Image astrometry parameters

RTS2 is able to support on-line astrometry, with corrections being fed back to the telescope as correction parameters, thus significantly improving the telescope pointing. Following is list of parameters which affect astrometry, and a guide how to set them up. These parameters are arguments to the rts2-camd-* device drivers and are best set in the file /etc/rts2/devices.

• 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. If you have 0.3 arcsec in X and 0.2 arcsec in Y (=non-square pixel size), you specify –plate 0.3:0.2 (not supported on the astrometry side, though ). 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 (if you restart camera daemon, they will be lost). XPLATE and YPLATE are present only when –plate parameter is included among camera daemon parameters. Precision needed for the astrometry to run smoothly is relatively high (~5%), so it is best to get the values from dry runs of the astrometric routine. Crude estimate may be done in various ways, pixel_size[arcsec]=pixel_size[um]*206.265/focal_length[mm] being a quite useful formula. A good way is also getting some known field (with a nebula or a small galaxy) and trial-error with the astrometric routine.

• 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. It 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 the 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 an 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 changes on german equatorial mount (GEM) by 180 degrees if you cross pole and go to the opposite side of the sky - eg. from hour angle 2 to hour angle 14. Default mount ROTANG is specified by -r parameter, and shows as MNT_ROTA in rts2-mon. You can also edit it there. MNT_ROTA changes 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 the variable is created. RTS2 sums all variables with RTS2_DT_ROTANG flag set to form resulting ROTANG, which is then 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);
        }