NIRCam 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 NIRCam.

Additions to visit overhead

End-of-visit activities: 55s

Table 1. Detailed timing for end-of-visit activities

ActivityTimeComment
Script compilation30
ASIC sync time10
FPE Reset15


Subarray configuration (if subarray is not FULL): 50s (also added to initial overheads). If there is a TA, this time is 90s (a 40s configuration prior to the TA and 50s between the TA and science), even if the subarray is FULL for the science exposures.

Filter/pupil wheel timing model

Each NIRCam module-wavelength combination has two wheels, a filter wheel and a pupil wheel, for a total of 8 wheels. The OSS scripts use two commands, one that can move 1-4 pupil wheels and one that can move 1-4 filter wheels.

Time to move filter or pupil wheels:

8s * number of wheels commanded + 20s * (number of wheel pairs actually moved + 1) + 3s for OSS telemetry sampling latency

In general, the number of wheels commanded is:

module ALL: 4

module A or B: 2

single detector module (ALONG, B1, etc.): 1

Exception to model: if 4 wheels are commanded, but the only wheels that actually need to move are the two wheels on the same module, the command takes 12 seconds longer than predicted (87s instead of 75s). This was observed in CV3 data, but the reason for it is unknown.

The assumption is that all commanded wheels (filter and pupil) will need to be moved at the start of each visit (with some exceptions (see below)).

Exposure overheads

Table 2. Exposure overheads

ActivityTime (seconds)Comment
Per-pointing values
Setup30
Cleanup, module ALL22
Cleanup, other modules12
Readout-reset (SUBARRAY=FULL, read out using 4 detector outputs)ceil(frame read time * (nInts - 1))
Readout-reset (all other SUBARRAY)ceil(frame read time * nInts)

OSS compilation overhead

Unless noted otherwise, the NIRCam OSS compilation time is 40 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

NIRCam coronagraphic imaging

Only a single wheel is commanded for filter/pupil wheel commands. Time for initial filter/pupil setting for TA is subsumed into the flat 600 seconds for target acq. For moving from TA to science, the pupil wheel is commanded, but won’t have to move (31 seconds). Filter wheel will have to move if initial science filter is not the same as TA filter (51 seconds). Subarray configuration overhead is 90 seconds (40 seconds to configure subarray for TA, 50 seconds to configure subarray for science exposures). This time is a component of the initial overhead.

Table 3. NIRCam coronagraphic imaging overheads

ActivityTime (seconds)Comment
Initial filter/pupil setting for TA0subsumed into flat 600s for TA
Command pupil wheel31does not have to move
Move filter wheel for science51 secif science filter is not the same as TA filter
Subarray configuration (this time is a component of initial overhead)
Configure subarray for TA40
Configure subarray for science50

NIRCAM grism time series

Exposure setup time is 35 seconds instead of 30 seconds, due to an extra Data Acq configuration.

A 50-second subarray configuration time is included in the visit overheads and initial overheads for the case where the subarray is FULL and the number of outputs is 1.

NIRCam dark

Filter wheels are not commanded. The pupil wheel(s) are commanded to FLAT for each exposure spec, but the wheels are already there, as that is the home position.

NIRCam wheel exercise

OSS Compilation time is 20 seconds per observation.

Each wheel rotation is accomplished via 3 wheel move commands. Before the wheels are rotated, they are moved to a designated initial position (hardcoded in the OSS script). For pupil wheels, the initial position is FLAT, so those wheels will not have to move when commanded to the initial position.

If wheel ALL is selected, all four pupil/filter wheels are moved with a single command. Otherwise, single wheels are commanded.

Since this template has no pointings, the OSS, mechanism and visit overhead are not properly assigned to their categories. The visit overhead is 175 seconds (including 55 seconds to command pupil wheels, which will already be at FLAT).

Table 4. NIRCam wheel exercise overheads

ActivityTime (seconds)Comments
OSS compilation20
Move to initial position: 51s51
Rotate wheel (3 commands)15351s * 3
Visit overhead175
Total399

NIRCam focus

Prior to moving the focus mechanism, all four pupil wheels are commanded to FLAT. However, the two pupil wheels from the module not in use are already at FLAT and won’t move. This is a situation where 4 wheels are commanded, but only the two wheels on the same module will have to move, so the extra 12 seconds of overhead mentioned in section 3.1.2 is applied. However, the pupil FLAT is an option for the Long Pupil. If that pupil is selected, only one wheel has to move and the 12 seconds is not applied. So the pre-focus pupil move overhead is:

Long Pupil = FLAT (4 wheels commanded, 1 wheel moves): 32+20+20+3 = 95s

Long Pupil != FLAT (4 wheels commanded, 2 wheels on same module move):

32 + 20 + 20 + 3 + 12 = 107s

After the focus move, the pupil wheels on the selected module are moved back to the selected positions:

16 + 20 + 20 + 3 = 59s

The NIRCam script sends focus move commands that supply a delta number of steps for each of the three linear actuators. The delta is the difference between the input absolute step position and the previous absolute step position. For the first entry, the delta is the difference between the specified absolution steps and the specified starting position in steps for each actuator.

