QSI 520c Betriebsanweisung PDF herunterladen (Seite 35)

QSI 500 SERIES USER GUIDE

Reducing noise in CCD images

CCD imagers have developed a standard set of calibration techniques to reduce or

eliminate different types of noise from CCD images. Calibrating CCD images requires

taking some special kinds of exposures that are then applied to the “light frames” taken of

the night sky. The calibration frames are called Dark Frames, Flat Fields and Bias Frames.

MaxIm LE and other CCD camera control software help gather these extra frames. After

the frames are gathered, MaxIm allows you to calibrate your images either automatically or

manually.

All the calibration frames should be collected during each imaging session with the CCD at

the same temperature used for the light frames. This will ensure the best possible

calibration of the final images. Many CCD imagers plan their night of observing to begin

taking the calibration frames as dawn approaches. That way, you don’t waste precious

dark time.

The image above is a single raw 6-minute image of the diffuse nebula M78 in Orion. Some bright stars are clearly

visible along with some nebulosity but there are also scattered bright spots around the image caused by “hot” pixels.

Dark Frames

Dark frames are used to subtract the build up of dark current from a CCD image. Dark

current is caused by heat. Similar to how CCDs convert the energy from a photon into a

stored electron, CCDs also convert the energy from heat into stored electrons. CCDs build

up “dark current” whether the CCD is being exposed to light or not. The rate that dark

current builds up is dependent on the temperature of the CCD and can be dramatically

reduced by cooling the CCD. Dark current builds up more slowly as the temperature of the

CCD is reduced.

Most pixels on a CCD build up dark current at a constant rate but that rate will vary slightly

from pixel to pixel. A subset of the pixels in a CCD will build up dark current at a

dramatically different rate from the average. These pixels are called “hot pixels” or “dark

pixels”. Hot pixels and dark pixels are both the result of slight imperfections introduced into

the silicon substrate of the CCD during the manufacturing process. Hot pixels are very easy

to see in a raw CCD image as a series of bright dots placed randomly around the image.

1 2 ... 30 31 32 33 34 35 36 37 38 39 40 ... 86 87

Keine Kommentare

QSI 520c Betriebsanweisung Seite 35