Read Noise Experiment Preliminary Design Review October 5 2001 Don Figer

1. SPACE TELESCOPE SCIENCE INSTITUTE Read Noise Experiment Preliminary Design Review October 5 2001 Don Figer

2. Goals of Review • Demonstrate that we know how to measure read noise, gain, linearity • Choose preferred experiment setups • Choose items to purchase • Generate actions

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

4. Maximizing S/N 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). • 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.

5. Definition of Gain • Gain is the conversion factor between ADU and e-, and applies to the whole A/D chain

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.

7. Source of non-Linearity See Solomon (1998; PhD Thesis) and Benson et al. (2000; SPIE 4131, 171)

8. NGST RequirementsNDC0200 (from NGST Doc. #641)

9. RGL Experiment Requirements(from NDC0200)

10. Photon Transfer Method for Measuring RG • Method described in Mortara & Fowler (1981SPIE..290...28) • Gain is slope of line • Read noise is square root of (y-intercept/gain2)

11. Photon Transfer Method for Measuring RGL • Method described in Mortara & Fowler (1981SPIE..290...28) • g=0.5 ADU/e- • Ndot=100 e-/s • sR=10 e- • Npixels=100

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

13. Alternate Read Noise Experiment Procedure • Blank off detector • Read detector twice • Repeat sequence for range of variations, i.e. read modes

14. Proposed Experiment Variations • 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

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

16. Proposed Experiment Designs • Standard TFST hardware (dewar, Leach controller, etc.) • Light source (approximately spatially flat)

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

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

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

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

21. Costs (Shopping List) • nothing

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.

23. Recommendations • While waiting for detector, perform sample tests of read noise with controller and shorted cables

24. Actions • Test Leach noise with cables

25. References http://www.mso.anu.edu.au/observing/detlab/ccdlab/ccd/ccdchar/linearity/linearity.htm