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HubbleSOURCE

Informal Science Education Resources
from the home of NASA’s Hubble Space Telescope

Articles

Create your own Color Hubble Images
Zolt Levay, Office of Public Outreach

Color Hubble ImageUntil recently, making presentable images from Hubble data has been fairly cumbersome, requiring some experience with specialized software such as IRAF or IDL, used to analyze astronomy data. Now the process has been significantly streamlined with the introduction of the Photoshop FITS Liberator. If you have a basic understanding of astronomical data, digital photography and a working knowledge of Adobe Photoshop you can transform astronomical data in the FITS format into pretty pictures, in black and white or in color. The resulting images may be used as any digital photo: reproduced in print, displayed on a web page, emailed, etc.

FITS, the Flexible Image Transport System is a file format used for many years in astronomy to store and move images from telescope-based instruments. It has been designed to provide capabilities needed by astronomers but is not a generally known format such as JPEG or TIFF.

Adobe Photoshop is the industry-standard “digital darkroom” software for working with photos. It includes many tools to work with images to improve tonal range, color, contrast, in addition to combining multiple exposures, adding annotation, and a whole suite of more artistic capabilities. But Photoshop has not recognized FITS data, until now.

The Software

The primary tool in the process is a software plugin to Adobe Photoshop called the FITS Liberator. This free software was developed by the European Space Agency (ESA), European Southern Observatory (ESO) and the National Aeronautics and Space Administration (NASA) and is available for free download from ESA along with instructions for the simple installation, sample data, and much more information. See this page in our Content Creator’s Toolbox for more information.

The plugin is available for PC/Windows and Mac systems and works identically on both. It provides full functionality with Photoshop CS (also known as version 8.0) but it also works with Photoshop version 7.0 as well as Photoshop Elements versions 3.0 and 2.0, with more limited functionality.

After installing the plugin, Photoshop recognizes FITS files as a known, native format. To begin, start Photoshop and open a FITS image using the File > Open (keyboard shortcut: CTRL/CMD+O). You will see the FITS Liberator dialog:

Photoshop FITS Liberator Dialog

The main components of the dialog are:

  • The preview window at uper left shows a representation of the image as it will appear once opened in Photoshop. The image initially is sized to fit entirely within the window but the "+" and "-" buttons below may be used to zoom in and out.  In addition, clicking and dragging within the window will pan a zoomed image.

Image preview window

  • A “histogram” below plots the number of pixels of each data value in the image.

Histogram window

  • Sliders and numeric input to specify the range of data values to import.

Data value input

  • A menu of the available image planes/extensions, if applicable to the FITS data.
  • Image statistics include image size, minimum and maximum data value, etc. Moving the pointer within the display window shows the location and value of the pixel at the pointer's location.

Image statistics display

  • The “stretch function” specifies how the image data values should be transformed into pixel values in the Photoshop document. Selections are: linear, log, square root, sqrt(log) and log(log).

Stretch function dialog
Stretch function selection

  • Various switches control other functionality: whether to use 8-bit or 16-bit channels in the Photoshop document, whether to translate FITS null values to black or transparent pixels, whether to show the image preview, how to preview out-of-range pixels, and how to display the histogram.

The Data: Obtaining FITS Images

The FITS Liberator will accept valid FITS data from any source. Several sample datasets from various sources are available from the European Space Agency.

Data from Hubble Space Telescope and several other missions are available from the Multimission Archive at Space Telescope (MAST) . Because of the size and complexity of the archive and the data, only a very brief introduction will be given here. Various tools are available for searching and retrieving data from the extensive collections, from a very comprehensive cross-mission search, to more directed search by specific mission, coordinate, instrument, etc., or more sophisticated searches by pointings (HST only), scrapbook , etc. For a limited set of observations, initial processing has been performed to produce High-Level Science Products more suitable for display and analysis than individual datasets.

Once the desired observations are identified, datasets may be retrieved from the archive, made available via anonymous ftp (alternately, you may register as an archive user). Complete instructions and tutorials are online and help is available via email.

Be aware that HST image datasets are quite large, and observation sets generally include numerous exposures. Be prepared with plenty of disk space and RAM. A single HST ACS/WFC exposure from the archive is a 170MB FITS file. A single WFPC2 exposure is 10MB. Many HST datasets available from MAST can be used as-is. In some cases, "on-the-fly" processing (OTF) combines associated datasets into a single image. Others require more extensive processing after retrieval to produce useable data.

