Caveat: Zero-exposure Event File

In some observations, the images of aspect fiducial lights (projections of fiducial lights onto the aspect camera detector) were not acquired for part or all of the observation. The reported aspect quality for these observations is bad, which causes empty Good Time Intervals (GTIs). If there is zero good time defined for an observation, all the events are screened out of the final level=2 event file.

The lack of FID lights is not a simple matter of absolute coordinates being offset. During many observations, the Science Instrument Module (SIM) drifts by up to an arcsec; without FIDs, this does not get corrected. On account of this, spurious nebulosity, etc., may be revealed when analyzing these datasets. Additionally, combining data from a multi-obi observation affected by this problem may produce double sources in the final event file.

Similarly, in some other observations of either the Moon or the Earth, Chandra will be violating the allowed viewing angles. This violation will cause the observations to have empty GTIs and zero exposure time.

If you are attempting analysis that reveals one of these problems, email the CXC Helpdesk for guidance. To create a new event file for basic analysis procedures, use the provided workaround.

Workaround for both imaging and grating datasets:

The fact that a) there were no guide-stars to the OBC aspect solution was used and/or b) that the observation violated the pointing-angles is contained in the mission timeline (mtl1.fits) file. Specifically, users can look at the bit-encoded LIMIT_STATUS column in the MTL_S (smoothed mission time line) to see which limits were violated.

unix% % dmlist "acisf00963_001N005_mtl1.fits[mtl_s][cols limit_status]" data,clean | uniq
#  limit_status[3]
 1100000010000100000
 1100000000000100000
 1100000000001100000
 1100000000000100000

For the mission time line, we count bits starting at 1, going from left to right. All the rows have bits: 1, 2, and 14 set, meaning that those limits were violated; the limits are specified in the LIMITS extension

unix% % dmlist "acisf00963_001N005_mtl1.fits[limits][#row=1,2,14]" data,clean | tr -s " "
# gti_limits
 (img_diam_rms:1<=0.5) 
 (img_roll_rms:1<=40.0) 
 ((0<=asp_sol_status:1)&&(asp_sol_status:1<256)) 

These each relate to the fact no FID data was available. For Earth and Moon observations, the fabs(Point_MoonLimbAng:1) or fabs(Point_EarthLimbAng:1) limits are typically violated. Note: the row numbers/bit locations are not fixed and may vary between ACIS, HRC, and Gratings datasets.

Use the dmgti tool to create a new GTI file (flt1.fits) which does not include those limits from the MTL. For example:

unix% dmgti acisf00963_001N005_mtl1.fits new_963_flt1.fits \
      lkupfile=acisf00963_001N003_mtl1.fits"[limits][exclude #row=1,2,14]" \
      userlimit=""

Then reprocess the level=1 data to create a new level=2 event file using the new flt1.fits file in place of the one from SDP. For help reprocessing your data, follow the Create a New Level=2 Event file thread.

Finally, one may create new GTIs to remove background flares by completing the Eliminate High Background Times section of the Filtering Data thread.