This article will tell you how to use Adobe Photoshop to make high quality color images with your astronomical data.
When you make astronomical observations you end up with FITS files. A grey scale FITS files is produced for each filter used in your observation. Color images are a composite of observations taken with a red, a green(visual), and a blue filter. With Photoshop you can add the colors back and combine it with the images from other filters to produce a full color image.
When downloading pictures from the LCO website the green file will be called Green FITS. If you are making observations the green file will be called Visual and when selecting a filter it will be v.
You will need separate red, green(visual) and blue filter observations to follow this article. You can use observations from the LCO data archive. A selection of objects can be found in the table below, simply click on the object name to access the observations and download the three FITS files. Name them by their filter color (this will becomes helpful later on).
You will also need FITS Liberator which changes the file into something Photoshop can read. You can download the application from the NASA/ESA website, which has installation instructions.
Launch FITS Liberator, and open your three FITS files by going to File > Open. To open more than one at a time press Shift while you select the files.
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.
Experiment with the different Stretch functions to enhance fainter parts of the image. Different stretch functions produce different results with each image so see what works. (In general, x^1.5, x^2, x^3, x^4, x^5, and exp(x) don’t do anything.)
When you are happy with the way the image looks, tick the 8-bit button which is below the Autoscale button (If you forget you can always change it later in Photoshop by clicking Mode then pressing 8-bit).
Click Save As and then name your file and save it to somewhere you will remember.
Open the next image into FITS Liberator and repeat until all three images are edited. Import the three Images into Photoshop by opening photoshop and clicking import or by dragging and dropping the three images onto the photoshop icon.
The next stage is to begin to add some color information into these images. To do this launch Photoshop, open one of the color images and go to: Image>Mode>RGB Color.
Do the same for each image.
When all three images are open, select the red image. Then go to Image > Adjustments > Hue/Saturation.
This will open a window with three sections. First, tick the Colorize button in the lower-right corner. Now enter the following starting values. For the red filter image enter Hue: 0 Saturation: 100 Lightness: -50. Leaving Hue alone, adjust Saturation and Lightness until you have a high contrast, crisp image. (These settings can be changed later.)
Then select your visual (green) image. Go to Image > Adjustments > Hue/Saturation, and enter Hue: 120 Saturation: 100 Lightness: -50 and adjust as necessary.
Finally, select your blue image, Image > Adjustments > Hue/Saturation and enter Hue: 240 Saturation: 100 Lightness-50 and adjust as necessary.
You should now have three coloured images.
Now that the images are colored, they need to be combined into a single image. To do this, you copy and paste the Visual (green) and blue images on top of the red image.
So, select your Visual (green) image, and go to Select > All then Edit > Copy. You can also create a duplicate the layer by right clicking on the layer then clicking duplicate then drag and drop the layer onto the red image, or layer from background by right clicking then clicking layer from background then drag and drop.
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. Change the layer name to red if you wish.
Now, follow the same process with your blue image. Select > All, Edit > Copy. Then go back to the red image (with the visual(green) image copied on top of it) and select Edit > Paste.
You now have one image that combines the red, green(visual) and blue images.
The next step is to blend the images so the colors 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 option, Normal. This controls the mixing mode of the layers.
Highlight the blue image in the Layers window to select it. Then change the mode from Normal to Screen. You may also want to try other presets here, especially Lighten, or Linear Dodge.
The blue layer will then blend with the visual(green) layer, and your image will turn a cyan color. Select the visual (green) layer and change the mode to Screen or Lighten or Linear Dodge, etc.
This blends all the layers, and you have a complete RGB image.
Before your basic color image is complete, remove haloing of the stars by aligning the layers.
With your combined RGB image open in Photoshop, select the Zoom Tool from the toolbar, and zoom into a star.
As you can see, the layers are not quite aligned, and this causes color halos to form around stars.
Select the Move Tool.
The idea is to align the visual (green) and blue layers with the red. To do this, make the blue layer invisible, by clicking on the eye icon next to it in the Layers window.
Then with the visual(green) 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 (green) layer, make it invisible and make the blue layer visible again. Highlight the blue layer, and click and drag the image to move it over the red image in the same way.
When you are happy with the blue layer, make all the layers visible again.
Zoom out so you can see the whole image again by going to View > Fit on Screen. Moving the layers may have created roughness on the edges of the image. 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 red, visual (green) and blue layers into a single RGB image.
Go to Layer > Flatten Image (or right click then flatten)
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. NGC6946_rgb and click Save.