Stacked Observation Detections Image Files (img3.fits, img3.jpg)
FITS
The stacked observation detections image files consist of a single FITS format image file for each set of stacked observation detections and science energy band; they are the image equivalents of the stacked observation detections event files filtered by the appropriate science energy band, ACIS stacks are blocked by 1 and HRC stacks 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 stacked observation detections background image in the appropriate science energy band from the filtered and blocked stacked observation detections event data, and dividing the result by the stacked observation detections exposure map.
Stacked ACIS Full-Field Events Image
![[Thumbnail image: stacked ACIS full-field events image]](imgs/acisfJ0035508m433654_stk_field_img3.png)
[Version: full-size]
Stacked ACIS Full-Field Events Image
An example of a stacked full-field events image for a set of ACIS observations.
The stacked observation detections image FITS file is named: 〈i〉〈s〉〈stkpos〉_〈stkver〉N〈v〉_〈b〉_img3.fits
where 〈i〉 is the instrument designation; 〈s〉 is the data source; 〈stkpos〉 is the position component of the stack name, formatted as "Jhhmmsss{p|m}ddmmss"; 〈stkver〉 is the 3-digit version component of the stack name, formatted with leading zeros; 〈v〉 is the data product version number; and 〈b〉 is the energy band designation.
![[CAUTION]](../../imgs/caution.png)
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
JPEG
A set of JPEG format image files are also produced for each set of stacked observation detections. For ACIS observations, multicolor images are generated by combining the image equivalents of the stacked observation detections event file filtered by the soft, medium, and hard science energy bands, blocked at multiple resolutions in SKY coordinates, according to the algorithm of Lupton et al. (2004, PASP 116 133). For HRC observations, the images are generated from the image equivalent of the stacked observation detections event file filtered by the wide science energy band, blocked at multiple resolutions in SKY coordinates. In both cases, the images are exposure corrected by dividing the filtered and blocked stacked observation detections event data by the stacked observation detections exposure map.
The stacked observation detections JPEG image files are named: 〈i〉〈s〉〈stkpos〉_〈stkver〉N〈v〉_〈f〉_img3.jpg
where 〈i〉 is the instrument designation; 〈s〉 is the data source; 〈stkpos〉 is the position component of the stack name, formatted as "Jhhmmsss{p|m}ddmmss"; 〈stkver〉 is the 3-digit version component of the stack name, formatted with leading zeros; 〈v〉 is the data product version number; and 〈f〉 is the JPEG blocking factor. Each image will be 1024⨯1024 pixels in size.
Related Science Documents
- ahelp: dmbinning
- ahelp: dmfiltering
- CIAO: True Color Images
- CIAO: Single Chip ACIS Exposure Map and Fluxed Image
- CIAO: Multiple Chip ACIS Exposure Map and Fluxed Image
- CIAO: Match the Binning of an Image