While attempting to make a flat using many non-uniform images---I don't
have any proper flats for these data---I noticed an anomaly.
This was the effective command line.
makeflat in="*pre(~2000,~2000)" out=firsflat method=br keepin minpix=1 \
title='Output from MAKEFLAT' useset=f logto='Both' usevar genvar=f \
logfile=CCDPACK.LOG clean gamma=3 iter=3 boxsize="[15,15]"
Input Parameters: Mean Value %<Contribution>
...raw/010806-1113-pn0-pre(1999:3998,2001:4000) 111.17 15.11
...raw/011006-1160-pn0-pre(1999:3998,2001:4000) 109.99 48.51
...raw/011206-1265-pn0-pre(1999:3998,2001:4000) 159.05 36.27
...raw/020806-1117-pn0-pre(1999:3998,2001:4000) 172.71 18.58
...raw/020806-1118-pn0-pre(1999:3998,2001:4000) 106.75 45.71
...raw/021206-1270-pn0-pre(1999:3998,2001:4000) 123.52 22.61
...raw/031206-1275-pn0-pre(1999:3998,2001:4000) 110.87 19.19
...raw/031206-1277-pn0-pre(1999:3998,2001:4000) 75.87 20.42
...raw/041206-1279-pn0-pre(1999:3998,2001:4000) 120.68 52.36
.../raw/080306-331-pn0-pre(1999:3998,2001:4000) 113.99 66.66
...raw/100706-1092-pn0-pre(1999:3998,2001:4000) 183.50 63.36
...raw/181106-1229-pn0-pre(1999:3998,2001:4000) 114.58 37.71
...raw/191106-1225-pn0-pre(1999:3998,2001:4000) 291.30 11.17
...raw/200806-1131-pn0-pre(1999:3998,2001:4000) 133.62 18.29
...raw/200806-1133-pn0-pre(1999:3998,2001:4000) 102.83 34.79
...raw/201106-1233-pn0-pre(1999:3998,2001:4000) 203.69 38.71
...raw/231106-1241-pn0-pre(1999:3998,2001:4000) 211.89 39.45
.../raw/260306-481-pn0-pre(1999:3998,2001:4000) 106.44 17.52
...raw/281106-1252-pn0-pre(1999:3998,2001:4000) 153.19 17.97
...raw/291106-1259-pn0-pre(1999:3998,2001:4000) 233.46 8.80
Data combination method : BROADENED
Minimum number of contributing pixels : 1
Output Parameters:
Output NDF: /data/mjc/topsat/raw/firstflat
Output NDF data type : _REAL
> stats firstflat
Pixel statistics for the NDF structure /data/mjc/topsat/raw/firstflat
Title : Output from MAKEFLAT
NDF array analysed : DATA
Pixel sum : 3.89519e+06
Pixel mean : 0.973799
Standard deviation : 0.0393955
Minimum pixel value : 0.824687
At pixel : (2814, 2482)
Co-ordinate : (2813.5, 2481.5)
Maximum pixel value : 1.17688
At pixel : (2711, 3285)
Co-ordinate : (2710.5, 3284.5)
Total number of pixels : 4000000
Number of pixels used : 4000000 (100.0%)
The surprising thing is the mean being markedly different from 1.0. I
tried using the mean. The percentage contributions are far more even in
the 90+ percent region, yet the mean of the flat was 0.970. Looking at
the image the left-hand side has a mean below 0.96 with pixel-to-pixel
fluctuations of about 0.02 and the right half has a mean about 0.99,
0.045 noise (as seen from GAIA->X/Y Mean of the whole array).
Between 2 to 5% of 4 million pixels are rejected in the clipping stage
reducing the sigma by typically 15%.
Does anyone have any ideas why the mean flatfield value isn't very close
to 1.0?
Malcolm
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