Below is the (shortened) version of an email exchange on marx-help
For observed files RA_PNT & DEC_PNT, in the event files, is the mean-pointing of the HRMA. We usually pick this to be location of the tangent point so RA_NOM = RA_PNT and DEC_NOM = DEC_PNT (ROLL_NOM = ROLL_PNT too, but not critical here).
The marx2fits outputs (in v5.0 and 5.1) has different values for the (RA|DEC)_NOM and the (RA|DEC)_PNT keywords.
Hi Kenny,
RA_NOM and DEC_NOM are indeed just the user input values that are
carried over to the fits file output.
As you found out by experimenting RA_PNT and DEC_PNT are calculated from
RA_NOM and DEC_NOM with the PointingOffsetY|Z and the roll angle.
PointingOffsetY|Z set "the misaligment between the HRMA optical axis and
the nominal pointing".
Unfortunately, that means that I don't know exactly why the given
default values were chosen. It is possible that the represent an offset
between aspect camera and HRMA, but if so I cannot find the the source
of those numbers.
PointingOffsetY/Z is not used at any position in the code except to
calculate the RA_PNT and DEC_PNT for the output. If you have no
misalignment between the optical axis and the pointing direction, you
can set them to 0 with no side effects.
Today, there is no offset between the optical axis and nominal pointing in observed Chandra data.
Below is the (shortened) version of an email exchange on marx-help
For observed files RA_PNT & DEC_PNT, in the event files, is the mean-pointing of the HRMA. We usually pick this to be location of the tangent point so RA_NOM = RA_PNT and DEC_NOM = DEC_PNT (ROLL_NOM = ROLL_PNT too, but not critical here).
The marx2fits outputs (in v5.0 and 5.1) has different values for the (RA|DEC)_NOM and the (RA|DEC)_PNT keywords.
Hi Kenny,
RA_NOM and DEC_NOM are indeed just the user input values that are carried over to the fits file output. As you found out by experimenting RA_PNT and DEC_PNT are calculated from RA_NOM and DEC_NOM with the PointingOffsetY|Z and the roll angle.
PointingOffsetY|Z set "the misaligment between the HRMA optical axis and the nominal pointing". Unfortunately, that means that I don't know exactly why the given default values were chosen. It is possible that the represent an offset between aspect camera and HRMA, but if so I cannot find the the source of those numbers.
PointingOffsetY/Z is not used at any position in the code except to calculate the RA_PNT and DEC_PNT for the output. If you have no misalignment between the optical axis and the pointing direction, you can set them to 0 with no side effects.
Today, there is no offset between the optical axis and nominal pointing in observed Chandra data.
closes #10