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ASTR 3130, Majewski [SPRING 2015]. Lecture Notes

ASTR 3130 (Majewski) Lecture Notes


CCD QUANTUM EFFICIENCY ISSUES

Despite the huge advantage that CCDs have over photographic plate detectors, astronomers continue to drive the creation of devices that are even closer to the ideal of a detector with 100% QE.

  • The demands on large telescopes makes this an important research investment:

    Every fractional increase in QE means an equivalent reduction in either light gathering power or integration time (at same light gathering power) needed to do same S/N work.

  • QUESTION: Why do you think astronomers are driving this kind of work and not the television/broadcast industry?

Understanding QE in detail:

  • Peak Q.E.'s for CCDs are from 40-80% (compared to a few percent for photographic plates).
  • The Q.E.'s of CCDs vary with wavelength.
  • An important source of this variation is the ability of photons of different wavelengths to penetrate the silicon.
    • Imagine an incoming photon stream (flux) at the front surface of a Si CCD chip as
      F(φ) in photons / sec. The flux at some depth z in the silicon is given by:

      where is the Coefficient of Intrinsic Absorption (which has units of inverse microns, or 1/μ), which describes how easily the photon is absorbed and is a function of temperature, T, and wavelength, λ.
    •   λ (1/μ)
          T=300 K T=77 K
      blue 4000 Å 5.0 4.0
      red 6000 Å 0.5 0.25
      infrared 8000 Å 0.1 0.005
      infrared 10000 Å 0.01 0.002

  • Note:

    • Photons are pretty well "stopped" by about four "scaleheights" (where an exponential scaleheight is given by 1/, also called an "optical depth").

      This means that in a silicon detector at liquid nitrogen temperature blue photons are pretty much totally absorbed by ~ 4/ ~ 4/(4μ-1) ~ 1μ.
    • On the other hand, a significant number of photons make it past only one scaleheight.

      This means that infrared photons can travel a significant distance, to > 1/, or > 200 μ
    • Note, λ > 11000 Å electrons do not have enough energy to make electron - hole pairs in silicon and pass completely through the device.
  • Thus, based on the above wavelength dependence considerations of the intrinsic absorption, , of silicon, we see how a CCD's blue and red limits of sensitivity are actually defined, apart from the bandgap energy:
    • Sensitivity in the blue is limited by weak penetration of photons (on order of only microns). Many photons absorbed before reaching depletion zone. Need to be worried about thinness of SiO2 and other layers to cross. To increase the efficiency in blue, decrease thickness of Si to be crossed.
      • THIN CCDs, "backside illuminated" desired.
    • Sensitivity in red requires substrate thick enough to have enough opportunity to absorb weakly interacting red photons.
      • THICK CCDs best, typically "frontside illuminated".

FRONTSIDE CCDs

  • Easier to make (cheap)
  • Thick substrate and surface layers OK for red photons to ~ 11,000 Å
    • low, can travel 500μ or more -- the thicker the CCD, the more sensitive -- i.e. more electrons intercepted by Si to make electron-hole pairs -- higher Q.E.

    • But note:
      1. Loss of resolution with increasing thickness -- electrons can "get lost" on way to depletion zone.
      2. More substrate = more chances for dark current.
  • Crossing gate and SiO2 layer limits best Q.E. to about 50%.
  • Ways to make a thick CCD sensitive in the blue, and even the UV, involve the applications of substances to the CCD that act as a wavelength converter.
  • Most commonly used substances are fluorescent.
  • How does wavelength conversion take place in fluorescence?

    • Fluorescent materials contain molecules that have a variety of vibrational and rotational modes:

      More about vibrational modes...if you're interested.
    • These modes have associated energy states that are quantized.

