Source detection can be applied to image data and to photon event data.
In the local and the map detection
a 3 
3 image pixels cell is slided over the image.
Source counts are determined within the cell, background counts
either from an area surrounding the cell (local background) or
from a (smoothed) background image within the same cell.
A maximum likelihood detection can be applied to photon event file data.
All of these tasks can be done using a single command
(see next chapter).
Here the steps are performed individually for a better understanding
whats going on.
There are some preparatory tasks to be done before a source detection can be started, this is the creation of an image and the creation of a mask.
pixels and the image pixel size must be
30 sky pixels (15 arcsec) for ROSAT PSPC. For ROSAT HRI and WFC
the binsize is different
(see Table 5.2).
There is an automatic command to bin an image fulfilling these
requirements and it is highly recommended to use this command.
We also pass the name of the eventrates table in order to fill
a descriptor LIF_TIM which is later on evaluated in the
maximum likelihood analysis task to determine the count rates
of the sources.
Midas 001> create/source_detect_image testdata1:events 11-235 -
? ? testdata1:eventrates
An image image1.bdf will be created in the calibrated amplitude channel range 11 to 235.
Mean xoffset, yoffset and roll are determined in an automatic manner from the attitude table (in the testdata area testdata1).
Midas 002> adjust/mask image1 EXSAS_CAL:mask0thick ? ? ? -
testdata1:attitude load
An adjusted mask called mask.bdf is created. It is strongly recommended to control at this point the correct overlay of the mask on the image (alwaysuse the default load display option).
An alternative way to create an adjusted PSPC mask is via the command CREATE/WOBBLED_MASK (see section 5.2.35 for details).
Now the local source detection can be performed. For this task we create a parameter file dlpar.par with the proper defaults for the ROSAT PSPC in POINTING mode and start the local detection:
Midas 003> create/parfil EXDL dlpar ROSAT,PSPC Midas 004> detect/local dlpar editThe parameter file dlpar.par is created and offered to be edited. The parameters were set for the POINTING mode of the ROSAT X-ray telescope with the PSPC detector (ROSAT,PSPC). Other allowed experiment identifiers for CREATE_PARFIL would have been ROSAT,HRI, ROSAT,WFC and ROSAT,SURVEY.
! ! parameter file for DETECT/LOCAL - task EXDL ! MISSION,DETECTOR ROSAT,PSPC ! mission and detector OBS_MODE POINTING ! pointing or survey NUM_IMAGES 1 ! number of images IMAGE_1 image1.bdf ! first image MIN_ML 8.00 ! minimum of ML MASK_FLAG T ! flag to use mask MASK mask.bdf ! mask LDETECT_LIST_1 lslst1.tbl ! first LDET source list
After editing the file (if necessary) the local source detection begins and a source table lslst1.tbl containing the sources found is created. Next a smoothed background image will be created for the map detection. This can be done with the command CREATE/BG_IMAGE.
Midas 005> create/parfil EXDB dbpar ROSAT,PSPC Midas 006> create/bg_image dbpar editThe parameter file dbpar.par is created and offered to be edited.
! ! parameter file for CREATE/BG_IMAGE - task EXDB ! MISSION,DETECTOR ROSAT,PSPC ! mission and detector OBS_MODE POINTING ! pointing or survey NUM_IMAGES 1 ! number of images (1-6) IMAGE_1 image1.bdf ! first image MIN_ML 8 ! minimum ML CUT_RADIUS_FWHM 2.000 ! cut radius in FWHM LDETECT_LIST_1 lslst1.tbl ! first LDET source list MASK_FLAG T ! flag to use mask (T/F) MASK mask.bdf ! mask EXPOSURE_FLAG T ! flag to use exposure EXPOSURE_IMAGE_1 exposure.bdf ! exposure image BG_IMAGE_1 bacmp1.bdf ! first backg. imageThe exposure image can be the broad band exposure from the ROD or the exposure created by the CREATE/EXPOSURE_IMAGE command. Note, that the exposure image is not used to correct the output background image, which has to be uncorrected. The exposure map is only used to obtaina smoother spline fit to the background. The CREATE/BG_IMAGE command is recommended to be used to create the background image. However, as an alternative another command to create a smoothed background image exists, which does not apply a spline fit procedure:
Midas 007> CREATE/SMOOTH_BG image1.bdf bacmp1.bdf lslst1.tbl 4 16The smoothed background image bacmp1.bdf obtained either by CREATE/BG_IMAGE or CREATE/SMOOTH_BG_IMAGE is used in the map detection which is started with the commands
Midas 008> create/parfil EXDM dmpar ROSAT,PSPC Midas 009> detect/map dmpar editThe parameter file dmpar.par is created and offered to be edited. The parameters were set according to the POINTING mode of the ROSAT X-ray telescope with the PSPC detector.
