Meade Image Processing Manual de usuario descargar pdf (Pagina 11)

The various parts of the resulting dark-subtracted images are not equally bright. There

are many optical surfaces in a telescope system, and dust can settle on each one. The linear

scaling will enhance tiny contrasts. It will bring out all little dust shadows and unevenness

of sensitivity. Some regions or just some pixels in the picture are darker than others. We

must find a way to compensate for these differences.

The procedure is called flat fielding. Again with the exact same device, we take pictures

of an even surface, such as a chunk of the clear blue sky, during the day. These pictures,

called flat frames, are supposed to be even, flat, without details on them, but in reality they

will show the same shadows and unevenness as our pictures. So we divide the pictures by

the corresponding flat frames. CCDOPS will also do this upon pressing of a few buttons.

The dark subtracted and flat-fielded images may still contain a few bright or dark single

pixels, which can be corrected by the appropriate filters called “kill warm/dark pixels”. This

advanced step is very useful when long exposures are taken.

Your images are now ready to use. It is conceivable however, that some images may

need a very long exposure, which cannot (for practical reasons) be done at one go. In these

cases more images are taken and they are added together to form one. Also, images are

taken with various color filters, and these images can be assembled into one color picture.

However, it is almost certain that the telescope could not maintain the exact same aim while

all these images were taken. They will be offset (shifted) from each other, so they cannot be

simply added together. You need to align the images first.

The image alignment routine is quite awkward in CCDOPS. We have another, more

sophisticated, program called CCDSOFT, which can align a whole set of images at the same

time with subpixel accuracy. All you need is to mark the same star on all images and click

the “align centroid” tab.

Fig. 8: Dividing by a flat frame removes uneven shadows. On the raw picture far left, notice the darker

background top right, as well as the blob bottom right of the image. Flat fielding divides it off.

Red Green Blue composed into a

color image

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Meade Image Processing Manual de usuario Pagina 11