FGS 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 the Fine Guidance Sensor (FGS).

Exposure overheads

Table 1. Exposure overheads

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

OSS compilation overhead

Unless noted otherwise, the FGS OSS compilation time is 5 seconds per exposure specification.

FGS internal flat

Previous APT versions included the entire time for this template as instrument overheads and counted the time for the full-frame exposures twice. APT 24.4 now includes the time for the three full-frame exposures as science duration and no longer counts it twice.

FGS focus

The OSS compilation time for this activity is 20 seconds.

There is an additional 20s 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 positive offsets are performed first, in ascending order. Then a single negative move is performed from the most positive offset to the most negative offset, and the remaining negative offsets are processed in ascending order, so that they are traversed with moves in the positive (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 * 0.5 + 6.03s backlash correction for a negative 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 * 0.5 + 6.03) / 11)) * 11 = 297s.

Additionally, there is an overhead associated with dropping and reacquiring guiding for each focus move. This overhead is 130 seconds (15s to drop guiding and 115s to reacquire it) and is assigned to exposure overheads.

Change log

Version 1 (June 6, 2017)

  1. Initial version.