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Telescope pointing algorithm

All coordinates entered to telescope control are in J2000. Using 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 telescope_driver for description how to add new telescope driver.

code/telescope_poiniting.txt · Last modified: 2009/10/07 00:00 (external edit)