Fixed az driftPattern at transit
The fixed azimuth drift routine within cima is
used for various drift surveys. It allows you to start a drift
scan at a particular az and declination. To use the routine:
- the observer provides the azimuth and declination of the source.
- Cima computes the za and hour angle to start the drift.
- To do this, it precesses the declination to current coordinates
- It computes the ha,za given the az, dec Current, and the
- Note that cima is using the aoLattitude that is .5 arc
seconds less than what the pointing program uses.
- Cima also uses the feed coordinates before applying the
model. The final az,za could be slightly different after the model is
applled (this shouldn't be a problem except for hitting any limits).
- Cima moves to the az,za at the start of the strip and then waits
for the hour angle to arrive before starting the drift.
Users have been having trouble with the fixed drift routine when
observing declinations close to that of the AO lattitude (18:21:14.2).
The problems are:
The problem occurs because the declination strip of a source across the
dish may cross the same azimuth twice (at different za's and hour
angles). This occurs when the source declination is close to the
lattitude of ao.
- Setting az=90 or 270 will cause a divide by zero in cima. The
computation is dividing by the cos(az). We're stuck with this for
- Close to the aoLat (a degrees or so) cima seems to go to a za
that is different than what the observer expected.
- Cima also seems to not be self consistent. For example:
- user picks source from the pointing menu
- the pointing menu reports an az,za for the current position of
- users starts fixed drift routine and telescope moves to a za
that is lower than what was just reported.
shows the za,vs azimuth for sources close to the ao lattitude
- the top frame shows feed azimuths close to 90 degrees. The bottom
frame has feed azimuths close to 270 degrees.
- Each colored line is the track for a source of a given dec
(labeled in the upper right corner).
- The vertical dotted lines show lines of constant az.
- There are two za values for the same azimuth when:
- az > 90, dec > ao lattitude, source setting
- az < 270, dec > ao lattitude, source rising.
- this is just how the ra,dec coordinates map onto the az,za
- When the user selects the source from the catalog and asks for
the current az,za they can get the larger or smaller za.
- When the fixed drift routine has to choose between the two za's,
it always picks the smaller za,hour angle with the smaller za.