Full-Field Image Files (img3.fits, img3.jpg)
The full-field image files consist of a single FITS format image file for each observation and science energy band; they are the image equivalents of the full-field event files filtered by the appropriate science energy band, ACIS observations are blocked by 1 and HRC observations are blocked by 2 in SKY coordinates, background-subtracted, and exposure-corrected. The images are recorded in flux units of photons s-1 cm-2 by subtracting the full-field background image in the appropriate science energy band from the filtered and blocked full-field event data, and dividing the result by the full-field exposure map.
ACIS Full-Field Events Image
The full-field image FITS file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_〈b〉_img3.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
A set of JPEG format image files are also produced for each full-field observation. For ACIS observations, images are generated each science energy band, equivalents of the full-field event file filtered by the soft, medium, and hard bands, blocked by 1 in SKY coordinates. For HRC observations, the images are generated from the image equivalent of the full-field event file filtered by the wide science energy band, blocked by 2 in SKY coordinates. In both cases, the images are exposure corrected by dividing the filtered and blocked full-field event data by the full-field exposure map.
The full-field JPEG image files are named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_〈f〉_img3.jpg
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 〈f〉 is the JPEG blocking factor. The optional discriminator identified in square brackets is included only for ACIS alternating exposure (interleaved) mode observations. Each image will be 1024⨯1024 pixels in size.