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
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.