CCD Image Smear

Warning

Please note this section describes a CCD-specific issue.

If an image is shifted while the shutter is open, the charge that collects while the image is moving makes the image look smeared. Smearing can occur in several situations: if the camera is set to read out without closing the shutter, if the shutter is set to close too slowly, or in frame transfer sequences where the shutter stays open while the image is shifted to the storage array.

In most frame transfer applications, the shutter opens before the sequence begins and closes after the sequence ends. The charge gathered during the shift creates a smear across the image array.

Although the frame transfer time is usually small compared to the smallest useful exposure time, smearing cannot be eliminated when the shutter is left open for the entire sequence. The higher the ratio of the exposure_time to the frame transfer time, the brighter the image is in comparison to the pattern caused by smearing. An exposure_time that is too long will saturate the pixels and cause the image to lose all contrast.