Using dmcoords to Convert between CHIP and SKY Coordinates

Background Information

Sample ObsID used: 1838 (ACIS-S, G21.5-09)

File types needed: evt2; asol1

To connect sky and chip coordinates, we need to take into account not only the celestial pointing direction (RA and Dec) of the telescope optical axis, but also the position of the instrument bench in the frame of the telescope mirror.

• the motion of the SIM which puts ACIS or HRC in the focus at a particular location
• the time-variable bending of the telescope tube and other drifts in the Chandra geometry.

This latter correction is measured by the fiducial optical system and is contained in the aspect solution (asol1.fits) files as columns dy and dz, giving shifts in mm of the focal plane relative to the optical axis. These values change very slowly over the course of an observation (about one or two pixels); it is taken into account in the calculation of sky coordinates in the pipeline by acis_process_events.

Because the mean values of dy and dz were also small early in the mission, dmcoords does not by default take them into account. However, five years into the mission the mean value has now grown to the order of 10 arcseconds and so cannot be neglected.

Accounting for the Mean Offsets

To get an accurate chip to sky conversion with dmcoords, one must determine the mean offsets and supply them to the tool. You can do this very simply by providing the aspect solution file (pcad_asol1.fits) in the asolfile parameter:

unix% pset dmcoords asolfile="pcadf084244404N001_asol1.fits"

This lets dmcoords pick up the correct conversion from the aspect solution.

In some cases there will be more than one aspect solution file asol1.fits file. All the files must be input to the asolfile parameter, either as a list or as a stack (see ahelp stack for more information).

Verifying the Results

You can check the results by finding a bright source (for good statistics) and using dmstat on the events in the event file. Then check that the results agree with those given by dmcoords. Since acis_process_events has always applied the full aspect solution correction, including the dy/dz correction, the chip and sky coordinates for individual photons in the event files are fine - the problem only arises in dmcoords when you are trying to predict positions.

unix% dmstat "acisf01838N001_evt2.fits[sky=circle(4070,4249,14.024608)][cols chip]"
chip(chipx, chipy)[pixel]
    min:        ( 336 353 )           @:        ( 1278 391 )
    max:        ( 395 412 )           @:        ( 1649 2567 )
   mean:        ( 365.80169554 382.8641725 )
  sigma:        ( 12.5906856 12.709801407 )
    sum:        ( 1984840 2077421 )
   good:        ( 5426 5426 )
   null:        ( 0 0 )

unix% pset dmcoords asolfile=pcadf084244404N001_asol1.fits
unix% dmcoords acisf01838N001_evt2.fits x=4070 y=4249 option=sky mode=h
unix% pget dmcoords chipx chipy
371.0480782140089
381.5545050249841

The discrepancy is about 5 pixels. Compare this to the results obtained from dmcoords without the aspect solution:

unix% punlearn dmcoords
unix% dmcoords acisf01838N001_evt2.fits x=4070 y=4249 option=sky mode=h
unix% pget dmcoords chipx chipy
374.6091485412975
391.6940783299734

The difference is greater than 10 pixels without the aspect solution applied.