! ! parameter file for DETECT/MAP - task EXDM ! MISSION,DETECTOR ROSAT,PSPC ! mission and detector OBS_MODE POINTING ! pointing or survey NUM_IMAGES 1 ! number of images (1-6) IMAGE_1 image1.bdf ! first image MIN_ML 8 ! minimum ML MASK_FLAG F ! flag to use mask (F/T) MASK mask.bdf ! mask BG_IMAGE_1 bacmp1.bdf ! first backg. image MDETECT_LIST_1 mslst1.tbl ! first MDET source listThe mask need not be used (parameter MASK_FLAG set to F) as the background spline has been done using the exposure image (containing the wobbled support structure). Source tables created by the local and the map detection lslst1.tbl and mslst1.tbl can be merged to one single table mplst.tbl by the commands
Midas 010> create/parfil EXSM smpar ROSAT,PSPC Midas 011> merge/source_tables smpar editSources will be merged to a new source in case their detection cells have a distance of less than MIN_DIST_IN_CELL or they overlap in the FWHM
the factor MIN_DIST_IN_FWHM.
The parameter file smpar.par is created and offered to be
edited. The parameters were set for the ROSAT X-ray telescope
with the PSPC detector.
! ! parameter file for MERGE/SOURCE_TABLES - task EXSM ! MISSION,DETECTOR ROSAT,PSPC ! mission and detector OBS_MODE POINTING ! pointing or survey NUM_IMAGES 1 ! number of images (1-3) LDETECT_LIST_1 lslst1.tbl ! first LDET source list MDETECT_LIST_1 mslst1.tbl ! first MDET source list MIN_DIST_IN_FWHM 1.00 ! min source dist in FWHM MIN_DIST_IN_CELL 2.00 ! min source dist in Cell BG_IMAGE_1 bacmp1.bdf ! first backg. image MERGED_LIST mplst.tbl ! merged source listPhotons lying in circles centered at the merged source positions can finally be analysed by a maximum likelihood detection technique. One has to submit the commands
Midas 012> create/parfil EXSP sppar ROSAT,PSPC Midas 013> detect/maxlik sppar editThe parameter file sppar.par will be created and offered to be edited. The parameters were set for the ROSAT X-ray telescope with the PSPC detector.
! ! parameter file for DETECT/MAXLIK - task EXSP ! MISSION,DETECTOR ROSAT,PSPC ! mission and detector OBS_MODE POINTING ! pointing or survey INPUT_DATASET testdata1:events.tbl ! name of events table AMPLITUDE_LOW 11 ! lower amplitude bound AMPLITUDE_HIGH 235 ! upper amplitude bound CUT_RADIUS 2.50 ! cut radius in FWHM EXCLUDE_SIGMA 5.00 ! thresh.f.source exclude OFF_AX_MIN 0.00 ! min off-axis angle OFF_AX_MAX 57.00 ! max off-axis angle ML_THR 10.0 ! threshold of -ln(p) START_POSITION merged_list ! detection mode MERGED_LIST mplst.tbl ! merged source list USER_LIST uslst.tbl ! user source list FIT_POSITION not active ! fit source position BG_IMAGE_1 bacmp1.bdf ! first background image ML_LIST_ACCEP solst.tbl ! list accepted sources ML_LIST_ALL mllst.tbl ! list all sourcesThe starting positions of the maximum likelihood fits are taken from table mplst.tbl which was created in the previous step. Alternatively one could have used a user-defined source list (in this case parameter START_POSITION would have had to be set to user_list).
The output table solst.tbl contains all sources accepted by the maximum likelihood technique and having a likelihood larger than the threshold value ML_THR. We read the table which contains 12 sources.
