NIRISS 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 NIRISS.

Additions to visit overhead

Table 1. Additions to visit overhead

ActivityTime (seconds)Comment
End-of-visit script compilation6
Transition to STANDBY and back to IMAGING4
Two event messages2

Filter/pupil wheel timing model

Wheel timing is modeled as 12s + 4s/position moved + 10s. If a wheel is already in the desired position, the time required for the command is 12s. Each wheel has 9 optical elements, so a move is generally modeled as 2.5 positions, or 32 seconds. There are exceptions on templates where it is known that all possible wheel moves are 1-position moves (see below).

NIRISS filters are contained on both wheels. It is generally assumed that both wheels will move for the first exposure in a visit. The wheel that does not contain the selected filter will move to a clear position.

Exposure overheads

Table 2. Exposure overheads

ActivityTime (seconds)Comment
Per-pointing values
Readout-resetceil(frame read time * (nInts + 0.5))

OSS compilation overhead

Unless noted otherwise, the NIRISS OSS compilation time is 30 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.


NIRISS imaging

The six short-wave filters (F090W – F200W) are located on the pupil wheel, while the six long-wave filters are on the filter wheel. A mechanism move overhead for both wheels (64s) is incurred when switching from a filter on one wheel to a filter on the other wheel, as the first wheel has to move to the clear position, while the second wheel has to move from the clear position to the selected filter. Switching between two filters on the same wheel incurs a 44s overhead (moving one wheel (32s) and commanding the other wheel to its current position (12s).

NIRISS wide field slitless spectroscopy

Moving from the initial direct image to the first grism image requires a 1-position pupil move (26 seconds) if the grating is GR150R or BOTH and a 2-position pupil move (30 seconds) if the grating is GR150C.

If the grating is BOTH, the sequence of the gratings is reversed at each dither position so that there is a single 1-position pupil move (26 seconds) at each dither position.

If the dither option is not NONE, there is a final direct image after the last grism image. If the grating is GR150C (for all dither options) or BOTH (3-point dither only), this is a 2-position pupil move from GR150C to CLEAR. Otherwise, it is a 1-­position move from GR150R to CLEAR.

NIRISS single-object slitless spectroscopy

For the target acq exposure, the filter is F480M and the pupil is either NRM or CLEARP. For the science exposure, the filter is CLEAR (adjacent to F480M in the wheel) and the pupil is GR700XD (between the two possible target acq pupils in the wheel). Threrefore, there is a 1-position move for both the filter and pupil wheels (52 seconds) between target acq and science.

NIRISS aperture masking interferometry

The NRM pupil is used for target acq if the AMIBRIGHT option is selected for the Acq

Mode parameter. The CLEARP filter is used for the AMIFAINT option.

The pupil for the science exposures is NRM.

The pupil for the direct images is CLEARP.

NIRISS focus

The OSS compilation time for this activity is 10 seconds.

There is an additional 95s of mechanism overhead charged for the time required to read the focus mechanism’s current position.

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 commands is modeled as (26s + ceiling(offset in mm * 4.5 + 6.03s backlash correction for a positive move) / number of focus positions)) * number of focus positions.

If the Use Default Focus Scan option is selected, the sorted focus offsets are:

-0.4, -0.8, -1.2, -1.6, -2.0, 2.0, 1.6, 1.2, 0.8, 0.4, 0.0.

The total distance that the focus mechanism is moved is then 0.4*5 + 4.0 + 0.4*5 = 8.0mm.

The focus mechanism move overhead is then (26 + ceiling((8.0 * 4.5 + 6.03) / 11)) * 11 = 330s.

NIRISS internal flat

There is additional mechanism overhead for powering lamps on and off for each exposure specification:

Table 3. NIRISS internal flat

ActivityTime (seconds)Comment
Turn off 4 lamps prior to exposure8421s * 4 lamps
Turn on specified lamp prior to exposure40
Turn off specified lamp after exposure21
Total14521s * 5 + 40s

See NIRISS Imaging section for note on overhead incurred when selecting filters on different wheels on consecutive exposure specifications in the same visit.

NIRISS external calibration

As for SOSS and AMI, the two BRIGHT options for the Acq Mode parameter use the NRM pupil, while the FAINT options use the CLEARP pupil.

Change log

Version 1 (June 6, 2017)

  1. Initial version.