NIRSpec 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 NIRSpec.

Additions to visit overhead

Table 1. Additions to visit overhead

ActivityTime (seconds)Comment
End-of-visit script compilation30
Filter wheel to OPAQUE30if not already OPAQUE
MSA to ALLCLOSED90if not already ALLCLOSED

Filter/grating wheel timing model

Before moving the grating wheel, telemetry is checked to see if the wheel is already in the desired position. If it is, the wheel is not commanded.

Table 2. Filter/grating wheel timing model

ActivityTime (seconds)Comment
Grating telemetry check10
Move grating wheel70
Move filter wheel30

Exposure overheads

Table 3. Exposure overheads

ActivityTime (seconds)Comment
Per-pointing values
Setup, normal mode17
Setup, IRS^2 mode45
ceil(frame read time * (nInts + 0.5))

OSS compilation overhead

Unless noted otherwise, the NIRSpec OSS compilation time is 65 seconds per exposure specification, not including target acq exposures. The OSS compilation time for target acq is subsumed into the flat 1200-second TA overhead.

Additional instrument overheads

Table 4. Additional instrument overheads

ActivityTime (seconds)Comments
MSA config overhead90
MSA config, arm only20where only the magnetic arm must move
Readout mode switching overhead100IRS^2 or normal
Arm CAA and cycle lamp power30included in mechanism overhead

Since the readout mode is not known at the start of a visit, half of the switching overhead (50s) is applied for each visit, to represent the statistical average of initial switching.

The filter is set to OPAQUE for autocals, so an overhead for this is included for each exposure spec that has an autocal specified. Performing autocals also incurs the 30-­second overhead for the CAA and lamp power. If the last exposure in a visit has an autocal, the visit overhead will not include the end-of-visit filter wheel move to OPAQUE, as the wheel is already in that position. There is always a filter move overhead charged when moving from an exposure spec with an autocal to the next exposure in the same visit, as the filter wheel has to move from OPAQUE to the filter specified for the next exposure.

If an MSA configuration is not selected for MSA target acq or verification exposures, the OSS scripts will configure the MSA to ALLOPEN for these exposures.

LINE lamp wait time

Each LINE lamp (used in science templates when autocal option WAVECAL or BOTH is selected and in Internal Lamp template) is required to be off for at least 1000 seconds before it can be powered on again. If the minimum off time has not been satisfied when a lamp is about to be used, the OSS script will wait until the required time has elapsed. This wait time is modeled within a visit as a component of mechanism overhead, but not between visits.

The LINE lamps used for WAVECALS are based on the grating:

G140[MH] = LINE1

G235[MH] = LINE2

G395[MH] = LINE3


Note that this will be changing in OSS Build 6 (APT 25.1), such that all gratings except PRISM will use the TEST lamp, which does not have an off-time requirement.

Once a LINE lamp has been powered off, its off time is tracked by the model. Within an activity, the lamp is powered on just prior to the exposure is started, after OSS compilation overhead, wheel move overhead, readout mode switching overhead, MSA configuration overhead and exposure setup overhead. If there is dithering, the SAM overhead between pointings is also included as part of the calculated lamp off time.

Internal lamp example

Here is an Internal Lamp example where there are 3 exposure specs in a visit. The first uses lamp LINE1, the second uses FLAT4 and the third uses LINE1 again. All exposures in this example use the same grating, filter, MSA configuration and readout mode.

LINE1 is turned off at the end of exposure spec #1. Elapsed time between this power off and the next time the lamp is to be used is:

Table 5. Example - internal lamp durations

Internal Lamp example
ActivityTime (seconds)Comment
OSS script compilation time for exposure spec 265
Mech move time for exposure spec 210
Exposure duration for exposure spec 2729photon collect time + setup + cleanup + reset frame time
Time to arm CAA and power FLAT4 lamp on and off30
OSS script compilation time for exposure spec 365
Mech move time for exposure spec 310just a check to see if grating needs to be moved
Exposure setup for exposure spec 317

At this point, the OSS script is ready to turn on LINE1. Total off time is the sum of the above, or 926 seconds. Script will wait until 1000 seconds have elapsed, or an additional 74 seconds.

A warning was added to APT to inform the user when lamp wait time has been accrued.


NIRSpec fixed slit spectroscopy

The MSA configuration for the science exposures is ALLCLOSED, so there is an MSA configuration change overhead charged in moving from the target acq to the science exposures.

NIRSpec IFU spectroscopy

The MSA configuration for the science exposures is ALLCLOSED (with the magnetic arm in the IFU position), so there is an MSA configuration change overhead charged in moving from the target acq to the science exposures.

The MSA closure at the end of a visit only needs to move the magnetic arm to the primary park position, as the shutters are already closed with the arm in the IFU position. This requires 20 seconds instead of 90 seconds.

NIRSpec multi-object spectroscopy

Grating is set to MIRROR for optional confirmation image, which is taken after the target acquisition and before the science.

If the target acq option is TACQ, there is no overhead charged for this grating, as the target acquisition exposures also use the MIRROR grating and the selection of this grating for the TA is part of the flat target acq overhead.

If the target acq method is VERIFY_ONLY, there is an overhead charged for moving the grating wheel for the initial exposure (confirmation or science).

NIRSpec bright object time series

The MSA configuration for both the science exposures and the target acq exposures is ALLCLOSED, so there is no MSA configuration change overhead charged in moving from the target acq to the science exposures or at the end of visit.

NIRSpec dark

Filter wheel is commanded to OPAQUE (the home position) and the grating wheel is not commanded. MSA is configured to ALLCLOSED, the home position. As a result, there is no mechanism or MSA overhead for this template.

NIRSpec focus

The OSS compilation time for this activity is 55 seconds.

Grating is set to MIRROR for the exposures.

Focus mechanism offsets are sorted by OSS scripts so that all negative offsets are performed first, in descending order. Then a single positive move is performed to from the most negative offset to the most positive offset, and the remaing positive offsets are processed in descending order, so that they are traversed with moves in the negative (preferred) direction.

A return to the original position is added by the OSS script if the number of offsets is greather than 2 and this 0 offset is not already included in the input.

The timing for the focus move command is modeled as (number of offsets * 8s) + (total number of steps moved/60)s.

NIRSpec imaging

The MSA configuration is ALLOPEN for these exposures.

NIRSpec internal lamp

Filter wheel is commanded to OPAQUE, the home position, which incurs no overhead. Lamp/CAA overhead is applied for every exposure spec for which a lamp is specified.

Change log

Version 1 (June 6, 2017)

  1. Initial version.