This section explains some basic things you can do with images in MaxIm DL.
Selecting Images
You can quickly switch between images using the tabs running across the top of the main window.
Accessing Images During Processing Commands
For most image processing commands, you can still access the image while the command is open. You can pan and zoom it, or use tools such as Screen Stretch and Quick Stretch, shown on toolbars or in the context menu displayed when you right-click on the image. In many cases statistical analysis and image inspection tools such as the Information and Line Profile windows can be used while an image processing command is open.
Usually just the window you are working on is active; but for some commands that operate on multiple images, all open image windows are available.
Note that the small preview window on the dialog box can be adjusted for Screen Stretch, as well as the main image window.
High Bit Depth Images
Most point-and-shoot digital cameras produce 8-bit data. DSLR cameras can produce 12-bit or higher data in Raw mode. Scientific-grade CCD cameras typically produce 16-bit data. Moreover, if you stack 16-bit data, you can end up with more than 16 bits.
For this reason, MaxIm DL works with images in 32-bit floating-point format. This requires more memory than a standard ”paint” program, but it means you have almost unlimited data range available.
This has two major effects on how you work with the package:
When you view images, you have to ”Screen Stretch” the image – that is, adjust its brightness and contrast so you can see it
When you save images, you have to make sure the file format can accommodate the bit range
Screen Stretch
Let’s assume we have a 16-bit image. The brightest pixel in such an image has a value of at most 65535. For an astronomical image, we might have a few stars this bright, but much of the information - especially nebulosity - will be much fainter.
Unfortunately your computer screen can only display 8-bit images, or a data range of 0 through 255 (in reality it is not even this good; you cannot distinguish 256 different brightness levels on a computer monitor). How do we bridge this gap?
Simple - by scaling the data so the interesting parts are in the correct range. In MaxIm DL, we have the Screen Stretch window. You set values for the Minimum and Maximum levels. When an image is displayed, any pixel value below the minimum is set to 0 (black), and any pixel above the maximum is set to 255 (white). The values between minimum and maximum appear as shades of gray. This means adjusting the sliders or numbers in the screen stretch window will change how the image looks on the screen, but will not actually modify the image.
MaxIm DL will try to automatically figure out the best values for you. When you load a new image, it typically defaults to the Medium Auto Stretch mode. (Files saved in FITS format may contain a recommended initial Screen Stretch, while 8 bit images automatically display the entire range.) You can change to different auto-stretch modes using the Screen Stretch window. These automatic modes have to be a compromise; they will produce decent results for almost any image, but not necessarily the ”best”.
You can easily adjust any image manually. The fastest and easiest way is to use Quick Stretch. Simply point the mouse at the image. Hold down the Shift key and then press and hold the left mouse button and drag it on the image. Up/Down will adjust the brightness of the image up/down. Right/Left will increase/decrease the contrast of the image. The image adjusts continuously as you move the mouse.
When using Quick Stretch, it is best to use short motions, lifting the mouse button in-between. This gives you the best control. Practice a bit on the sample images to get the hang of it - you will be using it a lot. If you get lost, just flip it back to Medium in the Screen Stretch window.
Saving High Bit Depth Images
Suppose you take a 16-bit image with your camera, and want to save it as an 8-bit JPEG that you can post on your web site. How can you make sure the image appears correctly?
Often the data in a 16-bit image will never go as low as 255; if you simply save the image in JPEG, it will come out as all white!
To fix this, the File menu Save As command has an option called Auto Stretch. What this does is apply the Screen Stretch setting to the file before it saves it to the disk. If you then load up the picture in your web browser, it will now look exactly the same as it did in MaxIm DL.
You can also do these adjustments manually, if you so desire, using the Process menu Stretch command. That said, it is usually easiest just to set the Screen Stretch and save using Auto Stretch.