After making a successful pointing measurement and applying the CORRECTIONS, check the axial focus with the ZFOCUS command. And the ZFOCUS measurement should also be done several times to be sure that the measurement is consistent to 0.1 mm. If there's a choice, the focus procedure is more sensitive if the angular diameter of the planet is small. However, even focusing on Jupiter is better than nothing. The CORRECTIONS command will adopt the last focus fit if one types
CORRECTIONS * * F
Or one can substitute a number for F as with COL* and NULE.
If the above ZFOCUS was successful, a few measurements of the vertical focus with the XFOCUS command is wise (after checking and correcting any pointing errors with another POINTING observation and a CORRECTIONS). The fourth parameter for CORRECTIONS is the X focus value. So to accept the XFOCUS fit, type
CORRECTIONS * * * F
Again, one can substitute a number for F as with COL* and NULE.
When choosing the next object for pointing, one should select an object based upon its being positioned in an area of the sky with few or no other measurements and which will not require a long slew of the telescope. Ideally, one should systematically work one's way from one sky area to an adjacent area until the whole sky has been measured. But the few objects available for pointing rarely are positioned to make this possible.
The minimum number of objects required for a good pointing model fit is about 30 if they are well spaced over the sky. The more objects, the better the fit and the less suseptible the fit is to being spoiled by a spurious measurement. Although the signal to noise ratio goes down with elevation, it is important to make measurements at as low an elevation as is feasible. But note that the ``horizon'' as seen by the HHT is not at zero elevation, but ranges from 15 degrees in the south to 30 degrees in the north. A plot of this horizon vs azimuth is posted on the bulletin board.