    • When a photon is initial absorbed,

      1. ...it can put the molecule into an excited state of higher vibrational energy levels.
      2. Collisions of the molecule with its environment allows some shedding of kinetic energy with no photon emission.
      3. The molecule can ladder down vibrational modes until, if the process takes long enough, it can at some point instead undergo a spontaneous emission of a (lower energy) photon.
    • Note a "fluorescent" process involves very quick timescales -- it is "instantaneous" so that the effect stops if excitation source removed (e.g., the cool black-lite poster in your friend's dorm room).

        Compare to "phosphorescence", wherein the molecules can absorb a photon and find a metastable intermediate, semi-forbidden state, that only slowly leaks off energy (e.g., "glow in the dark" materials).

  • Polycyclic Aromatic Hydrocarbons (PAHs)
    • The most common materials used to make CCDs more sensitive is a class of molecules containing carbon rings called Polycyclic Aromatic Hydrocarbons (PAHs).

    • PAHS have a number of possible vibrational modes involving the ring nodes.

      • A vibrational mode of a dehydrogenated Pyrene molecule
    • For related reasons to their effectiveness as wavelength translators, PAHs also represent an important molecule found in interstellar dust, where they are commonly seen emitters of infrared radiation.

    • Space telescope CCDs (early) used Coronene - absorbs UV (1000 - 3500 Å ), re-emits at 5500 Å (chips good > 4000 Å).
      • Vibrations of a dehydrated Coronene molecule:
    • Another good substance is Lumigen Phosphorous (Highlighter ink!) -- Absorbs < 4800 Å, re-emits at 5250 Å.
    • "Laser dyes" are most efficient, when different ones are chosen carefully to convert photons to progressively lower energies (longer wavelength). These cascades can reach 20% efficiency, but tricky to apply and some very toxic.
  • Note that these PAH substances are generally very carcinogenic, which makes them unpleasant to use in the lab.


EXAMPLE OF A BLUE ENHANCED, THICK CCD: THE ASTR 3130 CCDs

The heart of one of the CCD cameras we are using for ASTR 3130 is a thick CCD chip made by Kodak.

The heart of the new ST-8E camera is an enhanced ("E") version of the KAF-1600 detector from Kodak

A few years ago we sent our ST-8 cameras to the camera manufacturer for the chips to be coated for increased blue/UV sensitivity.

Note the improvement in the QE response.

What does this mean for the gain in blue, red, green sensitivity for the same integration time?

RELATIVE SPECTRAL RESPONSE THROUGH RED, GREEN & BLUE INTERFERENCE FILTERS

ABG-B2.tif
(26817 bytes) 2ABG-G.TIF
(26817 bytes) 2ABG-R.TIF
(26817 bytes)

STANDARD ABG 30 SEC BLUE

STANDARD ABG 30 SEC GREEN

STANDARD ABG 30 SEC RED

ENH-BL2.tif (26817 bytes) 2ENH-GL.TIF (26817 bytes) 2ENH-RL.TIF (26817 bytes)

ENHANCED NABG 30 SEC BLUE

ENHANCED NABG 30 SEC GREEN

ENHANCED NABG 30 SEC RED

colorchart.gif
(22456 bytes)

RGB + IR BLOCKING FILTER PASSBANDS (nm)

THE IMAGES IN THIS TABLE WERE ALL TAKEN ON THE SAME NIGHT THROUGH THE SAME TELESCOPE
WITH THE SAME FILTER SET AND WITH THE CAMERAS SET TO THE SAME TEMPERATURE.
EACH FRAME IS A 30 SECOND EXPOSURE.

Below is the information from the ASTR 3130 manual explaining the characteristics of the ST-8 CCD. You should now be able to understand what most of this table means!