Midas 014> read/tab solst
Table : solst
Sequence INDEX IND_MP TOTNUM X_POS Y_POS CTS_MP BG_MP
-------- ----- ------ ------ --------------- --------------- -------- ------
1 1 1 974 309.64 59.34 186.76 0.20
2 2 3 1342 273.80 106.41 300.37 0.31
3 3 4 943 313.24 119.96 137.35 0.34
4 4 11 405 126.81 196.10 75.43 0.12
5 5 13 130 274.94 202.30 16.33 0.46
6 6 14 506 378.57 203.40 51.54 0.19
7 7 15 2983 285.03 204.92 2321.00 0.42
8 8 16 97 293.97 211.25 13.56 0.37
9 9 17 162 295.00 215.47 13.68 0.37
10 10 21 626 399.52 247.99 131.23 0.17
11 11 25 84 283.98 273.16 27.12 0.29
12 12 32 469 120.56 367.90 36.26 0.08
-------- ----- ------ ------ --------------- --------------- -------- ------
Sequence ML_MP X_IMA Y_IMA CTS EXI_ML XERR YERR CERR EXT
-------- ----- ------ ------ -------- ------- ------ ------ ----- ------
1 13 307.42 60.82 163.25 37.5 1.54 1.09 22.3 0.00
2 60 272.98 106.45 835.81 920.8 0.22 0.20 32.7 1.99
3 25 313.22 120.55 177.43 83.1 0.53 0.53 18.4 0.00
4 14 127.55 197.02 138.61 87.5 0.62 0.64 14.5 3.83
5 13 275.38 202.52 88.81 73.7 0.23 0.31 9.2 0.87
6 23 375.58 203.28 79.15 10.6 2.46 1.78 16.6 5.69
7 8 284.28 205.05 20721.82 17605.2 0.03 0.20 54.5 0.80
8 11 294.02 212.52 34.20 17.4 0.46 0.58 7.8 1.32
9 12 292.05 213.32 66.04 19.8 0.49 0.50 10.8 2.30
10 21 399.65 247.85 267.42 200.4 0.45 0.48 20.1 1.95
11 8 283.98 273.18 41.62 43.3 0.30 0.28 7.4 1.00
12 16 121.18 367.75 109.68 65.6 0.88 0.87 13.5 2.77
-------- ----- ------ ------ -------- ------- ------ ------ ----- ------
Sequence EXTERR EXT_ML FA PRIO VIG_COR RATE ERATE
-------- ------ ------ ------ ---- ------- --------- ---------
1 0.000 0.0 0.580 48 1.846 0.14435 0.01968
2 1.006 8.9 0.685 48 1.510 0.60450 0.02367
3 0.000 0.0 1.000 48 1.453 0.12345 0.01278
4 1.808 14.3 1.000 48 1.316 0.08734 0.00916
5 0.616 2.9 0.580 0 1.110 0.04723 0.00488
6 3.094 4.8 1.000 32 1.422 0.05390 0.01133
7 0.212 154.8 0.580 32 1.139 11.30528 0.02974
8 0.959 1.0 0.580 32 1.145 0.01876 0.00429
9 1.044 15.0 1.000 32 1.136 0.03592 0.00590
10 1.570 1.2 1.000 48 1.481 0.18961 0.01424
11 0.735 3.1 1.000 48 1.056 0.02105 0.00373
12 2.324 1.1 1.000 48 1.499 0.07876 0.00971
-------- ------ ------ ------ ---- ------- --------- ---------
The parameters of the source tables are described in Table
5.29.
We convert the image pixel coordinates to
sky pixel,
equatorial,
ecliptic and
galactic coordinates for equinox 2000 with the command
Midas 015> transform/coord solst solst_2000 2000.0and we read the table solst_2000.tbl
Midas 016> read/tab solst_2000
Table : solst_2000
Sequence INDEX X_IMA Y_IMA X_SKY Y_SKY
-------- ------ ----------- ----------- ---------------- ----------------
1 1 307.42 60.82 1527.00 -5871.00
2 2 272.98 106.45 494.00 -4502.00
3 3 313.22 120.55 1701.00 -4079.00
4 4 127.55 197.02 -3869.00 -1785.00
5 5 275.38 202.52 566.00 -1620.00
6 6 375.58 203.28 3572.00 -1597.00
7 7 284.28 205.05 833.00 -1544.00
8 8 294.02 212.52 1125.00 -1320.00
9 9 292.05 213.32 1066.00 -1296.00
10 10 399.65 247.85 4294.00 -260.00
11 11 283.98 273.18 824.00 500.00
12 12 121.18 367.75 -4060.00 3337.00
-------- ------ ----------- ----------- ---------------- ----------------
Sequence RA_DEG DEC_DEG RA_HMS DEC_DMS LAMDA BETA
-------- --------- --------- ------------ ------------ --------- ---------
1 84.65453 -69.14319 05H38M37.08S -69D08M35.4S 305.51321 -86.72667
2 85.05576 -69.33425 05H40M13.38S -69D20M03.3S 301.52170 -86.66479
3 84.57896 -69.39181 05H38M18.95S -69D23M30.5S 303.18921 -86.51739
4 86.79947 -69.70490 05H47M11.87S -69D42M17.6S 289.28500 -86.63853
5 85.02320 -69.73449 05H40M05.56S -69D44M04.1S 297.97925 -86.32777
6 83.81789 -69.73201 05H35M16.29S -69D43M55.2S 303.12567 -86.08572
7 84.91598 -69.74487 05H39M39.83S -69D44M41.5S 298.37677 -86.29890
8 84.79817 -69.77573 05H39M11.56S -69D46M32.6S 298.64059 -86.25076
9 84.82181 -69.77911 05H39M17.23S -69D46M44.8S 298.50769 -86.25242
10 83.51328 -69.91503 05H34M03.18S -69D54M54.1S 302.65878 -85.87723
11 84.91508 -70.02877 05H39M39.61S -70D01M43.5S 296.12500 -86.05788
12 86.93250 -70.41533 05H47M43.79S -70D24M55.1S 284.84705 -85.98586
