Announcements• After lecture, adjourn to the observatory to

start set-up for Dark Sky Observing night• Forecast for Thursday doesn’t look good

(60% chance of rain during day falling to 20% chance in the evening)

SOmeet here to start doing basic image processing. I have canned images we can use.

Digital Imaging

Basics: The Photoelectric Effect

Photoelectric Effect: The Theory

The MOS works on the photoelectric effect

By “catching” the liberated electrons in the MOS device you lose the frequency information. Thus, the MOS simply

measure the intensity (# of photons), not the color.

CCD Versus CMOS

The difference is in the on-pixel amplifier

Each pixel is an MOS photodiode

The liberated electrons are held in the potential well until they are read out

CMOS includes amplifiers on each pixel while CCD’s do not

As a result, not all the area on a CMOS pixel is available to collect light. The amplifiers also tend to be nonlinear so they aren’t useful for photometry

A CCD array has millions of individual devices

The smaller the pixel, the higher the resolution. But, the smaller the pixel, the fewer the number of electrons that can be stored before it fills up

Downloading the array is like running a series of conveyor belts

Downloading the array

If a pixel gets overfilled

(saturated), it “blooms” into pixels down

the line

Anti-blooming Architecture helps eliminate booming

The download a numerical array of how many photons hit each pixel

Number of electrons in each pixel is proportional to the number of photons that hit that pixel. The result is a spreadsheet that is easy to mathematically manipulate.

The numbers in the array can be translated to a grayscale

Once pixel is assigned a shade of gray, a black & white image is created from the spreadsheet

Color imaging requires filters

For pretty pictures we use red, green, blue and luminance. For photometry we use ubvri (Johnson set) or ugriz (SDSS set). Other systems extend into the UV or IR ends of the spectrum

It is possible to put filters directly on the CCD chip

All color digital cameras use microfilters. Notice there are more greens than reds or blues. Different chips have different wavelength sensitivity.

A color chip has a lower resolution since each “pixel” is actually four

Silicon chips are most sensitive in infrared and red and least sensitive in blue. An infrared filter will cover the entire chip.

The “white balance” adjusts the amplification of each of the colors.

A color image download has to account for the colors