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projects:omegacam:portal:illuminationcorrection [2011/12/30 15:52] gverdoes |
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| - | = Illumination variations: characerization and correction | + | = OmegaCAM Illumination Variation and Correction |
| - | == Introduction | + | == Reports |
| - | Flat field observations can have a straylight component. The flatfielding operation does not distinguish between true variations in sensitivity and apparent ones, mimicked by gradients in this straylight. Gradients in straylight are called illumination variations. Illumination variations can be characterized a posteriori from the residuals in observed stellar magnitudes compared to their reference value. The corresponding procedure is called Illumination Correction. | + | * OmegaCAM Commissioning - DFS - Illumination Characterisation and CorrectionIllumination Report v2.1 VST-TRE-OCM-23100-3608 {{:projects:omegacam:portal:ocam_illum.pdf|}} |
| + | == Supplements | ||
| - | == Illumination Correction Model 1: global 2nd-order polynomial | + | * Accompanying movie of twilightflat residuals vs rotator angle: |
| + | {{:projects:omegacam:portal:deltaflatsky-ocam_g_sdss-rotwithangleindicated.gif?250|}} | ||
| - | * An algorithm has been implemented that solves simultaneously for illumination variations and chip-to-chip gain variations for 32 chips. It models the illumination variations as a second order polynomial. As input it takes 32 dithered observations of SA fields that place a star on all different chips. The data are flatfielded using a flatfield that is a combination of a twilight flat and a domeflat. The large scale variations are set by the twilight, the small scale by the domeflats. Figures 1-4 show the illumination modelling and resulting magnitude residuals for Sloan u,g,r,i. Table 1 lists the resulting residuals. We define an internal and external rms in the residuals. External is with respect to the stellar reference magnitude in the standard star catalog, which is SDSS DR7 photometry in this case. Internal is wrt the mean of the observed magnitudes per star. Table 2 list the polynomial description and gives links to the data in Astro-WISE. | + | == Links |
| - | + | ||
| - | + | ||
| - | <box> | + | |
| - | ^filter ^rms internal ^rms external^ | + | |
| - | ^ ^(mag) ^(mag)^ | + | |
| - | |u| 0.019| 0.032| | + | |
| - | |g| 0.012| 0.022| | + | |
| - | |r| 0.011| 0.024| | + | |
| - | |i| 0.014| 0.026| | + | |
| - | Table 1: Residual magnitudes in Sloan u,g,r,i filters left after global illumination correction. Input data are the 32 dithered observations on SA fields. The data are flatfielded using a flatfield that is a combination of a twilight flat and a domeflat. The large scale variations are set by the twilight, the small scales by the domeflats. We define an internal and external rms in residuals. External is with respect to the stellar reference magnitude in the standard star catalog (SDSS DR7 stellar photometry). Internal is wrt the mean of its observed magnitudes. | + | |
| - | </box> | + | |
| - | + | ||
| - | <box 1000px> | + | |
| - | ^band ^data in Astro-WISE ^coeff x*x ^coeff x*y ^coeff y*y^ | + | |
| - | | u | [[http://dbview.test.astro-wise.org/DbView?project=OMEGACAM%40VST&set_cookie=1&QSort=&QDesc=descending&numrows=1000&fcontext_project_only=on&show_expanded=no&Exportselect=NoDownload&class_str=IlluminationCorrectionFrame&mode=table_res&depth=1&python_dict=%2528dp0%250AS%2527query%2527%250Ap1%250A%2528dp2%250AS%2527IlluminationCorrectionFrame.instrument.name%2527%250Ap3%250A%2528S%2527%253D%2527%250Ap4%250AS%2527OMEGACAM%2527%250Ap5%250Atp6%250AssS%2527expanded%2527%250Ap7%250A%2528dp8%250AS%2527IlluminationCorrectionFrame.filter%2527%250Ap9%250A%2528lp10%250AS%2527prefilled%2527%250Ap11%250AaS%2527link_property%2527%250Ap12%250AaS%2527Filter%2527%250Ap13%250AasS%2527IlluminationCorrectionFrame.instrument%2527%250Ap14%250A%2528lp15%250AS%2527prefilled%2527%250Ap16%250AaS%2527link_property%2527%250Ap17%250AaS%2527Instrument%2527%250Ap18%250AasS%2527IlluminationCorrectionFrame.chip%2527%250Ap19%250A%2528lp20%250AS%2527prefilled%2527%250Ap21%250AaS%2527link_property%2527%250Ap22%250AaS%2527Chip%2527%250Ap23%250AasS%2527IlluminationCorrectionFrame%2527%250Ap24%250A%2528lp25%250AS%2527open%2527%250Ap26%250AaS%2527root%2527%250Ap27%250Aag24%250AassS%2527derived%2527%250Ap28%250A%2528dp29%250As.