The Process: Making a Black & White Image

The key element in producing presentable images from data is how to scale the image from the pixel values in the input file into values in the image that are displayed as a range of gray brightness levels. This is accomplished by selecting the range of input values and specifying the function to transform the input data into displayed pixel values. Astronomical images tend to include a very large range of brightness levels, from the very dark sky background to much brighter stars, galaxy nuclei, etc. Often small numbers of pixels have values well outside the usual range. "Dead" detector elements or other causes can result in values of zero or much less than the normal sky background. Cosmic rays can produce much higher counts than moderately bright stars. Using a data range including the minimum and maximum data values often does not produce a very exciting image. Although celestial objects appear very dim in the sky to our eyes, long exposures with sensitive detectors on large telescopes yield a large range of numerical values in the resulting images (technically referred to as large dynamic range, made possible by image formats with high bit-depth).

Select the function from the “stretch function” menu.

Stretch Function dialog
Stretch Function selected

In many cases, log scaling provides the best results for astronomical images. This conveniently serves to enhance contrast and bring out details in the darker areas of the image, where the interesting stuff often goes on in astronomical images. In addition, it suppresses the much brighter areas such as the centers of stars, where the detail is not so critical. Sometimes square root (sqrt) stretch works better for certain types of images. Try different functions to see which result appears more pleasing in the preview.

FITS Liberator will select an initial data range based on the distribution of data values, but will most likely not produce the best image. Adjust the data range using either the sliders under the histogram plot or the numeric input values labeled “Black level” and “White level”. You will see the result in the preview display. As you increase the black level the image will darken overall and whole areas will become black (clipped). If the “Black clipping” check box is selected those pixels whose value is smaller than the black level will show as blue in the preview but will become black in the Photoshop document. Conversely, as you decrease the white level, the image will brighten overall and more pixels will become white (saturated). If the “White clipping” check box is selected the pixels whose value is greater than the white level will show as green in the preview and will become white in the Photoshop document. You can experiment with the data range to achieve a pleasing result.

Data Value dialog

By paying attention to the out-of-range indicators in the image preview, it is possible to scale the image such that the background (shadows, in photo terminology) is dark, but not completely clipped and the brightest areas (highlights) are not saturated or rendered totally white.

Select whether to scale the data to 8-bit or 16-bit pixels in Photoshop.

Channel bit-depth dialog

The advantage of using 16-bits/pixel channels is that there are many more gray levels available. The disadvantage is that it makes the resulting image files twice as large in disk storage. Having more gray levels available means that you can adjust the image a great deal.

When you are satisfied with the result, click “OK” to accept the image as adjusted and read the image into a black and white (gray) Photoshop document. Note that there is no provision for automatically recording the settings, so you may wish to write down the scaling function and black and white levels. The result will be a gray image in a Photoshop document window:

Grayscale Photoshop image

Photoshop Adjustments

Once the image is in Photoshop, a vast suite of tools is available to adjust and manipulate the image. Perhaps the most useful of these are the “levels” and “curves” adjustments. Levels (Image > Adjustments > Levels or CTRL/CMD-L) allows you to adjust the brightness and contrast of the image overall by specifying the minimum data value to assign black and the maximum data value to assign white (similar to the black and white levels in the FITS Liberator).  In addition, it permits an intermediate adjustment (sometimes referred to as “gamma”) that changes the functional transformation between the pixel values and displayed pixel brightness (something like the selection of the stretch function in the Liberator but with continuous variation rather than fixed functions).

Photoshop Levels dialog

Curves (Image > Adjustments > Curves or CTRL/CMD-M) permits much more flexible changes to the brightness and contrast of the image with a graphical representation of an arbitrary transform function.

Photoshop Curves dialog

Note that it is usually preferable to apply adjustments as an "adjustment layer" rather than modifying the image layer directly. Create an adjustment layer using Layer > New Adjustment Layer or the New Adjustment icon on the Layers Palette.

A further refinement is to apply adjustments only to areas that need it. For example, you can brighten fainter regions without blocking up (saturating) brighter areas by applying a mask to the adjustment, if it is applied using an adjustment layer. Paint in the layer mask with black to prevent the adjustment from applying.

