MIRI Overheads

JWST's Astronomer’s Proposal Tool (APT) estimates duration for each step in the proposal. This page provides an accounting of durations specific to MIRI.

Additions to visit overhead

MIRI has no default end-of-visit activities, except for certain templates where the CCC must be closed/opened and/or lamps powered on/off. These are described in subsections for the relevant templates.

Filter/pupil wheel timing model

MIRI filter wheel has 18 positions, so a move of 4.5 positions is assumed. Timing model is 0 seconds if wheel is already at desired position, 14s + number of positions moved * 8s otherwise.

Moving each of the IFU grating wheels requires 25 seconds.

Table 1. Filter/pupil wheel timing model

ActivityTime (seconds)Comment
Move filter wheel4s + number of positions moved * 8s
Moving IFU grating wheels25per wheel

Exposure overheads

Table 2. Exposure overheads

Activity
Time (seconds)
Imaging setup
20
MRS setup (mismatched number of groups and integrations for short & long)
30
MRS setup (matched number of groups and integrations for short & long)
20
Imaging + MRS (3-detector operation) setup


40
Imaging cleanup
12
MRS or 3-detector cleanup
16
Readout-reset
ceil(frame read time * 0.5))

MIRI does not perform reset frames, so the readout-reset overhead is just the half-­frame exposure start synchronization time.

OSS compilation overhead

Unless noted otherwise, the MIRI OSS compilation time is 25 seconds per exposure specification, not including target acq exposures. The OSS compilation time for target acq is subsumed into the flat 600-second TA overhead.

Templates

MIRI low-resolution spectroscopy

The filter for the science exposure is P750L. This is not one of the legal target acquisition filters, so there is a filter move overhead in the transition from target acq to science.

MIRI medium-resolution spectroscopy

There is an overhead to set the filter for the target acq exposure, followed by overhead to set both the 1_4 and 2_3 gratings for the science exposure. If detector ALL is selected, there are 3 exposures and both gratings must be set for each exposure.

MIRI dark

The CCC is closed at the start of the visit (70s) and opened at the end (70s) if the value of the Detector parameter is MRS or ALL. These times are allocated to visit overheads, and the time for the closure is included in initial overheads. If the value of the Detector parameter is IMAGER or ALL, the filter wheel is set to OPAQUE (50s).

MIRI external flat

Visit overhead includes time to power lamp(s) on or off.

Power lamp on: 45s

Power lamp off: 20s

If the value of the Detector parameter is ALL, both the IFU and Imager lamps must be powered, so the above times are doubled.

If the value of the Lamp Use parameter is ON ONLY, the time to power on the lamp(s) is included in initial overheads.

MIRI MRS cross grating engineering

Visit overhead includes time to power IFU lamp on or off if the value of the Lamp Use parameter is not OFF ONLY. If the value of the Lamp Use parameter is ON ONLY, the time to power on the lamp(s) is included in initial overheads.

MIRI Anneal

The MIRI Anneal overhead timing will be of the form:

((T1+T2) * #Detectors) + T3 + Overheads

Table 3: Components of MIRI Anneal overhead timing

ActivityDescriptionValueComment
T1warming time80smiri_anneal_warming_duration from timing_model_parameters as of PRDOPSSOC-L-023 (link to PRD)
T2cooling time300smiri_anneal_cooling_duration from timing_model_parameters as of PRDOPSSOC-L-023 (link to PRD)


#Detectors

Imager1
MRS2
ALL3
T3heat exchanger time1800smiri_anneal_max_heat_exchange_duration from timing_model_parameters as of PRDOPSSOC-L-023 (link to PRD)
Overhead durationMIRI anneal overhead duration120smiri_anneal_overhead_duration from timing_model_parameters as of PRDOPSSOC-L-023 (link to PRD)

Change log

 Click here to expand...

Version 4

  1. PROPINSTJWST-91387 - Appendix I: Added MIRI Anneal section.


Version 3 (December 4, 2018)

  1. PROPINSTJWST-91049 - Updated Table 2 overheads; removed outdated description of setup time in MIRI medium-resolution spectroscopy section.


Version 2 (May 2, 2018)

  1. PR 89606 - Updated MIRI MRS cross-grating engineering section.


Version 1 (June 6, 2017)

  1. Initial version.