LAB #4

The Importance of Average Stacking in Planetary Imaging

M. Nicolini

We want to show how much important is the technique of the average stacking in the planetary images and how is applied with Astroart. For this LAB it will be useful to use a complete set of images (raws, dark frames and flat fields).

1. Introduction

Who possesses a ccd camera will surely have already heard to speak of " noise " and of its consequences: it is an enough complex problem that deserves a description more detailed in the next episodes of Astroart in LAB.
For the time being, let's satisfy with to see the effects of the noise. In the figure below you can compare the same image processing steps shown in the
LAB #3 Planetary Imaging With The "Linked Unsharp Masking" Processing (L.U.M.) applied to a single image, 4 averaged images, 9 averaged images and finally the average of the whole set of 20 images: averaging reduces noise by the square root of the number of frames you average. If you average 4 frames, the amplitude of the noise with respect with the single image is halved; averaging 9 frames you cut noise by a factor of three; sixteen frames and you gain a factor of four and so on. In this case with a set of 20 images we can reduce the noise by a factor of about 4.5.

Fig. 1 - Comparition table of the effects of average stacking

Now we will see how much simple is to apply all the preprocessing steps needed to calibrate and average a set of images using the powerful feature of the Preprocessing Tool in the full version of Astroart.

2. Image Processing Steps
  1. Run Astroart and load one frame of your set of raw images: drag a large square on the object with the left button of the mouse. This will tell to Astroart to search in that area for the objects to be aligned.

    Fig. 2 - Selecting an area around the object.

  2. Open the preprocessing window with Tools | PreProcessing. This window is composed by three sheets: Files, Options and Operations. Click on the Files Sheet: this is a files browser that will help you to select all the files needed for the preprocessing: raw images, the dark frames for the raw images; flat fields and the dark frames for the flat fields. Use the Shift or Ctrl Button to select more than one file at once and drag&drop them to the corresponding box as shown in the figure below.

    Fig. 3 - Selecting files for preprocessing.

  3. Now click on the Options Sheet in the PreProcessing window (Fig. 4): this sheet contains a lot of options: for example, for each box of frames you can select the final result of the preprocessing, i.e. a Median or an Average stack for Flats, Darks or Biases involved in the process, except for the final image where you can select also the sum of all the frames (Add radio button) or to keep each single preprocessed image (Keep radio button). In my case I want to do an average stacking of the 20 images of Mars then I selected the Average radio button.

  4. These 20 images, before the stacking, must be exactly aligned with each other: click on the Auto alignment check box and a new Align Window will prompt on the screen. Note that the coordinates of the rectangle selected in Step 1 will appear in the four text boxes and you do not have to select or correct them anymore. Just select the Correlation radio button because we have to align a set of planetary images and not of stars fields.

    Fig. 4 - Selecting of the automatic aligning method

  5. Click on the OK button of the Align Window and on the OK button of the PreProcessing Window and the process of the files will start; if the Confirm each image check box was selected, Astroart will ask you a confirmation for each step of the preprocessing: in a few seconds the processed file with the average stack of 20 images wil be created and called NoName00.FIT. Finally, you can process it following the steps shown in the LAB #3.

NOTE

You can easily and rapidly create a comparition table like that shown in Fig. 1: infact Astroart keeps in memory the last preprocessing set-up (Fig. 3) so you can modify it deleting a subset of images from the Images Files Box clicking on it with the right mouse botton.

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    2000 M. Nicolini - All rights reserved