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