RTS2 is capable to support on-line astromety, with corrections being feed to the telescope as correction parametes, thus significantly improving telescope pointing. Following is list of how to setup parameters which affect astrometry.

• XPLATE and YPLATE - size of pixel in arcseconds in X and Y direction. This is specified by –plate <x>:<y> parameter, passed to camera daemon (preferably in /etc/rts2/devices file). If you have 0.3 arcsec in X and 0.2 arcsec inY (=non-square pixel size), you specify –plate 0.3:0.2. You must calculate this value, most probably from taking some know field (M42, ..), by diving catalogue star angular distances with measured pixel distances. XPLATE and YPLATE are normal variables, displayed in rts2-mon. Camera daemon change them when you change binning - their change should be immediately visible. You can also change them directly in rts2-mon - but those changes will not be recorded. XPLATE and YPLATE shows only when –plate parameter is present.

• FLIP - that depends on readout direction and used optics. Can be also regarded as indication of negative XPLATE - e.g. arcseconds rose in different direction along X axis then is common. Is 1 for most cameras, the only exception so far is Andor, which have 0 flip for EM channel and 1 for non-EM channel. This is set in driver and unless you put extra mirror in optics, should be correct. If needed, you can set flip with –flip parameter passed to camera daemon. Again, FLIP is visible in rts2-mon, and can be changed from rts2-mon.

• ROTANG, which is angle of north direction from vector Y axis. It is 0 when north is up on images, 90 when north is towards left (and FLIP is 1). ROTANG change on german equatorial mount (GEM) by 180 degrees if you cross pole and go to opposite side of the sky - eg. from hour angle 2 to hour angle 14. Default mount ROTANG is specified by -r parameter, and show as MNT_ROTA in rts2-mon. You can also edit it there. MNT_ROTA change on GEM by 180 degrees automatically. Please be aware, that ROTANG written to the FITS file can be sum of different variables, all having RTS2_DT_ROTANG flag set when creating variable. RTS2 automatically sum all variables having RTS2_DT_ROTANGE to form resulting ROTANG, which is written to FITS headers. Check your mount source code for details - e.g. for OpenTPL mount with derotator, derotator angle is added by this code:

        // nasmith derotator
        if (opentplConn->haveModule ("DEROTATOR[3]"))
        {
                createValue (derotatorOffset, "DER_OFF", "derotator offset", true, RTS2_DT_ROTANG, 0, true);
                createValue (derotatorCurrpos, "DER_CUR", "derotator current position", true, RTS2_DT_DEGREES);
                createValue (derotatorPower, "derotatorPower", "derotator power setting", false);
        }