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Cosmic Ray Removal
Individual "CR split" images
Bright, sharp spots and streaks from cosmic rays appear on every HST exposure. Multiple exposures at the same pointing allow the randomly-appearing noise to be removed relatively easily, as long as nothing else changed in the meantime (easier for nonvariable deep space targets such as nebulae and galaxies rather than closer fastmoving solar-system targets). "Good" pixels from one exposure replace "bad" (cosmic ray) pixels from another exposure in a combined image. Otherwise, the exposures add together exactly as if it were one longer exposure.

Resulting combined image


WFPC2 consists of four cameras
Individual WFPC2 Chips

PC1

WF4

WF2

WF3 Mosaic Image

WF4 WF3

HST's WFPC2 instrument contains four CCD detectors ("chips"), each producing an image 80 0x80 0 pixels. Three "wide-field" (WF) chips have a field of view of 2.5 minutes of arc. The fourth "planetary camera" (PC) chip sees an adjacent, narrower field of 35 seconds. Because of the optical path, the resulting images are rotated with respect to each other. The separate images must be rotated and scaled to the same pixel scale before being combined into a mosaic. The fields overlap somewhat but "seams" remain in the resulting mosaic images.

R136, 30 Doradus F555W (V) HST · WFPC2

WF2

PC1


Image scaling renders maximum visible detail
Full data range Black: -207 White: 16143
Automatically setting the smallest image value to be black and the highest value to white can force important areas of the image to be rendered invisible. By selecting a minimum and maximum data value to render as black and white, repsectively, (clipping) the significant regions of the image can be made visible. Clipping the bright end to a lower pixel value shows some detail, but fainter details still may not visible. Clipping to still lower levels brings out fainter details but "saturates" (forces larger areas to be white) the bright regions. Applying a non-linear (log, square root, etc.) transformation can compress the dynamic range so that more detail becomes visible. Fainter details can become visible without saturating the brihgtest regions.

Clipped Black: 0 White: 60 0

Clipped Black: 0 White: 150

Logarithmic scale Black: 5 White: 550

Histograms­plots of the numbers of pixels at each brightness value, showing the relative distribution of intensities­can be useful to guide the selection of clipping and intensity scaling. A flatter histogram reflects more detail visible in all intensity levels. Intense peaks indicate a concentration of values at a particular brightness.

R136 (30 Doradus), F555W (V), WF2 HST · WFPC2


Combining separate exposures to produce a color image

Separate filter images

Grayscale

[HeII] F469N

[OIII] F502N

H F656N

[NII] F658N

Colorized

Resulting color composite
Separate images are exposed through different color filters. The resulting black and white or "grayscale" images are assigned a color that can be viewed or reproduced (that may or may not be the color of the filter used for the exposure). The separate images are combined in color image "channels" that together produce a full range of hues. Using filters that match the eye's response can result in a natural or "true" color image. Otherwise, the colors are enhanced or shifted from what we would be able to see.


Color composite using Screen layer blend mode
Layers Palette
Screen layer blend mode
U band

Grayscale

Colorized

Levels adjustment layer grouped with U image layer

B band

Hue/Saturation adjustment layer grouped with V image layer

V band

I band

Hue = 130 (green) Saturation = +10 0% LIghtness = ­50% Colorize

Hue/Saturation Dialog

Resulting color composite image


Adjusting image layer using Levels
Layers Palette Levels Dialog
Channel

White level

Black level

Gamma Levels adjustment layer applies to red layer only

Levels not applied

Levels applied

Various adjustments may be made to individual layers or the composite image as a whole to enhance the appearance. These adjustments include brightness and contrast, color, saturation, etc. The adjustments may be made directly on the image layers or to "adjustment layers" which do not change the pixel values and permit later modification. Any adjustment layer may be "grouped" with a particular image layer so that that adjustment applies only to that layer. Otherwise the adjustment will apply to the composite image resulting from the appearance of all layers below.


Adjusting image using Curves
Layers Palette Curves Dialog

Curves adjustment layer applies to color composite image

Curves not applied

Curves applied

Various adjustments may be made to individual layers or the composite image as a whole to enhance the appearance. These adjustments include brightness and contrast, color, saturation, etc. The adjustments may be made directly on the image layers or to "adjustment layers" which do not change the pixel values and permit later modification. Any adjustment layer may be "grouped" with a particular image layer so that that adjustment applies only to that layer. Otherwise the adjustment will apply to the composite image resulting from the appearance of all layers below.


Color-mapped images
Positive black and white (grayscale) 8 bit images contain 256 different values that may be represented by 256 shades of gray or 256 arbitrary colors.

Negative black and white (grayscale). Features are often easier to see in black against a lighter background.

Color-mapped, variations in hue.

Color Table Dialog


Combining multiple images to produce a wider field
Ground-based image with location of WFPC2 pointings WFPC2 images from separate pointings

Global adjustment layers Levels adjustments for pointing 1 (grouped with image layer) Levels adjustments for pointing 2 PC chip only Layer mask

Pointing 1

Pointing 2

Layers Palette
The relatively narrow field of view of the WFPC2 camera means that many targets overfill the image. Several carefully aimed exposures ("pointings") may be used to cover the image but must be combined afterward. Overlapping areas can be cut around or combined. The resulting images retain the resolution of the individual WFPC2 fields.

Resulting composite panorama

Pointing 3

IC 2163 and NGC 2207 · HST/WFPC2


Combining and registering images Using Layers
Images of different pixel scales may be registered using multiple layers. Inverting and rendering the foreground image layer partially transparent (Opacity < 10 0%) makes possible precise alignment using Edit | Free Transform. Sub-pixel alignment may be accomplished by resampling the images to finer pixel scale and zooming in to a large factor. If the pixel scales are known or measurable (using the Measure Tool), then one image may be resampled to the other's scale using Edit | Transform | Numeric.

Transform bounding box with "handles"

Crab Nebula VLT and HST/WFPC2 Outline

Layers Palette

Foreground image, partially transparent

Layer mask Background image


Photo Layout in Illustrator

Crop marks

Text block

Illustrator Artboard
Detail image Illustrations or photo layouts are combinations of images, text and other graphics that explain something. They may be produced in Photoshop or Illustrator. Illustrator produces resolution-independent results and permits more flexibility in setting type, graphics, etc. Images produced and edited in Photoshop exist at some particular size and resolution, but may be made large enough for satisfactory reproduction. Photoshope permits more extensive image manipulation and effects and permits precise features such as borders for a specific image size and resolution, such as web content. Drop shadow layer effect

Guide (margins)

Background

Layers Palette


Photo Layout in Photoshop
Editable type dialog
Pixel type layer Editable type layer

Drop shadow effects dialog

Editable type layer

Detail image Drop shadow layer effect Image border

Layers Palette

Background image and layer mask Pixel type layer

Photo Layout