Full-Field Background Image File (bkgimg3.fits)
The full-field background FITS format image files consist of a background counts image for each observation and science energy band. ACIS observations are blocked by 1 and HRC observations are blocked by 2 in SKY coordinates. The images are recorded in units of counts.
ACIS Background Map
The full-field background image FITS file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_〈b〉_bkgimg3.fits
where 〈i〉 is the instrument designation, 〈s〉 is the data source, 〈obs〉 is the observation identification, 〈obi〉 is the observation interval identification, 〈v〉 is the data product version number, 〈c〉 is the cycle, and 〈b〉 is the energy band designation. The optional discriminator identified in square brackets is included only for ACIS alternating exposure (interleaved) mode observations.
In many L3 image files, "NaN" values are used to represent data that is outside the field of view (e.g., in the full-field exposure-corrected images where [0 counts / 0 exposure]).
Users should be cautioned that performing seemingly simple operations with the affected files may cause programs to run very, very slow, e.g. using DS9's built in 'smoothing' operation or the tool dmimgcalc. The reason is that whenever the NaN values are encountered, a SIGFPE is generated and has to be dealt with; most applications choose to ignore SIGFPEs, but mechanically, they are still processed and use up significant CPU time.
Furthermore, performing any FFT-based operation (e.g., csmooth, wavdetect, aconvolve, apowerspectrum, etc.) will (a) take a very long time, and (b) will yield an output that is all NaNs. Since every pixel contributes to every frequency in Fourier space, and any operation with a NaN yields another NaN, all the output pixels are NaN.
CIAO users can replace NaN values with zeros (or any other value) accordingly:
ciao% dmimgthresh in.fits out.fits cut=INDEF value=0