| | 1 | == May 14, 2013 == |
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| | 3 | Now that stack to chip unwarping is available, I've been looking into this problem using that as an additional view point. I've again constructed 2D FFT power spectra, using the central 500x500 square of a set of individual cells. |
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| | 5 | The following table shows a series of three cells on OTA27 at different stages of processing. The top panel shows the "quickmosaic" processing, which removes the overscan, but does no further detrending. Next is the "chip" processed image, which has had all detrends applied, including the PATTERN.ROW, which is applied to cells A and B, but not cell C (as is evident from the remaining horizontal banding). The bottom panel shows the "defect" image, constructed by scaling and subtracting the unwarped stack from the chip image. The stars that fall in the masked regions are visible as over subtracted objects, and the cores of some bright stars remain. In addition, this image presents are relatively pure example of the row-to-row offsets in cell C. |
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| | 7 | || raw || [[Image(raw_XY27.jpg)]] || |
| | 8 | || chip|| [[Image(chip_XY27.jpg)]] || |
| | 9 | || diff|| [[Image(diff_XY27.jpg)]] || |
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| | 11 | Further analysis will consider only cell A and cell C. The next table shows the power spectra for these two cells for the three stages of processing (the top half, as the bottom is identical due to symmetry). The main features to notice: |
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| | 13 | * vertical stripe at k_x ~= 0. This corresponds to DC offsets to a given row (k_x = 0), as well as power in the lowest frequency terms (abs(k_x) < ~5). These are certainly the frequency components that create the row-to-row offsets. There are a few peaks (k_y = 103, 170, 225) whose relative power likely control the widths and amplitudes of the individual stripes. Note that there is no obvious peak at k_y = 250, which would be the case if the row-to-row variations were entirely independent (each row arbitrarily different than its neighbors). |
| | 14 | * smaller horizontal stripe at k_y ~= 0. I believe this represents the corner glows visible on these cells. |
| | 15 | * constellation of three peaks in the +k_x, +k_y quadrant. These peaks are common to both cells, and likely represent a low level noise component that is undetectable to the eye in most exposures. I have not investigated multiple OTAs at this point to determine if this noise signal is found in all devices. |
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| | 17 | For cell A, the transition from raw to chip processing whows the removal of the first two features listed above. This suggests that the PATTERN.ROW correction is effectively filtering the vertical stripe on this cell (as is visible in the real image). The horizontal bar is removed as well, suggesting that the cause of this is removed by the detrending process. Finally, subtracting the stack removes the "cloud" around k_x = k_y = 0 that is caused by the presence of real objects (objects are all roughly PSF like, and therefore somewhat Gaussian, so their transform is also a Gaussian). The unknown noise pattern represented by the constellation is unchanged, as we do nothing that should alter it. |
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| | 19 | Cell C shows a similar reduction in the horizontal stripe and object cloud, but as we have not applied PATTERN.ROW to this cell, the vertical stripe is left unchanged. Therefore, an ideal replacement for PATTERN.ROW is an algorithm that removes the vertical stripe in the image power spectrum, while retaining the other properties of the image. One way to visualize what we're doing is shown in the bottom row. This shows the difference of the power spectra between the raw and chip stage (detdiff = log10(raw) - log10(chip) = size of the reduction in power at this frequency). This shows that we are reducing the vertical stripe in cell A, but there is no change in cell C. |
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| | 21 | || || Cell A || Cell C || |
| | 22 | || raw || [[Image(pow_raw_A.jpg)]] || [[Image(pow_raw_C.jpg)]] || |
| | 23 | || chip|| [[Image(pow_chip_A.jpg)]]|| [[Image(pow_chip_C.jpg)]] || |
| | 24 | || diff|| [[Image(pow_diff_A.jpg)]]|| [[Image(pow_diff_C.jpg)]] || |
| | 25 | ||detdiff|| [[Image(pow_detdiff_A.jpg)]] || [[Image(pow_detdiff_C.jpg)]] || |
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