User's Guide

As the ESA/ESO/NASA Photoshop Liberator is fairly self-explanatory this User's Guide will focus on explaining the various features, sliders and buttons in the Graphical User Interface (GUI) for the plug-in.

The main part of the GUI is taken by the preview window. It shows how the imported image will look with the chosen combination of Stretch function (8 + 9) and Black/White level (14). The coloured pixels are due to (6), see this below.
You may zoom up to 400% on the preview image, and down to 6.3% (depending on the size of the image). Default is a zoom factor that lets you view the entire image.
Some FITS files have several "Images" embedded or stacked on top of each other. And some FITS files also have several "Planes" in them. With this drop-down box you can select which Image or Plane to work on. If you want to import more than one Image or Plane you have to open the file multiple times. Default is Image 1, Plane 1.
The "Image statistics" box give you information about the Dimensions of the image (in pixels), the Minimum and Maximum pixel values, the Mean and Standard Deviations of the pixel values. If you move the cursor over the preview you will get information about the individual pixels (position and value). The Image Statistics box is updated when you choose another Stretch function (8).
If you want to perform several operations on an image and work with large images (or a slow computer) you can turn the preview off, and get all your choices calculated at once when you are done and click it back on. Default is Preview On.
You have a choice of getting three special types of pixel values marked individually with each their colour: Undefined values, White clipped values and Black clipped values.
- Undefined values are both so-called NULL values and 'real' undefined values. NULL values are values that are blank in the original FITS file. Undefined values are values that become undefined due to mathematical problems (e.g. division by zero) in the calculation of stretch function etc. Having these values marked is important as too many undefined pixels makes an image look less attractive.
- White clipped pixels are pixels that are completely white. You can adjust their number using the White level slider (14).
- Black clipped pixels are pixels that are completely black. You can adjust their number using the Black level slider (14).
You have a choice of scaling the graphical viewing of the histogram values (12) in Logarithmic was (default) or linear. Logarithmic usually gives the best overview as the number of pixels with certain intensities vary a lot.
The stretch function is the 'mapping' of your input image to the output image. The default is Linear and this maps the input image to the output image directly 1:1 without changing anything. If you apply for instance a logarithm the fainter areas of the input image gets enhanced.
The value a is a constant that offsets the image values. This can be important when you use a logarithmic stretch as Log for instance does not work on negative pixel values.
You can choose to import the image as an 8-bit image (256 greyscales) or a 16 bit image (65536 greyscales). Note: Photoshop 7.0 only has 15 bit support (32767 greyscales) and Photoshop Elements 2 only has 8 bit support.
If there are Null values in your image you can choose between having them imported as black or transparent. Note: Photoshop 7.0 doesn't have support for transparent in 15 bit.
The histogram shows the distribution of intensities. The higher the bars the more pixels there are of a given value.
The Apply button applies your typed-in values. Note: Do not use ENTER (Windows only) as this will make Photoshop import the image immediately .
With the Black and White level sliders you can adjust where you want to set the black and white levels of the imported image within the min/max range. The higher you set the black level the more pixels will appear black. The lower you set the white level the more pixels will appear white. Use the text fields to enter a value outside the min/max range of the image.