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NGST Window Dilemma: Optical Performance Versus Protection

This document outlines the decision-making process regarding the installation of a window over the NGST detectors, weighing the pros and cons of such a choice. It includes a detailed analysis of the impact on optical performance, throughput, complexity, and cost, comparing it with other missions like NICMOS/HST, IRAC/SIRTF, WFC3/IR, MODIS, and TES. Recommendations suggest eliminating the window to maintain detector integrity.

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NGST Window Dilemma: Optical Performance Versus Protection

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  1. SPACE TELESCOPE SCIENCE INSTITUTE Windows for NGST 14 February 2002 Don Figer

  2. Outline • Statement of Problem • Pros • Cons • Other Missions • Recommendations

  3. Statement of Problem • We must decide if there should be a window over the NGST detectors

  4. Pros for Window • reduces contamination pre-launch, i.e. hygroscopic effects • reduces “gettering” effect at L2 • protects against random mechanical damage • helps in associating secondary with primary radiation

  5. Cons against Window • Optical performance • reduces throughput (7% for NIR, 20% for MIR) • increases possibility of ghosts • increases possibility of fringing • reduces wavefront quality • induces chromatic focus shift • Program • increases interface complexity • increases cost and schedule

  6. Throughput • Uncoated • Coated:

  7. Other Missions • NICMOS/HST • IRAC/SIRTF • WFC3/IR • MODIS • TES

  8. Other Missions: NICMOS/HST • Has windows (one per each of three cameras) • 10-25 mm clear aperture, by 5 mm thick • Warm • Vacuum seal pre-orbit • Qualified to meet specs after 5 krad (SI) over 5 years • Material is fused silica or quartz

  9. Other Missions: IRAC/SIRTF • Does not have windows • Performed a 1-2 year humidity study of flight-like detectors and found no deterioration • The IRAC detectors are not the coldest part of the instrument,at least when in operation • In practice, there is no evidence of any contamination problems with the flight detectors after exposure to atmosphere.

  10. Other Missions: WFC3/IR • Has a window • 22 mm in diameter, by 4 mm thick Suprasil 311 • 93 per cent throughput when uncoated

  11. Other Missions: MODIS • Does not have windows • Did not keep detectors under vacuum/cryo conditions

  12. Other Missions: TES • Does not have a window • The first lens is very close to the detector with no direct path into the space in between. It is not hermetic but no particles could get inside because the detector is maintained under a dry nitrogen purge at all times and the mount is assembled in a clean environment.

  13. Recommendations • Require that there shall be no window over NGST detectors.

  14. Contributions and References • Contributions from: Steve Willner, Peter Love, Craig McCreight, Joe Hora, Judy Pipher, Massimo Stiavelli, Colleen Townsley, Mike Ressler, Bernie Rauscher • WFC3 ISR 2001-18 http://www.stsci.edu/instruments/wfc3/ISRs/WFC3-2001-18.pdf • NICMOS docs (drawing 517389)

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