MIRI Sensitivity
MIRI will have an enormous sensitivity advantage over current and foreseen mid-IR instruments
- >3 orders of magnitude more sensitive than the best ground based instruments on 8m class telescopes
- About 50 times more sensitive than Spitzer
- Order of magnitude increase in spatial / spectral resolution cf. Spitzer
- Mid-IR spectroscopy is especially important because it contains many unique spectral and diagnostic features.
- JWST sized telescopes enable good sensitivity for moderate resolution spectroscopy.
MIRI has a single, audited, controlled implementation of a sensitivity model, this model can be used to predict the sensitivity expected using the latest design information, and compare the results with the FRD Requirements.
Sensitivity Assumptions
- Zodiacal emission based on COBE data with thermal and scattered sunlight components
- Telescope properties (collecting area, temperatures, emissivity, etc.) as provided by GSFC
- "Simple" reflection properties - no polarization effects, etc.
- Thermal sources are gray bodies described by a temperature and emissivity (sometimes multiple gray bodies)
- Thermal emission from internal optics is negligible (confirmed by separate calculations)
- Well-baffled system so that stray light is negligible
Background Emission sources
JWST has worked to reduce emission from the telescope and sunshade as much as possible. MIRI adopts the provided background model as shown in the graph below:
Measured Filter Transmission Curves
- The measured profiles (at 7 K) for the 15 and 21 micron filters
- FRD bandwidth and OBA PCE budgets plotted for comparison
Quantum Efficiency Projections
- QE spectral measurements performed on non-AR-coated Process Evaluation Chips fromsame wafers as the detectors
- Reflectance measurements on test AR-coatings performed
- Projections above (combination of measured QE and reflectance) indicate requirements willbe met
- Rapid drop-off at 25 µm reduces the sensitivity margin of the 25.5 µm
Predicted Sensitvity Performance
25.5 micron filter
- All filters except the 25.5 µm filter have roughly 20% margin against the the FRD requirement
- The science team has accepted this potential reduction of performance at 25.5 µm, whilst looking for ways to recover some margin.
- One example of how this could be done:
- Increase integration time from 6 sec to 18 seconds to take advantage of the predicted x2 increase in well capacitance since the sensitivity requirements were written
- Gains 5 % margin
- Additional strategies will be investigated as more detector data becomes available.
