StoneEdge Astro Image Processor Help

Dave McGinnis

February 17, 2013

Introduction

The StoneEdge Astro Image Processor is a program designed for simple image processing operations on images taken with the StoneEdge telescope. However, the program will work work FITS files taken with any telescope. The program is meant to be straight forward and simple for students to use.

A typical sequence of image processing is the following:

1. Remove hotspots from the images. The hotspots are bad pixels on the telescope CCD

2. Subtract a dark image from the images. The CCD pixels have an offset or a pedestal that needs to be subtracted. The pedestal is measured by taking a “dark” image which is an image with the same exposure time but with the camera shutter closed. A dark image should always be taken during observing.

3. Divide a flat image from the images. The gain of the CCD pixels along the camera is not uniform or “flat”. The flatness of the CCD can be measured by taking an image of the bright part of the sky near twilight. Then the actual images are made “flat” by dividing the images by the flat image.

4. Align one image to another image. Before stacking images or making an RGB (RedGreenBlue) Plot, the images need to be aligned to compensate for telescope drift.

5. Stack or add multiple images together. Because of telescope drift or because of limited observing time, it is best not take extremely long exposures. Equivalent long exposures can be made by adding together multiple short exposures.

6. Normalize the images. The bandwidth of the filters on the StoneEdge telescope are not the same. For example, the “r” filter (green) has a much wider bandwidth than the “g” (blue) filter . When making a RGB (RedGreenBlue) plot, it is best to normalize the images so as not make the image too “green” or too “red”.

Starting the program

The program is written in Java which allows the program to run on any type of computer operating system. Because of the image size, the program uses a lot of memory. The memory needed for the program is set when the “AstroImageProcessorLauncher.jar” is opened. This program then opens the “AstroImageProcessor.jar”. If the user opens “AstroImageProcessor.jar” directly not enough memory will be set aside and the program might not behave properly. It is important to only run the program through “AstroImageProcessorLauncher.jar”.

Opening a File

The program can only read FITS file format which is the format in which the StoneEdge telescope stores the images. The extension on the image file must be .fits (preferred), .fts, or .fit. A file is opened under the File menu or using the keyboard shortcut Ctrl-O. When the file is opened the display looks as shown in Figure 1.

Figure 1. Program Display

Transfer Function

The CCD camera is a linear device. That is, the amount of signal it records is proportional to the amount of light it sees. Our eyes are not linear. That is, our eyes are more sensitive in dim light than in bright light. This nonlinear behavior of our eyes can be mimicked by the Transfer Function. The transfer function is the relationship on how much we see given by how much light the camera records. The program has four possible transfer functions; linear, square root (sqrt), logarithmic (log), and inverse hyperbolic sine (asinh). These transfer functions are shown in Figure 2.

Figure 2. Transfer functions

Display Type

By default the program displays the image with a black background and white pixels. There are a variety of display options as shown in Figure 3.

Figure 3. Various Display options. The display in the upper left corner is the default white pixels on a black background. The upper right is black pixels on a white background. The bottom left is a color spectrogram were colors indicate intensity. Thelower right is the inverse color spectrogram.

Intensity Scale

For view faint images, the intensity of the display can be changed with the slider bars in the Scale Panel.

Figure 4. Changing the scale. The image on the left has the scale set to 100%. The image on right has the maxim scale set to 6%

Zoom Panel

The panel on the lower left of the program is a 10x blow up of the full size image. The center of the zoom can be changed by clicking the mouse on the full size image at the location where the center of the zoom is desired.

Removing Hot Spots

Hot spots are removed from the image under the “Clean” menu. The program looks for single pixels that are a factor 10x greater than it’s neighbor and replaces the hot spot with the neighboring values. Once the hotspots are removed, the user must save the file to save the changes. The “Save As Fits” menu is underneath the “File” menu. In the file chooser dialog that pops up when the user saves the file, the program provides a suggested addition to the file name so as to not overwrite the original image.

Add, Subtract, Divide

Images can be added, subtracted, or divided by other images. These operations are under the “Action” menu. The program asks the user to select the file that is to be added, subtracted or to divide with a file chooser dialog. Once action has been performed, the user must save the file to save the changes. The “Save As Fits” menu is underneath the “File” menu. In the file chooser dialog that pops up when the user saves the file, the program provides a suggested addition to the file name so as to not overwrite the original image.