Meade Image Processing Manual do Utilizador descarregar pdf (Página 25)

IX. Now co-add the multiple exposures of the same color (if any) either in CCDOPS or in

CCDSOFT. When the same exposure time was used for all images, you want to use

CCDSOFT’s “Combine a folder of images” feature with the “average” setting instead of

simply adding. This will help avoid clipping stars when the pixel reading in their centers

exceeds 65,000. (Recall that clipped stars will lose their color in the final image!)

X. Depending on the quality of the images at this stage you should consider sharpening

(works when there isn’t much noise; use CCDSHARP, which is a slow but very powerful

piece of software), or blurring (needed when there is too much noise; try the “Median

Filter” in CCDOPS). You may have to use the “Remove image gradient” tool in CCDOPS

when there is too much uneven background light.

These steps are fairly standard, and provide you with a set of aligned images, one for each

color, in .fit file format (the standard picture format in astronomy). You will do the

remaining work in Photoshop. Before the transition to PHOTOSHOP will make sure all your

images are as good as can be; it is a real pain to have to come back later and make changes.

XI. For the conversion to PHOTOSHOP, open each of the three-color image components. The

LIBERATOR plug-in, which is installed on these computers, starts automatically. It gives

you a panel for each image on which you must very carefully adjust the black and white

points. The correctness of these numbers is crucial; you may end up with an incorrect

choice and have to come back to this stage to redo it. You must chose the back point so

that, even in 100% view, only a small percentage of the pixels is black-clipped (shown in

blue by LIBERATOR), and that none but a few pixels are white clipped (shown green by

LIBERATOR). These black and white point settings are now final, they cannot be undone in

PHOTOSHOP as they can in CCDOPS. Once open, immediately save these images in

PHOTOSHOP format (.psd)

The rest of the image processing is not specific to astronomy, and if you like you can do it

on any computer with PHOTOSHOP CS installed on it. For that, you need to put the image files

on a jump-drive, and take it with you (attaching to an email does not work; the files are too

large for that.

XII. Use the Merge Channels utility to produce a color image (the tab is hidden:

you’ll find it if you open the Channels palette and click on the little triangle in the top

right corner). Use the RGB mode and check if the correct files are used for each color.

XIII. At this point much of the detail that is actually present on the images is still

hidden; your deep-sky object may not even be visible yet. To “dig out” objects from the

usually very large background you will need to create adjustment layers. In PHOTOSHOP,

use “Layers → New Adjustment Layer → Curves”, and change the curves. Take care of

using only linear scaling in each color channel to balance the overall color as well as to

equalize the color of the remaining background. (Linear scaling means straight curves;

nonlinear scaling invariably produces ugly colors.) Use nonlinear scaling only in the

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