Changes between Initial Version and Version 1 of PS1_GPC1_Magic_And_Short_Exposures

Timestamp:

Jun 21, 2009, 4:59:09 PM (17 years ago)

Author:

eugene

Comment:

--

PS1_GPC1_Magic_And_Short_Exposures

@@ +1 @@
+== Background ==
+
+The IPP automatic processing of PS1/GPC1 data is currently blocked by
+excessive false positive rate in Magic for short exposures.  The
+assessment by Paul Sydney as of late May was that Magic is able to
+detect satellite streaks, but that the effort has reached the limit of
+its ability to distinguish real streaks from other contaminating
+sources.
+
+At the time, the top candidate for the contaminating source was the
+correlated read noise seen especially in certain chips.  This signal
+has the appearance of positive or negative trend in the bias for
+each row, with a small amount of correlation between neighboring
+rows.  The cause of this signal has been extensively investigated by
+the camera group, and is apparently due to the interaction between the
+video signal of multiple chips readout at the same time on the same
+flex-print circuilt.  Without going too much into the underlying
+electronic issues, the signal consists of a row-by-row offset in the
+effective bias of the row, along with a possible slow drift of that
+bias.  The camera software currently subtracts a row-by-row constant
+bias to make a first-order correction for this effect.  However, the
+(unpredictable) slope results in a slow drift for each row from this
+corrected value.  As a result, the cells & chips which suffer most
+from this effect have a generally good appearance on the end of the
+row, trending to significal offsets for the beginning of the row.  The
+amplitude of the effect (the 'streakies') varies from chip to chip and
+from cell to cell within a chip.  Although the row-by-row amplitude
+varies and is unpredicatble, the range of tilts for a given cell
+appears to stay consistent over many reads and many nights.  
+
+The camera group is studying possible mitigations in the hardware,
+including use of the pedestal signal and/or measuring a pre-scan bias
+level as well as a post-scan bias level.  Both mitigataions will
+require research in the lab with the test setup, though it is
+reasonable to expect that a solution can be implemented without
+removing the camera from the telescope -- the implementation would be
+completely in software.  However, because of the research needed (and
+finite available labor by the camera group) no such implementation can
+be expected for 2-4 months.  In addition, any hardware mitigation will
+only allow for future improvements; none of the data taken would be
+addressed by this work.  To this end, we have been working in two
+veins to implement a solution in the processing, even if such a
+solution would be less that the perfect solution.
+
+The two processing fixes we have been pursuing attempt in one case to
+correct (remove) the signal and in the other case to incorporated the
+effect into a more complete description of the noise model.  
+
+In the first case, John Tonry has implemented a program called
+'tiltystreak' which attempts to subtract the slope from each row,
+while at the same time attempting to avoid contamination from real
+signals in the data.  This program takes the difference between the
+image and a 1 pixel shifted version, in order to avoid sensitivity to
+gradients in the input image.  What is left is the 1st derivative of
+ths signal.  The program attempts to determine the row-by-row slope.
+It attempts to reduce further the impact of real sources by performing
+the analysis on all 8 cells in a row at once.  It has the option of
+only applying the fitted trend to a specified subset of cells.  The
+IPP team has incorporated 'tiltystreak' into the IPP pipeline, and
+defined recipes that allow the analysis to be turned on optionally,
+and for the desired cells and chips to be specified for processing.
+
+The second processing mitigation we have pursued is at attempt to
+model more accurately the effective readnoise introduced by this
+structure.  By default, the camera reports a single read-noise value
+for each cell.  We have created a tool (ppNoiseMap) to measure a
+robust variance for collections of pixels in a grid across each cell.
+This measurement can be made on a number of (eg) very short dark
+exposures, with a single high-quality measurement generated from the
+median of the multiple measurements.  The resulting information can be
+used as a more accurate model of the read noise, with the noise level
+modeled on the subcell level.  We have added an option to the standard
+processing to use this noise model to determine the noise
+characteristics of any given science image (combining in quadrature
+for each pixel the read noise from the model and the contribution from
+the poisson noise). 
+
+== Testing ==
+
+We have preformed a series of tests applying each of the two
+mitigations to the same collection of data.  The input data for this
+test consisted of a series of short g-band exposures for which the
+Magic analysis had previously given very poor results.  Unfortunately,
+as discussed below, these tests show that neither of these two
+mitigations is sufficient, though it appears that they substantially
+improve the situation.  
+
+We have run 3 tests with the g-band sequence.  In the first test