If the absolute magnitude of each delta is less than or equal to 672 only a single command is sent. If any of the deltas are greater than 672, the OSS script will split the move up into multiple commands.

For a single command, each of the three step deltas should be considered as a multiple of 96-step ICE moves. The total number of ICE moves in the command determines the timing as follows:

Overhead per command: 22s

Time for each 96-step ICE command (or portion of such): 4s

As an example, for a command where the step deltas are 100, 212, 400:

22s + (ceiling(100/96) + ceiling(212/96) + ceiling(400/96)) * 4s

= 22s + (2 + 3 + 5) * 4s = 62s

For a command with a step delta greater than 672:

Number of commands n = ceiling(maximum delta / 672)

Step scaling factor: 672 / maximum delta

Step values for first n-1 commands:

 LAx = floor(original LAx delta * step scaling factor)

Step values for nth command: remainder needed to make the total number of steps equal to the original specified values.


Example: command with step deltas 1400, 700, 192

Number of commands = ceiling (1400/672) = 3

Step scaling factor = 672/1400 = 0.48

LA1 steps for 3 commands: 672, 672, 56

LA2 steps for 3 commands: 336, 336, 28

LA3 steps for 3 commands: 92, 92, 8

Total number of ICE moves in the 3 commands: (ceiling(672/96) + ceiling(336/96) + ceiling(92/96)) * 2 + ceiling(56/96) + ceiling(28/96) + ceiling(8/96) = (7 + 4 + 1) * 2 + 1 + 1 + 1 = 27

Time for commands: 3 * 22s + 27 * 4s = 174s

NIRCam IPR imaging

OSS Compilation time is 35 seconds per observation.

Exposures are performed which are a subset of this sequence. The subset is determined by the parameters MODULE and LED_LAMP as described in Table 6.  There are 3 identical exposures performed for each of these. Mechanisms are commanded to their positions at the start of each set of 3 exposures and are not commanded between the 3 exposures.

Table 5. NIRCAM IPR imaging exposure sequences

1

A

SHORT

REDUNDANT

F212N

MASKIPR

2

A

SHORT

REDUNDANT

WLP4

MASKIPR

3

A

LONG

REDUNDANT

F335M

MASKBAR

4

B

SHORT

PRIME

F212N

MASKIPR

5

B

SHORT

REDUNDANT

F212N

MASKIPR

6

B

SHORT

PRIME

WLP4

MASKIPR

7

B

SHORT

REDUNDANT

WLP4

MASKIPR

8

B

LONG

PRIME

F335M

MASKBAR

 The sequence of exposures performed is determined by the template parameters Module and LED Lamp:

Table 6. NIRCam IPR imaging exposure sequence

ALL

BOTH

1-8

ALL

REDUNDANT

1,2,3,5,7

A

REDUNDANT

1-3

ASHORT

REDUNDANT

1,2

ALONG

REDUNDANT

3

B

BOTH

4-8

B

PRIME

4,6,8

BSHORT

BOTH

4-7

BSHORT

PRIME

4,6

BSHORT

REDUNDANT

5,7

BLONG

PRIME

8

A single pupil and filter wheel are commanded for each mechanism move.

NIRCam PIL imaging

The number of filter/pupil wheels commanded is 1.

Filter selection options include some pupil+filter options. For the single-filter options, the pupil is CLEAR.

Visit overheads include 70 seconds to deploy the PIL at the start of the visit and 70 seconds to retract it at the end of the visit. The deployment time is included in initial overhead.

Change log

 Click here to expand...

Version 3 (March 20, 2019)

  1. PROPINSTJWST-91187 - Appendix I: Update readout-reset overhead for NIRCam full-frame exposures.


Version 2 (November 20, 2017)

  1. PR 85855 - Updated Additions to Visit Overhead section.


Version 1 (June 6, 2017)

  1. Initial version.