1. Preliminary Design Review
Read Noise Experiment
Preliminary Design Review
October Don Figer
SPACE
TELESCOPE
SCIENCE
INSTITUTE

2. Goals of Review
Demonstrate that we know how to measure read noise, gain, linearity
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13 November 2018

3. Definition of Read Noise
Read noise is the uncertainty in the measurement of charge in a pixel.
Note that system read noise is greater than the intrinsic read noise of the detector.
Contributions to read noise include:
shot noise in FETs
Johnson noise (resistive elements)
drifts in reference (ground) voltages
dB/dt in a ground loop
temperature variations
pickup of ambient dE/dt, i.e. local transmitters
settling effects
A/D noise
60 Hz noise
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4. Maximizing S/N
S/N can be increased by reading the detector many times (note that this is not reducing read noise, strictly speaking)
Read noise can be reduced by minimizing noise sources, i.e. by shielding, stabilizing temperatures, bandwidth-limiting, etc.
Figure 5. Non-destructive reading reduces effective read noise. Data (red) for ALADDIN array, as measured by Co-I Fowler, compared to 1/n1/2 (blue).
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5. Definition of Gain
Gain is the conversion factor between ADU and e-, and applies to the whole A/D chain
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6. Definition of Linearity
Linearity is a measure of the change in the rate that charge is accumulated under constant source flux as a function of the amount of charge already stored.
Ideally, the response would be perfectly linear, i.e. that charge would be accumulated at the same rate while a pixel is empty up to the point where it is near full capacity. Practically, pixels deviate from linearity and become less responsive as they fill up.
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7. Source of non-Linearity
See Solomon (1998; PhD Thesis) and Benson et al. (2000; SPIE 4131, 171)
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8. NGST Requirements NDC0200 (from NGST Doc. #641)
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9. RGL Experiment Requirements (from NDC0200)
Parameter
Requirement
Read Noise1
1.2 e- RMS
Gain NR
Linearity2
1using any sampling mode consistent with NGST read modes. Attributes 10% of NGST system noise goal to uncertainty in measuring read noise.
2although NGST photometric accuracy must be <1%
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10. Photon Transfer Method for Measuring RG
Method described in Mortara & Fowler (1981SPIE )
Gain is slope of line
Read noise is square root of (y-intercept/gain2)
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11. Photon Transfer Method for Measuring RGL
Method described in Mortara & Fowler (1981SPIE )
g=0.5 ADU/e-
Ndot=100 e-/s
sR=10 e-
Npixels=100
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12. Proposed RGL Experiment Procedure
Drain depletion regions by blanking detector and allowing enough time for trapped charge to randomly bleed out of traps
Stabilize detector bias and temperature
Blank off detector
Obtain bias/dark ramp
Remove blank to allow flux of a few hundred e-/s/pixel
Obtain another ramp
Repeat sequence for range of variations, i.e. read mode
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13. Alternate Read Noise Experiment Procedure
Blank off detector
Read detector twice
Repeat sequence for range of variations, i.e. read modes
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14. Proposed Experiment Variations
Read modes:
dwell
multiple sampling
Fowler
Fowler with multiple sampling
Temperature: 3 levels (a through c, b optimal) covering NGST range
Bias levels: 2 levels covering NGST requirement (a) and goal (b) for well capacity
Combinations: 1a2b3a, 1b2b3a, 1c2b3a, 1d2b3a, 1c2a3a, 1c2c3a, 1c2b3b
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15. Proposed Experiment Duration
Dominant step in terms of schedule is step 1 of primary proposed experiment. Alternate experiment will take less than a day to perform for all variations.
Time estimate: 7 days
Extended scope:
variations in wavelength
variations in bandwidth-limiting techniques, i.e. digital vs. analog filtering
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16. Proposed Experiment Designs
Standard TFST hardware (dewar, Leach controller, etc.)
Light source (approximately spatially flat)
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17. Data Reduction/Analysis Procedure
Subtract dark frames from illuminated frames
Locate sample area that is free of defects and large pixel-to-pixel variations
Compute variance over sample area
Compute average illumination over sample area
Plot variance versus signal
Fit a curve to extract read noise and gain
Plot signal versus time
Fit a curve to extract non-linearity
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18. Data Reduction/Analysis Procedure for Alternate Read Noise Experiment
Subtract bias/dark frames from each other
Locate sample area that is free of defects and large pixel-to-pixel variations
Measure standard deviation in difference frame over sample area
Single-frame read noise is standard deviation divided by root 2
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19. Expected Performance
The accuracy in the read noise measurement is a big question mark
We will be able to measure gain with errors less than a percent, using a few thousand pixels in the sample region
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20. Schedule
Because read noise and dark current noise will dominate all measurements that we make, we should perform these experiments first
Estimate that the RGL test set would take about 10 days
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21. Costs (Shopping List)
nothing
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22. Risks
Our system will be too noisy to perform the measurements. This is almost always true for a new system and results in a prolonged period of debugging.
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23. Recommendations
While waiting for detector, perform sample tests of read noise with controller and shorted cables
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24. Actions
Test Leach noise with cables
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25. References
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