Orion G3 instruction manual Image Processing, Vignetting

Figure 16. The Combine Images window allows StarShoot G3 Monochrome users to calibrate, align, all in a streamlined task.

Figure 17. The Calibration window lets you select your darks and flats (if applicable). Bias frames are not typically needed if you have dark frames.

imager, you may choose to skip this step for the time being.

Vignetting

Vignetting (Figure 15.1) in a telescope reveals edge-darkening in the astroimage. The sensitive CCD chip can easily detect vignetting through a telescope, even specialized astrographs. Vignetting is more apparent when the telescope’s illuminated field is not large enough to illuminate the full area of the CCD chip. As a result, more light is detected in the center of the image compared to the edge. In general, vignetting should not be a problem on the ½" format CCD inside the G3.

Dust and Particles

Dust and particles (Figure 15.2) will inevitably show up in your raw astroimages. Large particles on the CCD optical window sometimes look like unfocused circles or doughnuts in your images. It’s too late to clean your camera if you are already imaging in the field at night. And even when the camera is clean, dust usually finds a way to show up in your images.

To take a flat field image:

Figure 15.1. Larger format CCD cameras like the Parsec reveal vignetting through most telescopes. Vignetting occurs when the edge of the image plane has less illumination than the center.

Figure 15.2. Dust or other particles on the camera’s optical window can show up as distracting dark shapes in your images.

the Maximum to read somewhere around 10,000-15,000. Adjust the exposure time as needed until the Maximum is close to this range.

It’s a good idea to take several flat frames and try different exposure times until you find the correct exposure. If you are taking your flats near dusk or dawn, the sky brightness will change rapidly.

5. Image

Processing

After you have captured your astro- images (with or without dark frames), you will need to:

1.Calibrate Raws

2.Convert to Color (only applicable to StarShoot G3 Color)

3.Align

4. Combine

If you are using the StarShoot G3 Monochrome, all of these steps can be performed in the Combine Images window (Figure 16). For StarShoot

G3 Color users, the calibration and color conversion should be done separately before proceeding to align and stack.

1.Ensure that the telescope is focused and ready for astro-imaging.

2.Point the telescope at a uniform and featureless light source, like the sky at dusk or dawn, or a blank white sheet of paper. Make sure the camera orientation is exactly the same as it is or was for astro-imaging (Although the telescope is pointing at a featureless surface, the focus and orientation must be set as it normally would be for astro-images.)

3.Set the Exposure Type to Flat in the Capture tab. Or if you are taking a sequence of images, set the Type in the Sequence window to Flat, this will also assign a “F” suffix to your saved file name so you can easily identify your flats later.

4.Set the Exposure value to 0.1 seconds for now and click Start or Run Sequence. Looking at the Global section of the Analysis window, you want

Calibrate Raws

1.Open your saved astro-images. They should be saved as Raw to enable the following steps. Do not convert your raws to color yet.

2.Go to the Process menu and select Calibrate. The Calibration window will appear (Figure 17).

3.Click Add Files and select your saved dark frames and flat fields if applicable.

4.Click Calibrate all files, and you will notice most of the hot pixels and noise should disappear from your images.

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