&expand_leaf=&expand_leaf_type=&expand_leaf_class=&derived_leaf=&node_state=&IlluminationCorrectionFrame.filter.expanded=prefilled&IlluminationCorrectionFrame.instrument.expanded=prefilled&IlluminationCorrectionFrame.chip.expanded=prefilled&IlluminationCorrectionFrame.expanded=open&IlluminationCorrectionFrame.creation_date.op=between&IlluminationCorrectionFrame.creation_date=2011-10-20+15%3A00%3A00%2C2011-10-21&IlluminationCorrectionFrame.filter.name=OCAM_u_SDSS&IlluminationCorrectionFrame.filter.name.op=%3D&IlluminationCorrectionFrame.instrument.name=OMEGACAM&IlluminationCorrectionFrame.instrument.name.op=%3D | illumu]] | 1.61533894480598e-09 | 1.93208441084062e-10 | 1.46502463058307e-09| | + | |
| - | | g | [[http://dbview.test.astro-wise.org/DbView?project=OMEGACAM%40VST&set_cookie=1&QSort=&QDesc=descending&numrows=1000&fcontext_project_only=on&show_expanded=no&Exportselect=NoDownload&class_str=IlluminationCorrectionFrame&mode=table_res&depth=1&python_dict=%2528dp0%250AS%2527query%2527%250Ap1%250A%2528dp2%250AS%2527IlluminationCorrectionFrame.instrument.name%2527%250Ap3%250A%2528S%2527%253D%2527%250Ap4%250AS%2527OMEGACAM%2527%250Ap5%250Atp6%250AssS%2527expanded%2527%250Ap7%250A%2528dp8%250AS%2527IlluminationCorrectionFrame.filter%2527%250Ap9%250A%2528lp10%250AS%2527prefilled%2527%250Ap11%250AaS%2527link_property%2527%250Ap12%250AaS%2527Filter%2527%250Ap13%250AasS%2527IlluminationCorrectionFrame.instrument%2527%250Ap14%250A%2528lp15%250AS%2527prefilled%2527%250Ap16%250AaS%2527link_property%2527%250Ap17%250AaS%2527Instrument%2527%250Ap18%250AasS%2527IlluminationCorrectionFrame.chip%2527%250Ap19%250A%2528lp20%250AS%2527prefilled%2527%250Ap21%250AaS%2527link_property%2527%250Ap22%250AaS%2527Chip%2527%250Ap23%250AasS%2527IlluminationCorrectionFrame%2527%250Ap24%250A%2528lp25%250AS%2527open%2527%250Ap26%250AaS%2527root%2527%250Ap27%250Aag24%250AassS%2527derived%2527%250Ap28%250A%2528dp29%250As.&expand_leaf=&expand_leaf_type=&expand_leaf_class=&derived_leaf=&node_state=&IlluminationCorrectionFrame.filter.expanded=prefilled&IlluminationCorrectionFrame.instrument.expanded=prefilled&IlluminationCorrectionFrame.chip.expanded=prefilled&IlluminationCorrectionFrame.expanded=open&IlluminationCorrectionFrame.creation_date.op=between&IlluminationCorrectionFrame.creation_date=2011-10-20+1%3A00%3A00%2C2011-10-21&IlluminationCorrectionFrame.filter.name=OCAM_g_SDSS&IlluminationCorrectionFrame.filter.name.op=%3D&IlluminationCorrectionFrame.instrument.name=OMEGACAM&IlluminationCorrectionFrame.instrument.name.op=%3D | illumg]] | 1.7019894901137e-09 | 3.3104022217857e-11 | 1.67043927858479e-09| | + | |
| - | | r | [[http://dbview.test.astro-wise.org/DbView?project=OMEGACAM%40VST&set_cookie=1&QSort=&QDesc=descending&numrows=1000&fcontext_project_only=on&show_expanded=no&Exportselect=NoDownload&class_str=IlluminationCorrectionFrame&mode=table_res&depth=1&python_dict=%2528dp0%250AS%2527query%2527%250Ap1%250A%2528dp2%250AS%2527IlluminationCorrectionFrame.instrument.name%2527%250Ap3%250A%2528S%2527%253D%2527%250Ap4%250AS%2527OMEGACAM%2527%250Ap5%250Atp6%250AssS%2527expanded%2527%250Ap7%250A%2528dp8%250AS%2527IlluminationCorrectionFrame.filter%2527%250Ap9%250A%2528lp10%250AS%2527prefilled%2527%250Ap11%250AaS%2527link_property%2527%250Ap12%250AaS%2527Filter%2527%250Ap13%250AasS%2527IlluminationCorrectionFrame.instrument%2527%250Ap14%250A%2528lp15%250AS%2527prefilled%2527%250Ap16%250AaS%2527link_property%2527%250Ap17%250AaS%2527Instrument%2527%250Ap18%250AasS%2527IlluminationCorrectionFrame.chip%2527%250Ap19%250A%2528lp20%250AS%2527prefilled%2527%250Ap21%250AaS%2527link_property%2527%250Ap22%250AaS%2527Chip%2527%250Ap23%250AasS%2527IlluminationCorrectionFrame%2527%250Ap24%250A%2528lp25%250AS%2527open%2527%250Ap26%250AaS%2527root%2527%250Ap27%250Aag24%250AassS%2527derived%2527%250Ap28%250A%2528dp29%250As.