Detective Quantum Efficiency Preliminary Design Review 16 August 2001 Torsten Böker

1. SPACE TELESCOPE SCIENCE INSTITUTE Detective Quantum Efficiency Preliminary Design Review 16 August 2001 Torsten Böker (Revisions by Don Figer)

2. Goals of the DQE experiment • To measure the absolute DQE for (at least) one l • To measure relative DQE over the entire pass band (1-5 mm) • To measure DQE variations as a function of T (and l)

3. Definition of Detective Quantum Efficiency (allows for recombination losses, and multiple carriers per photon)

4. Examples of Detective Quantum Efficiency • QE maps for NICMOS detectors

5. DQE Experiment Requirements(from proposal, Sec. 2.4.5)

6. Questions • How to create a spatially uniform illumination? • How to vary l in a controlled and repeatable way? • How to measure absolute flux? • How to reach proper S/N ratio without saturating the detector? How to suppress unwanted background?

7. Option 1: Integrating Sphere

8. Integrating Sphere Theory 101 • Design Rule #1: Port fraction < 0.05 • For a 2048 x 27 mm detector, need sphere diameter of ~ 20 cm (are available off the shelf, e.g. from LabSphere) Surface brightness at exit port: L  M/D2 where M is the sphere multiplier and D is the sphere diameter. M is sensitive to the coating reflectance and the port fraction  need IR coating such as LabSphere’s InfraGold

9. Setup Options 1. Standard Design 2. External source: flexibility in wavelength and/or intensity 3. Diffuse input via 2nd IS: wider field of view

10. Option 2: Pinhole camera

11. Signal-to-noise Considerations • Signal from light source will (hopefully) be uniform, but thermal background will not (most likely not even inside th IS) • Need to remove background signal via on-off subtraction • Poisson noise from background will limit SNR: Nbg = 2*Sbg • Lamp signal needs to overcome this noise • As always, SNR increases as t • Need to guard against saturation  flux must not exceed 105 ph/s/pixel  might need cold neutral density filters of varying thickness

12. What wavelength resolution is needed? • based on NICMOS experience, DQE is fairly well-behaved • can measure at a number of “pivot” wavelengths (e.g. every 0.5mm), and interpolate • spectral resolution should be at least l/dl ~ 20 • resolution must be higher (R~100) if cutoff range is to be characterized • must control passband of emitter for lack of narrow-band filters  need monochromator

13. How to measure absolute flux? • need 2 calibrated photodiodes: one on inside of integrating sphere (or on dewar window), and one close to the detector at the focal plane • need to measure with narrow-band light, unless source spectrum and throughput curve of all optical components are accurately known • this will measure total system throughput vs. l • need to repeat this for each light source/filter combination

14. Proposed Experiment Procedure • Stabilize detector temperature and bias voltage • Set source flux • Illuminate detector with flat field • Reset/Integrate/Read detector using a “reasonable” read mode • Repeat sequence over variations.

15. Experiment Considerations • want to automate wavelength scanning • at fixed temperature, scan wavelength range • alternatively, if full wavelength range requires manual intervention (e.g. for changing filters and/or light sources), could either scan over separate wavelength regions or • could scan over T at fixed l (potential strain on hardware) • use PK50 at all wavelengths: suppresses background for l<3 mm, at l>3 mm, it simply acts as ND filter (might need variable thickness)

16. Proposed Experiment Variations • Variations • Wavelength: center wavelengths of RIJHKLM filters • Temperature: 5 levels (a through e, c optimal) covering NGST range • Combinations: 1R2c3a, 1I2c3a, 1J2c3a, 1H2c3a, 1K2c3a, 1L2c3a, 1L2c3a, 2a1K3a, 2b1K3a, 2d1K3a, 2e1K3a

17. Proposed Experiment Duration • Time estimate: 1 day • Extended scope: more wavelengths as temperature is varied

18. PK50