Read Noise Experiment Preliminary Design Review October 5
Read Noise Experiment Preliminary Design Review October 5 2001 Don Figer SPACE TELESCOPE SCIENCE INSTITUTE 12/19/2021
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Goals of Review • • Demonstrate that we know how to measure read noise, gain, linearity Choose preferred experiment setups Choose items to purchase Generate actions 12/19/2021 2 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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 • d. B/dt in a ground loop • temperature variations – – pickup of ambient d. E/dt, i. e. local transmitters settling effects A/D noise 60 Hz noise 12/19/2021 3 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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/n 1/2 (blue). 12/19/2021 4 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Definition of Gain • Gain is the conversion factor between ADU and e-, and applies to the whole A/D chain 12/19/2021 5 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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. 12/19/2021 6 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Source of non-Linearity See Solomon (1998; Ph. D Thesis) and Benson et al. (2000; SPIE 4131, 171) 12/19/2021 7 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization NGST Requirements NDC 0200 (from NGST Doc. #641) 12/19/2021 8 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization RGL Experiment Requirements (from NDC 0200) Parameter Requirement Read Noise 1 1. 2 e- RMS Gain NR Linearity 2 NR 1 using any sampling mode consistent with NGST read modes. Attributes 10% of NGST system noise goal to uncertainty in measuring read noise. 2 although NGST photometric accuracy must be <1% 12/19/2021 9 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Photon Transfer Method for Measuring RG • Method described in Mortara & Fowler (1981 SPIE. . 290. . . 28) • • Gain is slope of line Read noise is square root of (y-intercept/gain 2) 12/19/2021 10 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Photon Transfer Method for Measuring RGL • Method described in Mortara & Fowler (1981 SPIE. . 290. . . 28) g=0. 5 ADU/e. Ndot=100 e-/s s. R=10 e. Npixels=100 12/19/2021 11 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Proposed RGL Experiment Procedure 1. 2. 3. 4. 5. 6. 7. 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 12/19/2021 12 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Alternate Read Noise Experiment Procedure 1. 2. 3. Blank off detector Read detector twice Repeat sequence for range of variations, i. e. read modes 12/19/2021 13 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Proposed Experiment Variations • Variations 1. 2. 3. • Read modes: a. dwell b. multiple sampling c. Fowler d. 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: 1 a 2 b 3 a, 1 b 2 b 3 a, 1 c 2 b 3 a, 1 d 2 b 3 a, 1 c 2 a 3 a, 1 c 2 c 3 a, 1 c 2 b 3 b 12/19/2021 14 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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 12/19/2021 15 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Proposed Experiment Designs • • Standard TFST hardware (dewar, Leach controller, etc. ) Light source (approximately spatially flat) 12/19/2021 16 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Data Reduction/Analysis Procedure 1. Subtract dark frames from illuminated frames 2. Locate sample area that is free of defects and large pixel-to-pixel variations 3. Compute variance over sample area 4. Compute average illumination over sample area 5. Plot variance versus signal 6. Fit a curve to extract read noise and gain 7. Plot signal versus time 8. Fit a curve to extract non-linearity 12/19/2021 17 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Data Reduction/Analysis Procedure for Alternate Read Noise Experiment 1. Subtract bias/dark frames from each other 2. Locate sample area that is free of defects and large pixel-to-pixel variations 3. Measure standard deviation in difference frame over sample area 4. Single-frame read noise is standard deviation divided by root 2 12/19/2021 18 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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 12/19/2021 19 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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 12/19/2021 20 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Costs (Shopping List) • nothing 12/19/2021 21 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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. 12/19/2021 22 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Recommendations • While waiting for detector, perform sample tests of read noise with controller and shorted cables 12/19/2021 23 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Actions • Test Leach noise with cables 12/19/2021 24 Read Noise Experiment PDR
SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization References http: //www. mso. anu. edu. au/observing/detlab/ccd/ccdchar/linearity. htm 12/19/2021 25 Read Noise Experiment PDR
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