Fixed az driftPattern at transit

3apr10

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 observatory lattitude.

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:

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 awhile.
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 the source.
users starts fixed drift routine and telescope moves to a za that is lower than what was just reported.

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.

The plot shows the za,vs azimuth for sources close to the ao lattitude (.ps) (.pdf):

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 coordinates.

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.

processing: x101/100329/cima_fixedold.pro

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