1 / 18

Detective Quantum Efficiency Preliminary Design Review

Detailed overview of the Detective Quantum Efficiency (DQE) experiment, including goals, examples, requirements, and experiment variations. Covers methods, considerations, and variations for measuring DQE.

dudleyg
Download Presentation

Detective Quantum Efficiency Preliminary Design Review

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related