&expand_leaf=&expand_leaf_type=&expand_leaf_class=&derived_leaf=&node_state=&IlluminationCorrectionFrame.filter.expanded=prefilled&IlluminationCorrectionFrame.instrument.expanded=prefilled&IlluminationCorrectionFrame.chip.expanded=prefilled&IlluminationCorrectionFrame.expanded=open&IlluminationCorrectionFrame.creation_date.op=between&IlluminationCorrectionFrame.creation_date=2011-10-20+15%3A00%3A00%2C2011-10-26&IlluminationCorrectionFrame.filter.name=OCAM_r_SDSS&IlluminationCorrectionFrame.filter.name.op=%3D&IlluminationCorrectionFrame.instrument.name=OMEGACAM&IlluminationCorrectionFrame.instrument.name.op=%3D | illumr]] | 1.54534631642476e-09 | 2.55003412514956e-11 | 1.38537472866354e-09| | + | |
| - | | i | [[http://dbview.test.astro-wise.org/DbView?project=OMEGACAM%40VST&set_cookie=1&QSort=&QDesc=descending&numrows=1000&fcontext_project_only=on&show_expanded=no&Exportselect=NoDownload&class_str=IlluminationCorrectionFrame&mode=table_res&depth=1&python_dict=%2528dp0%250AS%2527query%2527%250Ap1%250A%2528dp2%250AS%2527IlluminationCorrectionFrame.instrument.name%2527%250Ap3%250A%2528S%2527%253D%2527%250Ap4%250AS%2527OMEGACAM%2527%250Ap5%250Atp6%250AssS%2527expanded%2527%250Ap7%250A%2528dp8%250AS%2527IlluminationCorrectionFrame.filter%2527%250Ap9%250A%2528lp10%250AS%2527prefilled%2527%250Ap11%250AaS%2527link_property%2527%250Ap12%250AaS%2527Filter%2527%250Ap13%250AasS%2527IlluminationCorrectionFrame.instrument%2527%250Ap14%250A%2528lp15%250AS%2527prefilled%2527%250Ap16%250AaS%2527link_property%2527%250Ap17%250AaS%2527Instrument%2527%250Ap18%250AasS%2527IlluminationCorrectionFrame.chip%2527%250Ap19%250A%2528lp20%250AS%2527prefilled%2527%250Ap21%250AaS%2527link_property%2527%250Ap22%250AaS%2527Chip%2527%250Ap23%250AasS%2527IlluminationCorrectionFrame%2527%250Ap24%250A%2528lp25%250AS%2527open%2527%250Ap26%250AaS%2527root%2527%250Ap27%250Aag24%250AassS%2527derived%2527%250Ap28%250A%2528dp29%250As.&expand_leaf=&expand_leaf_type=&expand_leaf_class=&derived_leaf=&node_state=&IlluminationCorrectionFrame.filter.expanded=prefilled&IlluminationCorrectionFrame.instrument.expanded=prefilled&IlluminationCorrectionFrame.chip.expanded=prefilled&IlluminationCorrectionFrame.expanded=open&IlluminationCorrectionFrame.creation_date.op=between&IlluminationCorrectionFrame.creation_date=2011-10-20+14%3A00%3A00%2C2011-10-21&IlluminationCorrectionFrame.filter.name=OCAM_i_SDSS&IlluminationCorrectionFrame.filter.name.op=%3D&IlluminationCorrectionFrame.instrument.name=OMEGACAM&IlluminationCorrectionFrame.instrument.name.op=%3D| illumi]] | 1.527014845765e-09 | 1.07778571149453e-10 | 1.13924080056116e-09| | + | |
| - | Table 2. An algorithm has been implemented that models the illumination variations as a second order polynomial. This table gives links to the illumination correction frames in Astro-WISE and the polynomial coefficients. x,y is in pixel units with x,y=0,0 at the mosaic center. | + | |
| - | </box> | + | |
| - | + | ||
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_rawzp_u.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_model_u.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_resi0.75_u.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_illum_u.png?500|}} | + | |
| - | + | ||
| - | Fig1. Illumination correction derivation for OmegaCAM Sloan u. Input data is 32-dither observations of SA113. The magnitude residual after flatfielding are modeled with one 2D-polynomial + 32 ZPTs as free parameters. SDSS DR7 is used as standard star catalog. Top left: magnitude residuals after flatfielding (ZPT per chip not subtracted). Top right: model. Bottom left: External magnitude residuals after illumination model has been applied. Bottom right: model with ZPT per chip subtracted. | + | |
