Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/~levay/color/handout2p.pdf Дата изменения: Wed Oct 27 23:50:00 2004 Дата индексирования: Tue Oct 2 07:42:16 2012 Кодировка: Поисковые слова: http astrokuban.info astrokuban

Preparing Colorful Astronomical Images II
Z. G. Levay and L. M. Frattare (ST ScI)

AAS · January 8, 2003 Seattle, WA · 91.10

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e present additional techniques for using mainstream graphics software (Adobe Photoshop and Illustrator) to produce composite color images and illustrations from astronomical data. These techniques have been used with numerous images from the Hubble Space Telescope to produce photographic, printed and web-based products for news, education and public presentation as well as illustrations for technical publication. We expand on a previous paper to present additional techniques and more detail, taking advantage of new or improved features available in the latest software versions. While Photoshop is not intended for quantitative analysis of full dynamic range data (as are IRAF or IDL, for example), we have had much success applying Photoshop's numerous, versatile tools to work with scaled images, masks, text and graphics in multiple semi-transparent layers and channels.

How do I get images into Photoshop?
Linear Square Root Log

How do I make the image look better?
Note that much of this is subjective rather than dictated by hard and fast rules... І A dark, neutral sky background is often preferred for astronomical images. І Photographs usually benefit from having "shadow " and "highlight" details. That is, the darkest area should be close to black and the lightest area should be close to white. І A broad "tonal range" with detail apparent at all brightness values usually looks best.

Tadpole Galaxy, UGC 10218 Ў HST/ACS Credit: NASA, H. Ford (JHU) and the ACS Science team

v<20000

sqrt(v>1<50000)

alog10((v+600)>525<70000)

Photoshop Notes
І Apply adjustments to exposure layers individually to benefit from maximum flexibility in hue and tonal range. І Avoid committing changes to a pixel layer unless necessary. All Photoshop modifications can be accomplished using adjustment layers, which permit unlimited changes without modifying the original pixel values. І These examples were produced using Photoshop 7.0 on a Windows 2000 workstation, but it fundions identically in MacOS and most features are available in previous versions. І There is no up-to-date Unix version of Photoshop. The freeware GNU Image Manipulation Program (The GIMP) includes many of the same capabilities.

v<2000

sqrt(v>1<50000)

alog10((v+600)>525<8000)

Examples of different clipping values with linear, square root and log transform functions applied to the same grayscale image. Log scaling applies the greatest comIn an analysis software package (IDL or IRAF): І Clip (apply minimum and maximum data values) to isolate the appropriate data range. І Transform (apply square root, log, or other function) if needed, to adjust contrast and compress dynamic range. І Convert to 8 bit grayscale. The IDL

pression of dynamic scale; it decreases contrast in bright areas while increasing contrast at the faint end. Different stretches can be combined by judicious masking to extend the dynamic range. bytscl function scales any data type, returning a single-byte array. The rgbsun task in the IRAF color package combines three images into RGB Sun rasterfile fomrat. І Write the image in a format recognized by Photoshop (TIFF recommended). The IDL write_tiff command generates a TIFF image from an IDL 2-D array.


How do I combine images in color?

Global Curves adjustment layer applies to all layers below.
F658N [N II]

Green

І Place each grayscale exposures in a separate Photoshop layer. І Set the layer blend mode to Screen, which "projects" each colorized layer independently; this allows three primary colors to combine to produce all possible hues. І Apply a Hue/Saturation adjustment layer grouped with each image layer separately; this renders the grayscale pixel layer in color. І Select Colorize mode. І Select the layer's color by specifying the Hue value (0-360). І Set the Saturation to 100% for maximum color. І Set Lightness to -50. І Optionally apply a Curves or Levels adjustment layer grouped to each image layer only, or to the entire image.
IC 4406 HST WFPC2 C.R. O'Dell ( Vanderbilt)

F656N H

Hue/Saturation dialog for the F656N (green) layer of the IC 4406 color composite; converts grayscale layer into single-hue color layer.

F658N [NII] F656N H F502N [OIII]

F502N [O III]

The individual grayscale images comprising the color composite

The Photoshop Layers palette for the color composite image.

Levels adjustment layer, grouped with the [O III] (blue) image, applies to that layer only.

Levay & Frattare · Page 2 levay@stsci.edu · http://opostaff.stsci.edu/~levay/color/handout2p.pdf

The composite color image