CCDs

CCDs—the good (+)

Linear response photometry is “simple”

+ High efficiency, compared to other detectors

+ Sensitive to many wavelengths

+ 2-D arrays possible in large formats

+ Can be shuttered, or “frame transfer”

+ High dynamic range (i.e., contrast)

CCDs—the bad (-)

- Read noise: electrons not transferred perfectly (but pretty good)

- Dark current– Temperature sensitive– Coolant/cooler?– Accumulates condensates (i.e., gook)

CCDs—the ugly

Electron wells are finite and imperfectLeakage

Saturation blooms

Cosmic rays

Pixel-to-pixel variationAge-dependent

Linearity (good)• Expose to more light, get more

electrons—linearly increasing

• Photometry made easier because signal can be expressed as (data numbers per second), unambiguously

• Makes comparison of different images easier

Linearity—one more thing

• Allows straightforward normalization and addition of images

Quantum efficiency• Generally, higher than most other

detection schemes—that’s good (especially photographic film)

• Wavelength dependent

Exposure metering

•Can be shuttered, or

•Can be “frame transfer”

Dark current• Thermal motions of electrons

produces a spurious signal that is not due to incident light

• Temperature-dependent, so most cameras are cooled

• The level of spurious signal is still linear w.r.t. temperature and exposure duration, so can be subtracted from the “real” images

• Examples…

Temperature dependence of dark current

Q: Why’d we do this?

A: The camera is cold, so any residue floating around in the telescope will condense on the CCD. Yuck! Therefore, periodic “bakeouts” to remove gook from the CCD.

Pixel-to-pixel variation

• Differences in charge transfer efficiency

• Differences in well depth (less important)

• Shorted pixels continuously leaking charge

• Age-dependent (see example)

• Compensate via flat fielding and subtraction of dark frames

CCD aging

Pixel saturation• Potential wells have a finite depth, can hold

only a finite number of electrons

• 100,000 to 200,000 electrons is typical limit

• When the well is full, where do those electrons go?

• They spill over into neighboring pixels

• Example…

Composite Example1. Raw data

Composite Example

2. Subtract dark frame

3. Correct for stray light (no true flat fields for X-rays)

4. Co-register the cleaned images, normalize for exposure time

5. Replace saturated parts of “long” exposure with pixels from “short” exposure, to yield the final product…

Why composite?• Trivial answer: It looks nice.

• Less trivial answer: Enhanced dynamic range. You get to see the faint parts and the brighter parts, with quantitative accuracy.

•Troublesome

•Aesthetically unpleasant

•Confuse morphology of imaged object

•How to remove?

One more thing: cosmic rays