Model ST-8 CCD Specifications
CCD Kodak KAF-1600 + TI TC-211
Pixel Array 1530x1020 pixels, 13.8x9.2 mm
Total Pixels 1,500,000
Full Well Capacity 40,000 e-
Dark Current 1 e-/pixel/sec at 0o C
Antiblooming Standard (non ABG as option) -- Limits "bleeding" of bright source photoelectrons into neighboring pixels


Readout Specifications
Shutter Electromechanical
Exposure 0.11 to 3600 seconds, 10 ms resolution
Correlated Double Sampling Yes
A/D Converter 16 bits
A/D Gain 2.3 e-/ADU
Read Noise 15 e- RMS
Binning Modes 1x1, 2x2, 3x3
Pixel Digitization Rate 30 kHz
Full Frame Acquisition under 60 seconds


Optical Specifications (on 8-inch telescope at f/10)
Field of View 24x16 arcminutes
Pixel Size 0.9x0.9 arcseconds
Limiting Magnitude Magnitude 14 in 1 second
for 3 arcsec FWHM stars) Magnitude 18 in 1 minute


Here is the information on the ST1001E CCD:

Here is the information on the Apogee Alta CCD:


BACKSIDE ILLUMINATED CCDs

  • Because blue photons absorbed by few microns it is desirable to avoid the gate / SiO2 MOS layers.
  • Illuminate CCD from behind.
  • But, in order for photoelectrons to be collected efficiently and without resolution losses, best to form in or very near depletion zone -- have to thin CCD.
    • Can get 80% Q.E. if substrate 10 μ thick CCD.
    • Obviously, lose some red sensitivity.
    • Can be hard to thin evenly - need 1 μ accuracy or get large Q.E. variations.
    • Use strong acids to dissolve substrate.
    • Thin CCD fragile - subject to bending as support structure cools and shrinks -- Glue to glass on backside.
    • Hard to make = expensive, but nice Q.E.!
  • After thinning, unavoidable oxidation of Si on backside -- 20 Å wide SiO2 layer that can trap shortest λ UV photoelectrons.
    • UV Flood - Use UV lamp to ionize O2 in air, settles negative ions on layer which then drive photoelectrons into substrate. Lasts several months in vacuum.
    • Flash Gate - Attach a fine metal layer with negative bias.


EXAMPLE OF A THINNED, BACKSIDE ILLUMINATED CCD:
THE FAN MOUNTAIN SITe 2048 CCD

We will use this CCD for the last lab of the semester.

Note how the QE of this chip has been enhanced by thinning and backside illuminating.

Spec Value
Dewar IR Labs
Liquid nitrogen cooled, 1-liter capacity
Heating resistor
Hold time is > 36 hours
Chip SITe 2048-ENG
Back-illuminated, thinned to enhance blue response
Operating Temp. Unstable about -100o C
Optimal operating temperature is ~ -110o C
Lowest achievable temperature is -134o C
Format 2048 x 2049; 24 μm square pixels
Overscan Default is columns 2049-2100
High Gain Inverse Gain: 1.5 e-/ADU; Read noise: 11.5 e-
Low Gain Inverse Gain: 2.8 e-/ADU; Read noise: 11.5 e-
Field of View 12.5' x 12.5' on FMO 1-m
1 pixel = 0.365"
CTE 0.99998 - 0.99999
Dark Current A good exercise for the student
Full Well > 150,000 electrons / pixel
Readout 150 s (2.5 min) for entire chip
maximum ADU = 65536
No on-chip binning
4 available readouts (A-D), but only one amplifier (A)


CCD MISCELLANEA

  • Linearity:

    A great advantage often quoted for CCDs over, e.g. photography, is their linearity. But, strictly speaking, CCDs do exhibit some nonlinearities, though usually (but not always) small.
    • For a given pixel, let:

        F be the incident flux (in photons per second)

        S be the recorded signal level (in ADU)

        Q be the quantum efficiency

        G be the gain

        t be the integration time (in seconds)

      Then, for a strictly linear system:

      In reality, Q is a function of the accumulated charge:

      Where the expression in parentheses is the accumulated charge in time t.

      Thus, we are left with a sensitivity curve much like that we saw earlier for photographic plates, except that the strictly linear part is a larger fraction of the unsaturated regime:

    • In general, one does not need to worry about linearity with good quality CCD cameras, except in the case when the pixels become very full...

      Deviations from non-linear in Sloan Digital Sky Survey CCDs.


