateur
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Default Why .avi format ?

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

That makes a great deal of sense to me. Especially in light of statements
that seem to imply that long exposures aren't really needed for CCD data
collection. Although I have to wonder why there aren't discussions
about readout noise/image. What are the costs and benefits of many short
exposure images over fewer long exposure images ? Of course tracking
issues are an obvious one.


There are many discussions about noise issues, just not a lot on this
forum. Readout noise is the primary noise source when you are doing any
kind of video astronomy, either with a webcam or a video camera and
frame grabber. It is primarily readout noise that determines your S/N.
Because of the short exposures, dark current noise is insignificant.

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. 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.

Once you start imaging DSOs, you need long exposures- many minutes is
usually required to maximize S/N.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
  #9  
Old November 28th 05, 07:49 AM posted to sci.astro.amateur
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Default Why .avi format ?

AVI is a videofile format. BMP is an image file format, typically. Same for
JPG. When using webcams for planets and the moon, you need to take hundreds of
images quickly. Taking individual images is just not the way to go, so video is
the choice, and I guess AVI is one standard that's widely used.

--- Dave
--
----------------------------------------------------------------------
Pinprick holes in a colorless sky
Let inspired figures of light pass by
The Mighty Light of ten thousand suns
Challenges infinity, and is soon gone




"Tom Rauschenbach" wrote in message
news


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 ? I undertstand that .jpg wouldn't
give one access to the data on a per pixel basis, but wouldn't a .bmp file
work ? Is the .avi format chosen only by Toucam users ?

I'm also surprised by the heavy prevelance of Windows software for this.
Are FreeBSD, OSX, Linux, Solaris really as rare as they seem to be within
the Astro imaging crowd ?


  #10  
Old November 28th 05, 12:40 PM posted to sci.astro.amateur
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Posts: n/a
Default Why .avi format ?


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.

 




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