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QSI 500 SERIES USER GUIDE

prevent light from falling on the CCD surface while the image is being shifted out of the

CCD.

Interline Transfer CCDs

Interline transfer CCDs work somewhat differently. In an interline transfer CCD, next to

every column of pixels is a specialized storage column that is covered by a mask to prevent

light from hitting the storage 'pixels' underneath. When an exposure is complete, the entire

image is shifted in a single operation into this masked storage column. The pixels which

are now under the mask stop building additional charge and are shifted out of the CCD in

the same fashion as a full-frame CCD. Interline transfer CCDs give up some sensitivity

because a sizable portion of the potential light gathering surface of the CCD is occupied by

the masked storage columns. The key benefit of interline transfer CCDs is that the shifting

of the image into the masked storage column acts like a very precise electronic shutter

allowing short, accurate exposures.

Anti-Blooming CCDs

CCDs are subject to an electronic artifact called “blooming” that results in bright vertical

streaks leading from bright objects.

The 60-second image above shows a portion of M42, the great nebula in Orion. The stars that make up the center of

the nebula are much brighter than the surrounding nebula. Taking an exposure long enough to show detail in the

nebula causes the bright stars to bloom. Note that some of the other brighter stars around the image also show

varying amounts of blooming.

Blooming occurs when taking images of bright objects because when a pixel reaches its full

well capacity, say 100,000 electrons, the electrons literally overflow into adjoining pixels

eventually causing them to fill and overflow as well. In a severely bloomed image, the bright

blooming trail can lead all the way to the edge of the image. Data under a “bloom” is lost

although there are a variety of processing techniques that can be used to hide pixel blooms

in a final processed image.

Anti-blooming is a feature available on many full-frame and most interline transfer CCDs.

Anti-blooming technology limits the number of electrons that can accumulate in a pixel by

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QSI 520c Guía de usuario Pagina 32