  • Blooming:

    For sources that overfill their respective pixels with photoelectrons that overfill the well capacity, the electrons need to go somewhere, and this is usually in adjacent, charge-coupled pixels. The result is called blooming.

    • Example of blooming:

      Blooming visible in the bright stars around M42. The black stripe is unrelated.

    • Additional problems arise with overfull wells having degraded CTE, so that the blooming affects those pixels whose electron packets must pass through the affected pixel to get read out.

    • Some CCDs, like the ST-8, have either or both an anti-blooming barrier and/or a drain between the photosites on the CCD.

    .
  • Electroluminescence
  • In some cases, diodes in the output amplifier can actually act as Light Emitting Diodes (LEDs) and can cause serious problems of excess light near the amplifier.

    • Solutions:

      • Turn off power to amp until needed for readout -- LED only on during readout.

      • Treat as "dark current" and subtract with "dark" exposure (if not strong LED that saturates pixels or that adds LOTS of Poisson noise even after mean level subtracted off).

      Example of electroluminescence caused by the output amplifier.


  • Cosmic Rays
  • Perhaps the most insidious problem one deals with when using CCDs are cosmic rays.

    • High energy particles of unknown origin (probably supernovae and solar flares) that interact with silicon and produce many electron-hole pairs -- sharp, star-like points at random places in image. See this site if interested in knowing more.
    • Set of zero second exposures (bias frames) showing different cosmic ray patterns. Note that the sizes of the blemishes depend on the strength of the CR, its angle of approach, and whether it hits near the edges or centers of pixels.

    • If CR comes in at a small angle to CCD, i.e. nearly parallel to surface, can leave a streak of affected pixels.
    • Close up of a Hubble Space Telescope CCD image showing a large number of cosmic ray streaks. Photo courtesy of Dominion Astrophysical Observatory, HIA.

    • Higher CR incidence at higher altitudes (less atmospheric attenuation).
      • Space telescope - severe (see image above).
      • Take "CR-splits" = multiple images of same scene, different CR pattern, "median stack" the frames (see next page).
      • QUESTION: Why might we expect more CR "streaks" in HST pictures and more compact, point-like CR events on ground-based CCD images?

    • Thicker CCDs have higher probability of intercepting CRs.
    • "CR-like" events can also come from surroundings, like radioactive glass and coatings in telescope/camera system. At McDonald Observatory, there is reputed to be a large uranium deposit under the mountain that contributes a high radioactive decay event rate.

  • Cosmetic Defects

    Take various forms:
    • Dead pixel - Pixel unresponsive to light due to defective gate, depletion zone, substrate, insulator, etc.
    • Hot pixel - Pixel with much larger dark current than neighbors.
      • Note that hot pixels can look like cosmic rays, but, unlike cosmic rays, hot pixels always produce blemishes in the same place from frame to frame.

      • If the hot pixels have a stable dark current rate you can subtract out their effects by taking a dark current frame of equal integration time:

        Raw frame

        Dark subtracted frame

    • Bad column - A defective pixel where the defect affects CTE and all charge packets that pass through the defective pixel will be destroyed (e.g., fall into a trap) resulting in a bad column in the final image.
    • CCD image of galaxy NGC4565 taken with UVa's Fan Mtn. Observatory SITe 2048 CCD showing several bad columns.

    • Note, one of the limitations on making large CCDs is the frequency with which defective pixels occur -- both a technical and economics problem.

      • If bad pixel occurs in MUX - disaster.
      • So, if chance of getting a bad pixel is 1/10000, say, then one of every five CCDs of size 2048x2048 will have a bad pixel in the MUX and is garbage.

      • Extremely hard to get perfect chip, and cost of chip goes something like (1 / # defects)
    Recall this webpage, which shows how CCDs can be "graded" based on their cosmetic quality.


  • Drifts in Readout Amplifier
    • During readout, reference voltage can drift, changes "bias" level -- results in varying "0-point".
    • Solution: Overscanning.

