[Last Change: 24 Feb 2012 (rev 3)]

Telescope Motion and Orientation

These functions specify telescope motion and orientation.

dither_asol

dither_asol() takes a single argument, a table, with the following recognized named entries:

name	type	description
file	string	the name of the Chandra asol1 file
ra	angle (default units of degrees) or sexigesimal	mean R.A. of the projection of the telescope optical axis.
dec	angle (default units of degrees) or sexigesimal	mean declination of the projection of the telescope optical axis.
roll	angle (default units of degrees)	mean telescope roll about the optical axis

Simulate motion of the telescope using a Chandra aspect solution file. Aspect solution files provide the absolute orientation of the spacecraft relative to the celestial sphere as it varies through an observation. This function instructs SAOTrace to use the aspect information in the specified file to modify the pointing of the telescope during the simulated observations. This does a number of things

The ra and dec parameters specify the coordinates of the projection of the mean position of the telescope's optical axis during the observation. Currently the best approximation to these (and the roll) for Chandra are the values of the RA_PNT, DEC_PNT, and ROLL_PNT header keywords in the Chandra observation's level 2 event file. For a very few observations these keywords are available only in the level 1 events file.

Example

To simulate dither using the HRC-I observation of NGC2516 (OBSID 27), extract the pointing data from the Chandra level 2 event file:

% dmkeypar hrcf00027_evt2.fits.gz RA_PNT echo+
119.53549452648

% dmkeypar hrcf00027_evt2.fits.gz DEC_PNT echo+
-60.763660833014

% dmkeypar hrcf00027_evt2.fits.gz ROLL_PNT echo+
331.21684998055

And then feed it to SAOTrace via:

dither_asol{ file = 'pcadf066989205N003_asol1.fits.gz',
             ra = 119.53549452648,
             dec = -60.763660833014,
             roll = 331.21684998055
           }

Caveats

The Chandra aspect solution files also provide the motion of the detectors relative to the optical axis during the observation. This motion is currently not applied and must be applied by the detector model.