The following MD09 warps were chosen to test how the backgrounds had improved using the PV3 processing tag (note these have been previously referred to as MD04, that was an announcement typo):
These were determined to have the following backgrounds and sigmas estimated from fitting a gaussian to the full pixel distribution:
sky sigma 0.1827 25.33 0.1796 24.49 0.1666 18.86 0.2090 25.67 -0.2605 64.61 0.1865 34.10 0.3667 47.37 0.3696 31.11 0.3599 30.55 0.3705 32.55
Here is a plot of the pixel distributions:
The histogram with a stepped line is for the warp with background -0.2605, that must be an image taken with a high original background given the sky sigma (this is exposure o6550g0059o).
So the mean background is ~0.2, which is roughly confirmed by PSPHOT runs on these warps that give a mean SKY of ~0.3. The following is a normalised cumulative plot of these SKY values as for all warps (ignore the black line).
Click on the image to see a full resolution version. The cumulative histograms all pass through 0.5 (the median) at roughly ~0.3, except for the one at -0.2605. This is the green line that is clearly below the others in the postive SKY value half, so PSPHOT SKY and the pixel distribution peak do not correspond well for this warp.
The question now is what background value will be seen in a stack of these warps, previous analyses had given backgrounds of several hundreds.
From a simple minded analysis we might expect something like ~0.2, but stacks are scaled to a zero point of 25 per second. Given the warp actual zero points of around 24.6 per second at best (some are worse going down to ~23), the scale factor we need for 10 exposures is around 14, so we might expect a mean background of 0.2*14, i.e. 2.8, which would scale ~80 for a stack of 400 such warps.
Using the following command:
ppStack \ -input MD04-input.mdc MD04-threepi \ -stats MD04-threepi.stats \ -recipe PPSTACK STACK_THREEPI \ -recipe PPSUB STACK_1DG \ -recipe PSPHOT STACK \ -recipe PPSTATS STACKSTATS \ -stack-type DEEP_STACK \ -F PPSTACK.OUTPUT PPSTACK.OUTPUT.NOCOMP \ -F PPSTACK.OUTPUT.EXP PPSTACK.OUTPUT.EXP.NOCOMP \ -F PPSTACK.OUTPUT.EXPWT PPSTACK.OUTPUT.EXPWT.NOCOMP \ -F PPSTACK.OUTPUT.VARIANCE PPSTACK.OUTPUT.VARIANCE.NOCOMP \ -F PPSTACK.UNCONV PPSTACK.UNCONV.NOCOMP \ -F PPSTACK.UNCONV.EXP PPSTACK.UNCONV.EXP.NOCOMP \ -F PPSTACK.UNCONV.EXPWT PPSTACK.UNCONV.EXPWT.NOCOMP \ -F PPSTACK.UNCONV.VARIANCE PPSTACK.UNCONV.VARIANCE.NOCOMP \ -F PSPHOT.BACKMDL PSPHOT.BACKMDL.MEF \ -F PSPHOT.OUTPUT PSPHOT.OUT.CMF.MEF \ -F PSPHOT.PSF.SAVE PSPHOT.PSF.SKY.SAVE \ -F SOURCE.PLOT.APRESID SOURCE.PLOT.SKY.APRESID \ -F SOURCE.PLOT.MOMENTS SOURCE.PLOT.SKY.MOMENTS \ -F SOURCE.PLOT.PSFMODEL SOURCE.PLOT.SKY.PSFMODEL \ -threads 16 \ -tracedest MD04-threepi.trace \ -log MD04-threepi.log
with ppStack version: trunk/ppStack@37009, and input file MD04-input.mdc contents:
INPUT0 METADATA IMAGE STR ../warps/o6538g0139o.652143.wrp.970252.skycell.055.fits MASK STR ../warps/o6538g0139o.652143.wrp.970252.skycell.055.mask.fits VARIANCE STR ../warps/o6538g0139o.652143.wrp.970252.skycell.055.wt.fits PSF STR ../warps/o6538g0139o.652143.wrp.970252.skycell.055.psf SOURCES STR ../warps/o6538g0139o.652143.wrp.970252.skycell.055.cmf END INPUT1 METADATA IMAGE STR ../warps/o6540g0157o.652952.wrp.970253.skycell.055.fits MASK STR ../warps/o6540g0157o.652952.wrp.970253.skycell.055.mask.fits VARIANCE STR ../warps/o6540g0157o.652952.wrp.970253.skycell.055.wt.fits PSF STR ../warps/o6540g0157o.652952.wrp.970253.skycell.055.psf SOURCES STR ../warps/o6540g0157o.652952.wrp.970253.skycell.055.cmf END INPUT2 METADATA IMAGE STR ../warps/o6540g0213o.653008.wrp.970254.skycell.055.fits MASK STR ../warps/o6540g0213o.653008.wrp.970254.skycell.055.mask.fits VARIANCE STR ../warps/o6540g0213o.653008.wrp.970254.skycell.055.wt.fits PSF STR ../warps/o6540g0213o.653008.wrp.970254.skycell.055.psf SOURCES STR ../warps/o6540g0213o.653008.wrp.970254.skycell.055.cmf END INPUT3 METADATA IMAGE STR ../warps/o6543g0266o.653384.wrp.970255.skycell.055.fits MASK STR ../warps/o6543g0266o.653384.wrp.970255.skycell.055.mask.fits VARIANCE STR ../warps/o6543g0266o.653384.wrp.970255.skycell.055.wt.fits PSF STR ../warps/o6543g0266o.653384.wrp.970255.skycell.055.psf SOURCES STR ../warps/o6543g0266o.653384.wrp.970255.skycell.055.cmf END INPUT4 METADATA IMAGE STR ../warps/o6550g0059o.655728.wrp.970256.skycell.055.fits MASK STR ../warps/o6550g0059o.655728.wrp.970256.skycell.055.mask.fits VARIANCE STR ../warps/o6550g0059o.655728.wrp.970256.skycell.055.wt.fits PSF STR ../warps/o6550g0059o.655728.wrp.970256.skycell.055.psf SOURCES STR ../warps/o6550g0059o.655728.wrp.970256.skycell.055.cmf END INPUT5 METADATA IMAGE STR ../warps/o6558g0328o.658638.wrp.970257.skycell.055.fits MASK STR ../warps/o6558g0328o.658638.wrp.970257.skycell.055.mask.fits VARIANCE STR ../warps/o6558g0328o.658638.wrp.970257.skycell.055.wt.fits PSF STR ../warps/o6558g0328o.658638.wrp.970257.skycell.055.psf SOURCES STR ../warps/o6558g0328o.658638.wrp.970257.skycell.055.cmf END INPUT6 METADATA IMAGE STR ../warps/o6567g0066o.661135.wrp.970259.skycell.055.fits MASK STR ../warps/o6567g0066o.661135.wrp.970259.skycell.055.mask.fits VARIANCE STR ../warps/o6567g0066o.661135.wrp.970259.skycell.055.wt.fits PSF STR ../warps/o6567g0066o.661135.wrp.970259.skycell.055.psf SOURCES STR ../warps/o6567g0066o.661135.wrp.970259.skycell.055.cmf END INPUT7 METADATA IMAGE STR ../warps/o6569g0065o.661989.wrp.970258.skycell.055.fits MASK STR ../warps/o6569g0065o.661989.wrp.970258.skycell.055.mask.fits VARIANCE STR ../warps/o6569g0065o.661989.wrp.970258.skycell.055.wt.fits PSF STR ../warps/o6569g0065o.661989.wrp.970258.skycell.055.psf SOURCES STR ../warps/o6569g0065o.661989.wrp.970258.skycell.055.cmf END INPUT8 METADATA IMAGE STR ../warps/o6569g0073o.661997.wrp.970260.skycell.055.fits MASK STR ../warps/o6569g0073o.661997.wrp.970260.skycell.055.mask.fits VARIANCE STR ../warps/o6569g0073o.661997.wrp.970260.skycell.055.wt.fits PSF STR ../warps/o6569g0073o.661997.wrp.970260.skycell.055.psf SOURCES STR ../warps/o6569g0073o.661997.wrp.970260.skycell.055.cmf END INPUT9 METADATA IMAGE STR ../warps/o6569g0081o.662005.wrp.970261.skycell.055.fits MASK STR ../warps/o6569g0081o.662005.wrp.970261.skycell.055.mask.fits VARIANCE STR ../warps/o6569g0081o.662005.wrp.970261.skycell.055.wt.fits PSF STR ../warps/o6569g0081o.662005.wrp.970261.skycell.055.psf SOURCES STR ../warps/o6569g0081o.662005.wrp.970261.skycell.055.cmf END
to create a stack, we get a slightly negative PSPHOT SKY value. This is the black line shown in the cumulative histograms that crosses 0.5 at ~-0.1. But note that this is divided by a factor of 14 so that it can be compared to a warp, so in fact we get:
as reported at the last DRAVG. So the mystery is why do we get a negative background, when previously it was (mainly) positive, and the only adjustments to the data values before combination where positive scalings?
For reference the ppStack log reports the following values:
Applying scale correction to image 0: -2.893520 mag (0.007611) (with inputMask value 0) Applying scale correction to image 1: -2.854084 mag (0.008413) (with inputMask value 0) Applying scale correction to image 2: -3.559891 mag (-0.005770) (with inputMask value 0) Applying scale correction to image 3: -2.848194 mag (-0.000447) (with inputMask value 0) Applying scale correction to image 4: -2.860443 mag (-0.007146) (with inputMask value 0) Applying scale correction to image 5: -2.870781 mag (-0.003460) (with inputMask value 0) Applying scale correction to image 6: -3.236738 mag (-0.004380) (with inputMask value 0) Applying scale correction to image 7: -4.022354 mag (-0.002026) (with inputMask value 0) Applying scale correction to image 8: -4.743792 mag (0.164291) (with inputMask value 0) Applying scale correction to image 9: -4.658020 mag (0.042329) (with inputMask value 0)
Which gives the scale factors (from the CMF file headers):
14.36839 13.85586 26.54338 13.7809 13.93725 14.07059 19.71039 40.63887 78.97991 72.9807
And also estimates the sky as:
image sky : mean 0.236319, stdev 24.309254, dQ 0.380525 image sky : mean 0.172495, stdev 23.207118, dQ 0.380015 image sky : mean 0.101928, stdev 17.963553, dQ 0.206238 image sky : mean 0.180381, stdev 24.455099, dQ 0.403623 image sky : mean -0.124442, stdev 62.213806, dQ 0.248667 image sky : mean 0.280146, stdev 32.540251, dQ 0.361924 image sky : mean 0.274243, stdev 45.880944, dQ 0.475878 image sky : mean 0.601617, stdev 29.982373, dQ 0.219379 image sky : mean 0.339365, stdev 29.517882, dQ 0.160456 image sky : mean 0.501157, stdev 31.656462, dQ 0.230390
Not too dissimilar to the means already found.