-------- --------- --------- ------------ ------------ --------- ---------
Sequence LII BII OFFAX FWHM DIST
-------- --------- --------- ---------- ---------- ----------------
1 279.51648 -31.67472 50.55 554.80 1524.78
2 279.72064 -31.51196 37.74 354.17 1088.73
3 279.81027 -31.67233 36.83 342.05 1088.75
4 280.08237 -30.86869 35.51 325.07 4247.82
5 280.18854 -31.47833 14.30 160.13 87.35
6 280.24686 -31.89288 32.61 290.21 1329.13
7 280.20569 -31.51401 14.62 161.01 87.36
8 280.24725 -31.55085 14.45 160.54 -126.57
9 280.25006 -31.54233 13.98 159.30 -126.57
10 280.47757 -31.97162 35.85 329.39 1329.18
11 280.53635 -31.48080 8.03 150.29 1621.97
12 280.90182 -30.76145 43.79 442.15 4935.35
-------- --------- --------- ---------- ---------- ----------------
The parameters are described in Table
5.24.
We load the image image1.bdf and display the source positions
into the overlay channel.
Midas 017> draw/source lslst1 Midas 018> clear/chan o Midas 019> draw/source mslst1 Midas 020> clear/chan o Midas 021> draw/source solstTo display the detected sources as circles with a radius given by the size of the point spread function as in Figures 5.3 to 5.5, a column specifying the FWHM of the point spread function (in units of image pixels) has to be created. This can be done with a set of commands:
Midas 022> transform/coordinates lslst1 lslst1_2000 Midas 023> compute/table lslst1_2000 :FWIMA=:FWHM/30. Midas 024> draw/source lslst1_2000 1 circle white :FWIMA Midas 025> transform/coordinates mslst1 mslst1_2000 Midas 026> compute/table mslst1_2000 :FWIMA=:FWHM/30. Midas 027> draw/source mslst1_2000 1 circle white :FWIMA Midas 028> transform/coordinates solst solst_2000 Midas 029> compute/table solst_2000 :FWIMA=:FWHM/30. Midas 030> draw/source solst_2000 1 circle white :FWIMAStandard result files are created by source detection tasks, which need not be kept on disk (see Table 5.31). These files can be deleted with the command
Midas 031> remove/detection_files NONow the source detection is completed and the analysis (spatial, spectral, timing) on individual sources can be started.
Figure 5.1:
Mask image EXSAS_CAL:mask0thick.bdf before adjusted by the
command ADJUST/MASK (left upper figure),
after adjustment(right upper figure),
and blinded mask image created by the command
CREATE/BG_IMAGE.
This image has been created with the setting MASK_FLAG = F.
Circular regions (with a radius of 2 
the FWHM of the
point spread function) are excluded at positions found by the
local detection (command DETECT/LOCAL) from a mask
with constant intensity.
Figure 5.2:
ROSAT PSPC image (left upper figure). Cheesed image
(right upper figure) created by the command CREATE/BG_IMAGE.
This is the image with the removed source positions,
found by the command DETECT/LOCAL in circles with
a radius of 2 
the FWHM of the point spread function.
Binned cheesed image before bicubic spline fit has been applied
(left lower figure).
Background image (right lower figure) created by the command
CREATE/BG_IMAGE.
It is a bicubic spline-fit to the binned cheesed image.
Figure 5.3:
Source positions determined from the local detection (command
DETECT/LOCAL). The circles drawn with the command
DRAW/SOURCE_POSITIONS are centered at the source
positions and the radius of the circles are equal to the FWHM of the
point spread function. Sequence numbers are displayed for the
detected sources.
Figure 5.4:
Source positions determined from the map detection (command
DETECT/MAP). The circles drawn with the command
DRAW/SOURCE_POSITIONS are centered at the source
positions and the radius of the circles are equal to the FWHM
of the point spread function.
Sequence numbers are displayed for the detected sources.
Figure 5.5:
Source positions determined from the maximum likelihood detection
(command DETECT/MAXLIK).
The circles drawn with the command DRAW/SOURCE_POSITIONS
are centered at the source positions and the radius of the circles
are equal to the FWHM of the point spread function.
Sequence numbers are displayed for the detected sources.