APOINT - Auto Pointing & Focusing

The program APOINT can be run to remove the normal tedium of pointing and focusing the telescope. It can be run in continuum or spectral line mode with at present, a limited number of SiO sources from the FCRAO source catalog (see list below). To use the program, the observer still needs to set the main and reference positions. For the SiO masers, use the FCRAO catalog since the program needs to match the name of the source exactly. These include:
IKTAU ORION TCEPH TXCAM VXSGR UHER SPER RXBOO RLEO UORI XCYG MIRA TCAS RCAS IRC10216
Note for IRC10216, the measurement is made at $^{12}$CO J=1-0 or some other strong molecular line transition. In addition, the integration times need to be set by the observer since the program will not know how strong a given source is or the state of the weather. Be sure there is sufficient integration time to obtain good signal to noise. Finally, one needs to put the telescope into PCNORMAL mode. The pointing offsets are controlled by the AZPC and ELPC keywords. During the course of the APOINT program, these offsets are adjusted automatically to find the optimum pointing of the telescope.

The parameters associated with the APOINT program are:

APMODE

= desired operation mode. 1=POINT, 2=POINT+FOCUS(PS)+POINT, 3=POINT+FOCUS(5)+POINT.

APTOL

= desired pointing tolerance in millidegrees. The nominal value is 1.5.

APWEAT

= weather condition (0=clear,1=partly cloudy, 2=cloudy 3=precipitation). This has no effect on the operation of the program but is used when the Pointing log is automatically updated.

APOINT does not do any calibration measurements so it is prudent to do a CAL at the start. Thus, to initiate the program at the observer's console, one types:
REF STAR CAL STAR APOINT (A04 optional)

For a given pointing operation, a FIVE observation is activated. When the FIVE program is completed, APOINT automatically reduces the data (spectral line or continuum), plots the positions and intensities onto a graphics device. By default, thie device is the A14 (MODGRAPH) terminal in the control room. To direct the graphical output to the serial port on the UNIX workstation (accessible via the mterma program), the user add the additional argument A04. APOINT attempts to calculate the center of the data points (akin to the PEAK function in SPA). At this point, there are several possibilities.

1. If 1 or no positions are detected at the 3$\sigma$ level, then the program promptly gives up. You are either so far from the source or more likely, did not integrate long enough to detect the source. If the latter, then increase the integration time accordingly and restart APOINT.
2. If 2 to 4 positions are detected, a center is estimated from from the centroid positions and another FIVE is activated.
3. If the center fitting converges and the vector distance between the calculated center and the observed center is greater than APTOL, then the offset positions are updated automatically and another FIVE is activated. The results from previous FIVE observations are included in the next fit for the center position. A maximum of 5 iterations is allowed. Note that the updated offset positions are not the same values derived from the fit. However, with increasing number of iterations, the updated offsets become more comparable to those derived from the fit.
4. If the vector distance between the calculated center and the observed center is less than or equal to APTOL, then it is a convergent pointing, the offsets are set equal to those determined from the fit and the program finishes.

There is a fair amount of information dumped to A14. Observers should view this information to check whether the program is operating correctly. The updated pointing offsets are displayed on screen 1.

If a focus measurement is requested (APMODE=2 or 3), then the program activates PS observations (APMODE=2) at 3 focus positions (starting with the present focus) until the peak is bracketed. For APMODE=3, the peak at each focus is obtained from FIVE observations. Once the peak is bracketed, then a parabola is fit to the data to obtain the best focus. The program then activates yet another FIVE to obtain pointing at the best focus.

At the end of the final FIVE (assuming it converges), APOINT automatically updates the Pointing Log.