Detailed information about how TCS calculates pointing, to get to a given object at a given time

Slalib is the library that almost all telescopes use to do their initial pointing calculations. Haven't found the PDF for this manual yet. (4 scanned pages attached at the bottom of this wiki page)

The first page shows the basic flow. We convert from mean to apparent by applying precession, proper motion, and possibly parallax. These are terms that don't change quickly but get us to the current calendar date.

The conversion from apparent to observed coordinates, the place you actually point your telescope, is where all the messy external variables come in. Things like time of day, your lat/long/elevation on the earth, the non-spherical shape of the earth, as well as the atmosphere. The refraction terms are described starting at the bottom of page 3 of the attachment. In a nut shell the A and B terms are calculated by integrating though layers of the atmosphere using the local weather conditions and an assumed lapse rate (how temperature and pressure change with altitude). You must do this integration for each position because the depth and length of the atmospheric layers changes depending on the altitude of your pointing.

Once we have the observed hour angle and declination the next step is to convert them into engineering coordinates. This is where Tech Report 43 comes in. It describes how of go from observed Ha/Dec to X/Y/Z/Theta/Phi/Rho. However, these X/Y/Z coordinates are for an assumed perfect tracker, what we refer to as the Ideal Tracker Frame (ITF). These ITF coordinates are turned into the Real Tracker Frame (RTF) by applying the mount model corrections which convert the nice orthogonal ITF coordinates into some warped RTF coordinate frame. Finally, the RTF coordinates are sent to the tracker were they are converted into encoder positions for all the motors in what is called the Tracker Sensor Frame (TSF).

The reverse transformations through all these steps are what is used to go from the TSF of the tracker back to J2000 Ra/Dec as reported on the Tcs Gui front panel. Steps that can be even messier that the forward transforms.

Details on the SLA_REFCO routine to calculate the coefficients A & B are here ​http://star-www.rl.ac.uk/docs/sun67.htx/sun67ss156.html

Our implementation uses the following hard-coded values:

• effective wavelength WL, is 0.55 microns
• lapse rate TLR is 0.0065 degK/meter.

These are values provided by the Slalib package.

We also use the temperature, pressure, and humidity values from from the external weather tower.