few words from Chip Coldwell about

IOTA POINTING

March 20, 1997

Apparently since the installation of the ``acquisition TV cameras'' on the telescopes, most of the folks using the IOTA instrument have not bothered to fit pointing models after moving the telescopes.  If you're satisfied with this method then there is no need for you to continue reading this document.  However, it is possible to make substantial improvements in the telescope pointing if one is willing to invest the effort to acquire the data necessary to fit a pointing model.

Collecting The Data

The ``Telescope'' program includes a facility for logging the topocentric positions of stars and will also log the position of any siderostat which is in the ``tracking'' state.  By a remarkable coincidence, this is exactly the data needed to fit a pointing model. To use this facility, use the ``File : Log'' menu item.  The first time this menu item is invoked, it prompts the user for a name for the new log file before creating the file and entering the relevant data therein.  Subsequent invocations of the ``File : Log'' menu item will just append new data to the existing file.  To collect the data for a pointing model then, one simply tracks a number of stars and invokes
the ``File : Log'' menu item after each one is acquired. Probably the most critical thing for a successful pointing model is a good distribution of stars.  I recommend at least thirty stars with as large a zenith angle as possible.  A strategy which has been very successful for me in the past is the following.  Start out by using an old pointing model for the same telescope positions (this is just a matter of making a copy of one of the old configuration files and changing it's name to ``Telescope.Config'').  Since the first star will be the hardest, start with something near zenith.  Then pick a star only a few degrees away in some direction.  As long as the next star is only a small distance from the last one, the pointing offsets will not change by much and it can be rapidly acquired.  Continue moving out in small steps in the same direction until you reach a zenith angle where the stars are likely to be vignetted by the telescope domes.  Now move in a circle of nearly constant zenith angle until you have circumnavigated the sky.  The ``Voyager II'' planetarium program (which is installed on most of the Quadras) is a big help in finding stars.  It's startup default is Mount Hopkins at local time.  By clicking the mouse on one of the stars displayed by this program, you will get a data window which gives, among other things, the HR number for the star.  You can then track this star by invoking the ``Target : Bright Star Catalog...'' menu item in the ``Telescope'' program and entering the HR number found using ``Voyager II''.  The entire process of acquiring thirty stars using this method takes, with a little luck, about an hour and a half.  Don't forget to make a log entry after acquiring each star; otherwise you are wasting your time.

Fitting the Data

The fitting is done by an entirely separate program, called (surprisingly enough) ``PointingModel''.  So once you have collected all your stars, quit the ``Telescope'' program and start the ``PointingModel'' program. Open your log file in the ``PointingModel'' program using the standard Macintosh ``File : Open...'' menu item.  There should now be four windows displayed, a ``Message'' window with a friendly greeting from the programmer, a text window displaying your log file, and a window for each telescope with two polar plots and six fields displaying the pointing model parameters. The check boxes next to the pointing model parameters in these windows allow you to fix any of the parameters at any value. My experience has been that this never helps, but feel free to try it. The parameter fields themselves are editable.  If the parameter is fixed then the value entered in the field is it's fixed value; otherwise it is the starting guess used by the least squares fit. To actually do a fit, just push the ``Go'' button in the window for the telescope whose pointing model you want to update. After a few seconds the fit is finished, and the two polar plots display the data points. The full circle plot shows the altitude and azimuth of the points in the log file; now you can see how good your sky coverage was. The half circle plot shows the roll and tilt positions of the points in the log, with a red line indicating the direction and magnitude of the error which would result from the new pointing model.  The RMS error over the points in the log file for the new model is also displayed below the new parameters. Once you've pushed the ``Go'' button for each telescope (assuming you want to change the pointing models for both), you can quit the ''PointingModel'' program. The new parameters are then written out to the file ``PointingModel.Config''.

Updating the Pointing Model

By yet another remarkable coincidence, the new parameters written out to the ``PointingModel.Config'' file are in exactly the same format as they would appear in the ``Telescope.Config'' file.  Therefore, to update the pointing model, you need only open both of these files in your favorite text editor (e.g. ``BBEdit''), and  replace the six lines appropriate to Telescope A in the ``Telescope.Config'' file with the corresponding lines from the ``PointingModel.Config'' file, and similarly for Telescope B. It might be a good idea to make sure that you're not going to clobber the only copy you have of the original parameters, in case they turn out to be better than the ones you just fit.  That's it, you're done, save the changes and quit the editor. Now you can restart the ``Telescope'' program and see how well it works.
 


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