| - | </box> | + | |
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_rawzp_g.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_model_g.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_resi0.75_g.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_illum_g.png?500|}} | + | |
| - | + | ||
| - | Fig2. Illumination correction derivation for OmegaCAM Sloan g. Input data is 32-dither observations of SA113. The magnitude residual after flatfielding are modeled with one 2D-polynomial + 32 ZPTs as free parameters. SDSS DR7 is used as standard star catalog. Top left: magnitude residuals after flatfielding (ZPT per chip not subtracted). Top right: model. Bottom left: External magnitude residuals after illumination model has been applied. Bottom right: model with ZPT per chip subtracted. | + | |
| - | </box> | + | |
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_rawzp_r.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_model_r.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_resi0.75_r.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_illum_r.png?500|}} | + | |
| - | + | ||
| - | Fig3. Illumination correction derivation for OmegaCAM Sloan r. Input data is 32-dither observations of SA113. The magnitude residual after flatfielding are modeled with one 2D-polynomial + 32 ZPTs as free parameters. SDSS DR7 is used as standard star catalog. Top left: magnitude residuals after flatfielding (ZPT per chip not subtracted). Top right: model. Bottom left: External magnitude residuals after illumination model has been applied. Bottom right: model with ZPT per chip subtracted. | + | |
| - | </box> | + | |
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_rawzp_i.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_model_i.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_resi0.75_i.png?500|}} | + | |
| - | {{:projects:omegacam:portal:ocam3_sa113_illum_i.png?500|}} | + | |
| - | + | ||
| - | Fig4. Illumination correction derivation for OmegaCAM Sloan i. Input data is 32-dither observations of SA113. The magnitude residual after flatfielding are modeled with one 2D-polynomial + 32 ZPTs as free parameters. SDSS DR7 is used as standard star catalog. Top left: magnitude residuals after flatfielding (ZPT per chip not subtracted). Top right: model. Bottom left: External magnitude residuals after illumination model has been applied. Bottom right: model with ZPT per chip subtracted. | + | |
| - | </box> | + | |
| - | + | ||
| - | *The internal rms listed in Table 1 demonstrates that illumination variations are characterized to a precision between 1 and 2% over the whole mosaic. The magnitude residuals in Figures 1-4 demonstrate that remaining systematic effects over a single chip are < 1%. The exception are areas near CCD edges with a width of ~200 pixels which display offsets upto ~0.06mag. In addition a few localized regions (typical sizes less than ~12% of a chip area) are apparent where offset can be up to ~0.03mag. | + | |
| - | + | ||
| - | *The robustness of the illumination correction is determined in two ways. First, illumination corrections derived from two different SA fields for Sloan r differ by less than 1% percent (see Figure 5). Second, the illumination correction derived from SA113 as applied to SA95 does not show any systematics in the residuals (Figure 6). | + | |
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:illumdiv.png?500|}} | + | |
| - | + | ||
| - | Figure 5: Ratio of illumination corrections in Sloan r as derived from two different SA fields. The color scaling | + | |
| - | from black to white corresponds to ratio values of 0.994 to 1.006. Largest variations over a single chip amount | + | |
| - | to < 1% min-to-max. | + | |
| - | </box> | + | |
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:sa113_polynome.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa95_polynomesa113.png?500|}} | + | |
| - | + | ||