+(noise.20090618), we only applied the resolved noise model; In the
+second test (tilty.20090619), we only applied the 'tiltystreak'
+correction.  The critical measurement of success or failure is the
+number of streaks detected by magic.  Both tests result in 1000 - 2000
+streaks detected.  Since there are only at most a few real streaks in
+these images, the vast majority of these are false positives.  (HOW
+MUCH DID it improve relative to neither applied??).  The
+tilty.20090619 test gave marginally better results than the
+noise.20090618 test, but both were clearly unacceptable.  We ran a
+third test in which both the noise model and 'tiltystreak' were
+applied, in addition to a somewhat modified static mask, as discussed
+below.  This third test (noise.20090620) was again a modest
+improvement over tilty.20090619, but the number of detected streaks
+was still too high (approaching 2000).
+
+== Detailed Inspection. ==
+
+Below are a series of plots and images which illustrate the noise
+properties of the GPC1 images and how they are affected by the
+mitigations discussed above.  The first series shows the evolution of
+the noise properties of a detrended chip from a single image in the
+native chip pixel coordinate system.  All three plots show the
+log(Npixels) as a function of the predicted signal-to-noise
+(background subtracted).  A Gaussian has been fitted to the core of
+this distribution, and is over-plotted in blue.  If the noise model
+and background subtraction were perfect, the fit would result in a
+Gaussian with sigma 1.0 and a mean of 0.0, and the profile would
+follow the fitted curve all the way down.  This image is a positive
+image of the night-time sky, so we expect pixels with high
+significance due to the presence of stars.
+
+The first plot shows the result for noise.20090618, which only the
+noise model applied.  The second plot shows the result for
+tilty.20090619, with only tiltystreak applied.  The third shows the
+result for noise.20090620, with both applied and the additional
+masking.  This chip is known to have significant correlated
+read-noise.
+
+The set of images below show the S/N image used for the measurements
+above.  In the first image, the correlated read noise is evident.  In
+the second, tiltystreak has improved the correlated read noise, but
+has introduced an artifact on the first and last few rows of the
+chip.  We noticed that this effect was a typical result from
+tiltystreak, so for the third experiment (noise.20090620), we extended
+the static mask by a few rows on each cell.  This was an attempt to
+make a crude fix, with the intention of doing a better job if the
+impact seemed promising.  Figure NN shows that this additional masking
+was not quite sufficient in all cases.
+
+The next set of plots and images are equivalent to the above set, but
+for the warped images generally corresponding to the above chips.  The
+goal is to show that the warping does not significantly affect the
+noise predictions, and indeed it does not seem to have an adverse
+impact.
+
+The final set of plots and images shows the equivalent to the above
+for the difference images for this exposure.  The critical difference
+between this set at the warped version is that the real astronomical
+source should be subtracted, so that the high end deviations should be
+removed.  It is quite clear that in all cases, the amount of signal at
+the high end is reduced.  There is also an increase in the effective
+noise level.  Although the third experiment (noise.20090620) is the
+best of the three, it still has a noise level which is unexpectedly
+high (sigma = 1.24).  
+
+Although the elevated Gaussian noie is a concern, and needs to be
+studies further, the observed magic-detected streaks do not seem to be
+the result of poorly estimated Gaussian noise.  Figure NN shows the
+location of the detected streaks, along with small segements
+corresponding to the pixels responsible for the streaks.  These
+streaks are not randomly distributed across the focal plane as would
+be expected if they were caused by Gaussian noise.  Closer examination
+of the images shows that other remaining linear features are at least
+partially responsible for triggering magic.  
+
+It seems that the static mask current in place is not sufficiently
+conservative.  The next few images show some of the types of features
+which are not covered by the static mask.  The first shows the
+vignetted edge of the focal plane.  The next shows a glowing pixels
+around a bad cell.  The third shows some hot columns.  Our next step
+is to improve the mask by examining a set of processed science
+images as shown here and to manually mark poor pixels where were not
+caught in the intial mask creation.
+
+At this point, I conclude both 'tiltystreak' and the noise model are
+needed to produce generally well-characterized images, but that two
+possible remaining issues are contributing to the poor behavior of
+magic:
+
+1) insufficiently conservative masking of some features in the camera.
+
+2) excessive Gaussian noise in the difference images relative to the warps.
+
+The former can be addressed with ~1 day of work by the IPP team in
+manually reviewing the static masks.  The latter needs some
+additional investigation, and may be due to a software error, or may
+simply be a result of the correlated noise properties of the pixels in
+this camera.