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Accumulating vs. Stacking

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#1 ccs_hello

ccs_hello

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Posted 24 December 2007 - 01:11 PM

Dear CNers,

May I propose using the terms with the following definiton:

- stacking: only select good frames (bad seeing, tracking problem, etc.) to be added/averaged to form the final image

- on-sensor-accumulating (commonly known as long exposure): give the photosite more exposure time to gather more photons. In video industry, the implementation (technology since '91 or earlier) is to clamp the "interline transfer" pulse. For example, one timing controller (from Sony) allows exposure time be extended up to 510 * field time (in NTSC, it's 510 * 1/59.94 = 8.5 second). Similar principle for Long Exposure is used in SteveChambers_mod #1 (SC1). Noise control on LE is essential such as TEC active cooling.
Note: always let the light in, no selection possible.

- post-sensor-accumulating: always let the captured images be accumulated/summed in the frame buffer (digital memory) for summing.
There is no selection on which image to be dropped.
This is near real time.

In another word, stacking can do some neat tricks while accumulating is realtime or near realtime.

Clear Skies!

ccs_hello

#2 rodelaet

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Posted 24 December 2007 - 03:24 PM

Which makes me wonder : The Watec 120N (aka Stellacam II), does it work on OSA or PSA?

#3 ccs_hello

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Posted 24 December 2007 - 04:10 PM

Rony,

WAT-120N has the max of 256 frame accumulation. This sounds to me done as OSA. From simplicity and less processing degradation point of view, if the accumulation task can be done on CCD (by changing the timing controller L.E. shutter timing), it should be done there. (And leave the frame buffer to perform one simple task: keeps playing back the same accumulated image at the video rate to keep the video monitor happy, until the next new image refresh at the same LE period.)

Clear Skies!

ccs_hello


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