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.
- If you have a position angle of the object with respect to North,
then use Aperture Position Angle (APA). For instance, an APA of 0° means
that the observation should be scheduled with the y-axis of the aperture
aligned with North and an aperture position angle of 90°
means that the observation should be scheduled with the y-axis of the aperture
aligned due East. If your value of the position angle came from the
literature, or you measured the value with a software tool other than
APT/Aladin, this is the special requirement you should use.
- If you have the position angle of the V3 axis of the telescope with respect
to North, then use the V3 Position Angle (V3PA). If your value of the
position angle came from a report for the Visit Planner, or you measured
the value with APT/Aladin, this is the special requirement you should use.
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
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
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
Last modified: December 4, 2017.