This article will tell you how to use Adobe Photoshop to make high quality color images with your astronomical data.
When you make astronomical obsevations you will end up with FITS files. One of these grey scale FITS files is produced for each filter used in your observation (a color image is a composite of observations taken with a red, a green and a blue filter). With photoshop you can color the image and combine it with the images from other filters to produce a full color image.
You will need a red filter observation, a green and a blue to follow this article. You can download some from our archive using our search tool.
You will also need the FITS liberator plugin for Adobe Photoshop. You can download it from the joint NASA/ESA website, which has full installation instructions.
Starting up
Launch Photoshop, and open your three FITS files by going to File > Open (you can open more than one at a time but holding control while you select the files).
Now you will see the FITS Liberator box appear. You need to set the white and black level sliders in the histogram graph to give a good amount of contrast without too much black or too much saturation.
Next click Auto scaling
Experiment with the different Stretch functions to enhance fainter parts of the image. Different stretch functions produce different results with each image so try everything.
When you are happy with the way the image looks, tick the 8-bit button which is below the Auto scale button.
Then click OK.
The FITS liberator will then import your image into Photoshop. The next image will then load into the FITS Liberator and you repeat the process.
Adding Colour
The next stage is to begin to add some colour information into these images. To do this select an image and go to: Image>Mode>RGB Color
Do the same for each image.
When the three images are open, select the red image.
Then go to Image > Adjustments > Hue/Saturation.
This will open a window with a three sections. First, tick the Colorize button in the lower-right corner. Now enter the following values. Hue: 0 Saturation: 100 Lightness: -50.
Then select your visual (green) image. Image > Adjustments > Hue/Saturation, and enter Hue: 120 Saturation: 100 Lightness: -50
Finally, select your blue image, Image > Adjustments > Hue/Saturation and enter
Hue: 240 Saturation: 100 Lightness-50.
You should now have three coloured images.
Combining the Images
Now that the images are coloured, they need to be combined into a single image. To do this, you copy and paste the visual and blue images on top of the red image.
So, select your visual image, and go to Select > All then Edit > Copy.
Select your red image, and select Edit > Paste.
You should see a Layers window to the right of your screen, if you do not, go to Window > Layers. This window shows that a new layer has been added to the red image, it will be called Layer 1, you can double-click on this name and changed it to visual.
Now, select your blue image, Select > All, Edit > Copy. Then go back to the red image (with the visual image copied on top of it) and select Edit > Paste.
Blending Mode
The next step is to blend the images so the colours mix, and not just lie on top of each other.
Above the list of layers in the Layers window, there is a drop down menu that says Normal, this controls the mixing mode of the layers.
With your blue image selected (highlighted blue in the layers window), change the mode from Normal to Screen.
The blue layer will then blend with the visual layer, and your image will turn a cyan colour. Select the visual layer (highlighted blue n the layers window) and change the mode to Screen also.
This will set all the layers to blend with each other, and you should be left with a complete RGB image.
Aligning Layers
There is one more step that needs to be done before your basic colour image is complete, and that is to align the red, visual and blue layers together.
Select the Zoom Tool from the tool bar, and zoom into a star.
As you can see, the layers are not quite aligned, and this causes colour halos to form around stars.
Select the Move Tool.
The idea is to align the visual and blue layers with the red. Make the blue layer invisible, by clicking on the eye icon next to it in the Layers window.
Then with the visual layer highlighted, click and drag with your mouse on the main image, to move the visual layer.
Align it as closely as you can to the red layer, using the star as a reference. For finer movements, you can use your keyboard arrows.
When you are happy with the visual layer, make it invisible and the blue layer visible again. Highlight the blue layer, and click and drag the image to move it in the same way.
When you are happy with the blue layer, make all the layers visible again.
Cropping, Flattening and Saving
Zoom out so you can see the whole image again by going to View > Fit on Screen. Moving the layers may have created some rough edges, you can remove these by cropping them out.
Select the Crop Tool, and draw a square around the regions of the image you wish to keep.
Then double click on the image.
The final step is to flatten the image, this merges the three separate red, visual and blue layers into a single RGB image.
Go to Layer > Flatten Image to do this.
N.B. Make sure you are happy with the layer alignment as you will no longer be able to move the individual layers after this.
Now you can save the image - go to File > Save as.
Enter a new name for the file, e.g. m57_rgb and click Save.
Written by Daniel Duggan, edited by Edward Gomez
