History of Long-Range Photography in Astronomy

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Bias Frame Demo Read Out Noise Demo

Slide 36

Dark Frames

Even when a CCD array is placed in total darkness, individual pixels build up a charge over time. Most of this noise is generated by heat, and is called thermal noise or dark current. Lowering the temperature of the CCD array can reduce this thermal noise. The colder you can make the array, the lower the thermal noise.

Slide 37

Dark Frames

Slide 38

Cosmic Rays

Slide 39

Flat Fields

Taking an image of an evenly illuminated surface will create a flat field frame. Even though the CCD detector has been exposed to an evenly illuminated surface, the charge built up on each pixel can vary; flat fields are used to correct for these variations. There are other circumstances, which can help create these variations such as internal reflections, central obstructions and dust on optical surfaces.

NOTE: CCD detectors and their electronics have natural variations in the full-well depth and gain settings so the average value of the brightest area in a flat field should fall between 33% and 50% of the CCD’s saturation point.

Slide 40

How do you find the saturation point?

Full Well Capacity / Gain = The Saturation Point

For the ST-7E ABG

50,000 / 2.3 = 21,740

21,740 * .33 = 7,250 21,740 * .50 = 11,000

Slide 41

Flat Fields

Slide 42

Flat Field Demo

Slide 43

Reduction of Dark and Flat Fields

Slide 44

Dark Frame Subtraction Demo

Slide 45

Spatial Filtering

Spatial filtering is widely used to extract detail and control noise. This filtering modifies a pixel according to the appearance of its neighbors.

Low-Pass Filtering

Sometimes called blurring or smoothing, low-pass filtering averages out rapid changes of intensity from one pixel to the next.

Why would you what to blur or smooth an image?

To suppress noise.

Slide 46

Convolution Kernel