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====== Telescope pointing algorithm ======

All coordinates entered to telescope control are in J2000. Using [[http://libnova.sf.net|libnova]], telescope driver can handle all coordinate transformations. With own class set, it can also handle most of TPoint telescope pointing terms.

Calculating coordinates on which telescope will be sent is a complex process. It is make more transparent by those variables, which are displayed in //rts2-mon// and also written to the FITS header of image. Writing takes place at the beginning of the exposure. Following list provides they name as written to FITS header. You will find them in //rts2-mon// when you remove substrings - e.g. **ORIRA, ORIDEC** pair is displayed as **ORI**.

Those values are important:

  *  **ORIRA**, **ORIDEC** contains original, J2000 coordinates. Those area usually entered by observing program (rts2-executor).
  *  **OFFSRA**, **OFFSDEC** contains offsets applied to ORI coordinates.
  *  **OBJRA**, **OBJDEC** contains offseted coordinates. //OBJRA = ORIRA + OFFSRA, OBJDEC = ORIDEC + OFFSDEC//.
  *  **TARRA**, **TARDEC** contains precessed etc. coordinates. Those coordinates do not contain modelling, which is stored in MODRA and MODDEC.
  *  **CORR_RA**, **CORR_DEC** contains offsets from on-line astrometry which are fed to telescope. Telescope coordinates are then calculated as //TARRA - CORR_RA - MORA//, //TAR_DEC - CORR_DEC - MODEC//
  *  **TELRA**, **TELDEC** contains coordinates read from telescope driver. In ideal word, they should equal to //TARRA - CORR_RA - MORA//, //TARDEC - CORR_DEC - MODEC//. But they might differ. The two major sources of differences are: telescope do not finish movement as expected and small deviations due to rounding errors in mount or driver.
  *  **MORA**, **MODEC** contains offsets coming from pointing model. They are shown only if this information is available from the mount (OpenTpl) or when they are calculated by RTS2 (Paramount).

Following auxiliary values are used to track telescope offsets. Due to their transient nature, they are visible only in //rts2-mon//, and aren't written to FITS headers.

  * **woffs** contains offsets which weren't yet applied. They can be set either directly or when you change OFFS value. When move command is executed, //OFFSRA += woffsRA, OFFSDEC += woffsDEC// and //woffsRA// and //woffsDEC// are set to 0.
  * **wcorr** contains corrections which weren't yet applied. They can be set either directly or by correct command. When move command is executed, //CORRRA += wcorrRA, CORRDEC += wcorrDEC// and //wcorrRA = 0, wcorrDEC = 0//.

They are two algorithms worth mentioning. First is how OFFSets are handled, so telescope offseting do not cause observatory to move telescope during exposures. Second, similar algorithm is used for applying corrections (wcorr and CORR_ variables).

If you are interested to see how the code works, please see startResync() documentation for functions available to retrieve various coordinates. startResync is the routine which is called each time coordinates written as target should be matched to physical telescope coordinates. Using various methods in the class, driver can get various coordinates which will be put to telescope. You can also see [[code:telescope_driver]] for description how to add new telescope driver.