This is probably where the fun begins. Pre-processing is where you stack the individual exposures to improve the signal-to-noise ratio and also where you try and remove various artefacts in the images using flat fields and dark frames. At the end of pre-processing (also called reduction) you will have a single FITS image which you can then post-process in a programme such as Photoshop (see Step 6).
To start pre-processing in AstroArt, select Pre-Processing from the Tools menu (or press the third toolbar from the left). That will open up the Pre-Processing window's Files tab:
First step is to select the files you want to pre-process by navigating through the folder structure top left. Once in the correct folder, select which files you want then drag them into the appropriate box. Here I've put a Master flat in the Flats pane and 10 image files in the Images pane. For my camera the dark noise is very low so I don't typically use dark frames, but other CCDs will need to use dark frames.
Next step is to tweak the options you want AstroArt to use when it pre-processes the files. The important options for me are:
- How you want to combine the images, flats, etc. I usually use Average or Sigma
- Whether you want to Confirm every image: this allows you to inspect each frame before including it in the final result. Useful for getting rid of poor frames with trailed images or satellite tracks, or some other defect. Can be a royal pain when there are lots of images to combine!
- CCD Color Synthesis : for my one-shot colour CCD I need to synthesise the colour from the raw images. This will pop-up a window where you need to enter (once only) the parameters AstroArt needs to do the colour synthesis, and this very much depends on the specific camera. The help files are quite good.
- Alignment : for most applications I used Auto. Selecting this will pop-up a window where you can select the type of alignment you want - I generally use All Stars, which means AstroArt will use all the stars to perform the alignment.
When you've set the options, press OK and the pre-processing will begin.
As AstroArt performs the pre-processing, the Confirm Image window will appear (assuming you selected the Confirm Image option above). The image below shows the second one in a sequence of 10 (look at the title bar, showing 2/10). For each frame you press the "Tick" or "-" button to accept or reject the frame.
Once all the files have been pre-processed, the stacked image will be displayed. I immediately save the image as a FITS file, with a suitably descriptive name, e.g. NGC 2420 10x60s.fit. I don't perform any further processing in AstroArt, but I do tend to separate out the Luminance, Red, Green and Blue channels to separate files. Under the Color menu, select Luminance, Red, etc in turn. This will open a new image for each channel; save each of them to a separate file (I use the _L, _R, etc which AstroArt appends to the name I entered above).
That's it. Onto Step 6 for final processing.
