History of Long-Range Photography in Astronomy

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Slide 9

Planetary Imaging

Different rules apply for planetary work

Dealing with bright objects

Short exposure times – No cooling required

Planets can be imaged using very small CCD arrays

Small arrays have an advantage in that their readout time is more rapid than larger arrays

Rapid readout facilitates taking many exposures, so you can be selective about which ones you keep

Slide 10

CCD Array Sensor

Slide 11

Imaging Questions

Why do the stars look square in my images?

Why do my exposures times have to be so long to get any signal?

What is the size of my field of view?

These questions illustrate the three major issues of matching a telescope to a CCD array

Sampling

Pixel Sensitivity

Field of View

Slide 12

Sampling

A CCD image is made up of tiny square-shaped pixels.

Sampling refers to how many pixels are used to produce detail.

Each pixel has a brightness value that is assigned a shade of gray color depending on how many photons strike the pixel during an exposure.

Since pixels are square, the edges of features in an image will have a stair-step appearance.

The more pixels and shades of gray that are used, the smoother the edges will be.

Slide 13

Sampling

The following formula can be used to determine sky coverage per pixel with any given pixel size and focal length:

(Sampling in arcseconds) = 206.265 / (focal length in mm) * (pixel size in microns)

Or

(Sampling in arcseconds) = 8.12 / (focal length in inches) * (pixel size in microns)

Undersampled Oversampled

Slide 14

Sampling

The CCD user has some control over focal length and pixel size

Average seeing is between 3 to 5 arcseconds

A good rule of thumb to avoid undersampling is to divide your seeing in half and choose a pixel size that provides that amount of sky coverage

Slide 15

Pixel Sensitivity

The larger the pixel, the more sensitive the camera will be for any given focal length.

Under excellent seeing conditions, a camera with 24 micron pixels on a telescope of 2000 mm focal length will produce images that are very close to being undersampled.

For bright planets, oversampling will provide better resolution and it will help cut down the glare that can saturate the CCD pixels.

On the other hand, for faint deepsky objects like galaxies or nebula, moving toward undersampling will give better sensitivity, allowing shorter exposure times.