The goal of such adjustments is an image with a more balanced range of tones, from black to white, making visible those features in the data that are of most interest. For example, a well-crafted curves adjustment can enhance relatively faint outer arms of a galaxy without saturating brighter areas near the nucleus.

Save your work. If you save as a Photoshop document all features will be available for recovering and modifying again, including full 16-bit image layers, adjustment layers, masks, etc.

The Next Step: Making a Color Composite

A more sophisticated exercise is to combine multiple exposures into a color composite. Registered exposures of the same object made using different color filters may be used to reconstruct a color image. In general the colors will not be “real” as in what we would see visually.  But the colors do represent actual physical properties of the subject. The basic idea is to combine the images using Photoshop's "layers" which permits multiple images to be combined in a single document. There are various ways to combine the images; the most flexible is to put the separate images in separate layers and colorize each. This method permits adjusting the hue more or less arbitrarily, combining two or more images, and applying numerous adjustments to each gray image separately.

In this case we will "colorize" the image by applying a different hue to each layer, making use of the "additive" color model, in which varying intensities of the primary hues red, green and blue add together to produce the range of all possible visible colors. This is accomplished by "projecting" each layer onto the virtual screen of a displayed image using Photoshop's "Screen layer blend" mode.

To begin, open each image as a gray Photoshop document using the FITS Liberator. Copy each to a separate layer by dragging and dropping each in one of the documents. Note that if you hold down the Shift key while dropping, the dropped image will be centered in the existing document. It's best if the images are registered (the features in all the images line up pixel for pixel) beforehand, which may be done using external software. However it is possible to register them manually in Photoshop, but that's a little beyond this exercise. The Photoshop "Layers Palette" will graphically show the order of layers in the document.

Photoshop Layers Palette

(Note that you can assign a color and name to each layer using the "Layer properties". In this case, the name (the text to the right of each thumbnail) was used to identify the source of each image. The color has nothing to do with the color applied to the image pixels but serves only to further label and identify the layer.)

Convert the grayscale document to RGB color: Image > Mode > RGB Color , select Don't Merge when asked. Set the "layer blend mode" of each layer to "Screen" using the drop-down menu in the Layers palette or Layer > Layer Style > Blending Options...:

Photoshop Blend Mode menu

To colorize each layer use "Hue/Saturation" adjustment, preferably using an adjustment layer (Layer > New Adjustment Layer > Hue/Saturation...).

Photoshop Hue/Saturation dialog

Select the "Hue" of the layer to convert the gray image to a color layer. The numerical hue values correspond to angles around a color wheel: 0 is red, 60 is yellow, 120 is green, 108 is cyan, 240 is blue, 300 is magenta, and intermediate values result in intermediate colors. Make sure the "Colorize" box is checked, set "Saturation" to 100 and "Lightness" to -50. This will ensure that the colors for that layer will range from black (for 0 pixel value) to the brightest possible value of the given hue (for the maximum pixel value, 255 for 8-bit channels). Finally, to apply the Hue/Saturation adjustement only to the appropriate image layer, "Group" the Hue/Saturation colorize layer with the corresponding pixel layer by highlighting the adjustment layer and using the menu Layer > Create Clipping Mask (or CTRL/CMD-G on the keyboard).

First try color imagePhotoshop layers that created the first try image

Note that applying the "additive primaries" red, green and blue to three layers provides the broadest range of colors in the final image.You may certainly use any combination of these, but the range of possibilities is seemingly endless. The values suggested here result in the most predictable results and the widest range of reproduced colors. In the example above we have used the colorize values as described above and have inserted a descriptive Layer Name to make note of the intent of each.

In general the resulting color composite is not very sastisfying, appearing dull, or having a particular color cast. Again, you can apply any of the numerous Photoshop adjustmets to improve brightness, contrast, color, etc., just as with any digital photo. Adjustments may be applied to each gray image layer by inserting a Levels, Curves, etc. adjustment layer between the image layer and its corresponding Hue/Saturation colorize adjustment layer. In addition, you may adjust the composite as a whole by inserting adjustment layers above all of the image and adjustment layers.

Final color compositePhotoshop layers used to create final color composite

Again, save your work, and don't forget to save often; you never know when a system crash or other disaster may strike.

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