      • If chip has N columns, clock the MUX register N+M "pixels" through amplifier. The M extra reads of the amplifier correspond to no actual pixels on CCD -- i.e. represents pixels with 0 charge.

      • Thus, the "charge level" one reads from these non-existent charge packets tells you what the system is currently putting out as ADU signal for a 0 voltage.

      • After the fact, measure this 0-point level for each CCD row and subtract off the ADU level for each pixel in the row. Do for each row in the image to rectify the wandering 0-point level.

    • Overscanning IS done on the Fan Mtn. CCD but is NOT done on the ST-8 or ST-1001E CCD at McCormick Observatory.


  • Other
    • Interference from surrounding electronics - most commonly 60 Hz noise - get diagonal stripes across image (from readout + 60 Hz combo).
    • 60 Hz noise in a bias image due to interference from improperly shielded electronics cables near the CCD.
      (Image from Apache Point Observatory.)

    • Sky pollution
      • Satellites, airplanes

      A good example of sky pollution. During the observation of the galaxy pair NGC 7443-4, an airliner left traces of its signal lights on the CCD even though the field covered by this image is only about 13 arcmin. Observation was made with a 280 mm telescope and a TH7863 CCD. The integration time is 600 seconds, but the duration of the passage of the airliner was only a fraction of a second. (If the CCD image is in the red part of the spectrum, can you tell which way the airplane was moving?? Hint: The navigation lights on an airplane are green on the right wing and red on the left from the perspective of the pilot.)

    • "Cosmic pollution"
      • Meteors, asteroids, scattered moonlight/starlight on chip dewar parts

      Scattered light streaking across a CCD image. In this case a bright star landed on the CCD chip carrier and is reflecting onto the silicon.


  • Fringing
    • Recall thinned CCDs:

      • Created to increase blue sensitivity and backside illuminated. e.g. RCA, TI CCDs.
      • To reduce potato chip ripple, often bonded to a glass plate with 1 μ of glue
      • This glass, if illuminated by monochromatic light, creates fringe patterns due to interference of in-phase and out of phase reflections in glass.
      • QUESTION: Why doesn't this happen for non-monochromatic light?

    • In what astronomical applications is this a problem?

      • Imaging with narrow wavelength filters -- both sky and sources appear as "monochromatic" to CCD.
      • Using a CCD for recording spectroscopy -- different parts of the CCD sees different wavelengths of "monochromatic" light.

      • Imaging with broad band filters that include wavelengths where the night sky emits auroral emission lines. This is particularly problematical because the auroral emission varies in intensity over the timescale of minutes!
      • Important optical wavelengths where the atmosphere emits strong auroral lines and the Johnson-Cousin passbands in which they occur:

        5577 Å [OI] V band airglow
        5890,5896 Å NaI V band airglow
        6300 Å
        6364 Å
        [OI] R band airglow
        >5900 Å O,OH lines R,I bands airglow

      • Emission lines in source object (e.g., planetary nebula).
    • Removal

      • Fringing is an additive effect of sky -- tricky to remove; subtract scaled fringe frames of blank sky -- adds noise
      • "Prevention is the best cure"
      • Nowadays put an anti-reflection coating on the glass -- fringes minimized
      • Click here to learn how antireflection coatings work.

      • Not generally a problem in thick chips
    • Example of fringing pattern seen in a thinned RCA CCD:

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All animated images from http://astro.uwaterloo.ca/~ssseahra/home.html. Images of KAF-1600 detector, QE curve for KAF detectors, and spectral response through different filters from www.sbig.com/sbwhtmls/blue_enhanced.htm. Sloan linearity, hot pixel and M42 blooming images from http://spiff.rit.edu/classes/phys559/lectures/ccd2/ccd2.html. 60 Hz noise image from http://www.astro.princeton.edu/APO/DISnoisefixed/. All other material copyright © 2002,2006,2008,2012,2015 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 313 and Astronomy 3130 at the University of Virginia.