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#1
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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 ? |
#2
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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
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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
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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
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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
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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
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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
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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 |
#10
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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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