doc: sbp_extract.org: add events usage notes

1 files changed, 20 insertions, 1 deletions

diff --git a/doc/sbp_extract.org b/doc/sbp_extract.org
index efd525e..5ce3330 100644
--- a/doc/sbp_extract.org
+++ b/doc/sbp_extract.org
@@ -1,7 +1,8 @@
 #+TITLE: Surface Brightness Profile Extraction
 #+AUTHOR: Weitian LI <liweitianux@live.com>
-#+DATE: 2016-06-03
+#+DATE: 2016-06-05
 
+* TODO sbp extraction procedure
 
 * NOTE: how =dmextract= calculate the area of extraction region
 The =dmextract= can take either *events* file or binned *image* file as the input file.  However, the extracted SBP may be *different*, mainly due to the area calculation.
@@ -46,9 +47,27 @@ Therefore, when =dmextract= calculate the area of the source extraction region,
 
 The exposure map is generated by the CIAO tool =fluximage= and has the identical dimensions as that of the input image.  So the =EXP_AREA= is the same as =AREA=.
 Thus the final =SUR_FLUX= is independent of the area of the extraction region, which is what we want.
+
 As a result, it is recommended to bin the events file into an image, and then extract SBP from the image.
 
 ** Event file as the input
+When using the cleaned events file as the input file to extract SBP, e.g.,
+#+BEGIN_SRC sh
+$ dmextract infile="evt2_c7_clean.fits[energy=700:7000][bin sky=@sbprofile.reg]" \
+            outfile="sbprofile.fits" \
+            exp="expmap_c7_e700-7000.fits" \
+            opt=generic
+#+END_SRC
+extra care should be taken to deal with the regions that beyond the CCD edges.
+
+Since the events file does not incorporate the CCD FoV information (=skyfov.fits=), =dmextract= has no idea about the CCD FoV and thus can just calculate the total area of the source extraction region (excluded the regions corresponding to the removed point sources), i.e., the =AREA= is the total area of the extraction region with point source areas subtracted.
+
+On the other hand, the exposure map is always an image with the CCD FoV information embedded, and has certain boundaries.
+When =dmextract= to calculate the =MEAN_SRC_EXP=, the =EXP_AREA= is the *intersection* between the above *source extraction region* and *exposure image*.
+If the *source extraction region* is beyond the *exposure image boundaries* (a un-rotated box that just enclose the CCD FoV), the =EXP_AREA= is then *smaller than* =AREA=.
+In consequence, the final calculated =SUR_FLUX= (with dimension of =EXP_AREA/AREA=) has dependency on the extraction regions and maybe underestimated.
+
+Therefore, using the events file as the input file for SBP extraction is *not* recommended, and if it is used, the original extraction regions should be additionally processed to account for the regions that beyond the CCD FoV, i.e., *intersect* then with the CCD FoV which is extracted from =skyfov.fits=.
 
 
 * References