Assigning Position Angles for JWST Observations

This document shows how to assign Position Angles for JWST, For some observations, it is necessary to set the position angle (orientation) of the telescope (e.g. to place a jet along a NIRSpec slit). To do this, one specifies an acceptable range (to be as broad as scientifically possible to maximize the schedulability of the Observation) of the position angle with respect to north (measured through east) around the target. There are 2 different special requirements you can use, depending on which reference frame you have for the position angle.

Our project is to obtain NIRSpec Fixed Slit observations of the jet in 3C 273. I have already created the Observation, and I have found a position angle in the literature of 50°. In this case, I have added an APA requirement (see Adding Special Requirements to JWST Observations ( document, movie) for details on working with Special Requirements). Note that the V3PA values are included, and for this template, the offset between APA and V3PA is large (~220°).

If we look at the Observation in Aladin, we see that the position angle is not quite correct. Note that the position angle shown in Aladin is the V3PA value.

If we now better align the slit to the jet (by rotating the aperture in Aladin), we see we need a position angle (V3PA) of 263.5°, which we need to insert into APT.

If we go back to APT and try to edit the position angle range, the see the V3PA values are not editable. However, if we select the "Set using V3" box, we can enter the updated V3PA values to those we measured.

Last modified: December 4, 2017.