| - | Figure 6: Illumination correction residuals for OmegaCAM Sloan r for polynome method. **Left**: residuals after applying to SA113 the polynome model derived from SA113. **Right**: residuals after applying to SA95 data the polyonome model derived from SA113. No systematic differences are seen. | + | |
| - | </box> | + | |
| - | + | ||
| - | == Illumination Correction Model 1: local linear planes. | + | |
| - | + | ||
| - | To address the localized systematic offsets in residuals for the global approach we applied a localized illumination correction. The input data is identical to the global approach. A zeropoint is determined per chip. Each chip is then split into bins, 32 for u and 50 for g, r and i. In each bin a plane (a + b*x + c*y) is fitted to the magnitude residuals. Figures 7-10 show the results and Table 3 lists the rms residuals. | + | |
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:sa113_binning_u_data.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_u_model.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_u_resi.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_u_illum.png?500|}} | + | |
| - | + | ||
| - | Fig7. Illumination correction derivation for OmegaCAM Sloan u. The illumination variations are modelled per bin with a plane. Top left: magnitude residuals after flatfielding (ZPT per chip not subtracted). Top right: model. The bins | + | |
| - | are sometimes visible. Bottom left: External magnitude residuals after illumination model has been applied. | + | |
| - | Bottom right: model with ZPT per chip subtracted. | + | |
| - | </box> | + | |
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:sa113_binning_g_data.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_g_model.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_g_resi.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_g_illum.png?500|}} | + | |
| - | + | ||
| - | Fig8. Illumination correction derivation for OmegaCAM Sloan g. The illumination variations are modelled per bin with a plane. Top left: magnitude residuals after flatfielding (ZPT per chip not subtracted). Top right: model. The bins | + | |
| - | are sometimes visible. Bottom left: External magnitude residuals after illumination model has been applied. | + | |
| - | Bottom right: model with ZPT per chip subtracted. | + | |
| - | </box> | + | |
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:sa113_binning_r_data.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_r_model.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_r_resi.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_r_illum.png?500|}} | + | |
| - | + | ||
| - | Fig9. Illumination correction derivation for OmegaCAM Sloan r. The illumination variations are modelled per bin with a plane. Top left: magnitude residuals after flatfielding (ZPT per chip not subtracted). Top right: model. The bins | + | |
| - | are sometimes visible. Bottom left: External magnitude residuals after illumination model has been applied. | + | |
| - | Bottom right: model with ZPT per chip subtracted. | + | |
| - | </box> | + | |
| - | + | ||
| - | <box 1100px> | + | |
| - | {{:projects:omegacam:portal:sa113_binning_i_data.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_i_model.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_i_resi.png?500|}} | + | |
| - | {{:projects:omegacam:portal:sa113_binning_i_illum.png?500|}} | + | |
| - | + | ||
| - | Fig10. Illumination correction derivation for OmegaCAM Sloan i. The illumination variations are modelled per bin with a plane. Top left: magnitude residuals after flatfielding (ZPT per chip not subtracted). Top right: model. The bins | + | |
| - | are sometimes visible. Bottom left: External magnitude residuals after illumination model has been applied. | + | |
| - | Bottom right: model with ZPT per chip subtracted. | + | |
| - | </box> | + | |
| - | + | ||
| - | <box> | + | |
| - | ^filter ^rms internal ^rms external^ | + | |
| - | ^ ^(mag) ^(mag)^ | + | |
| - | |u| TBComputed| TBC| | + | |
| - | |g| TBC| TBC| | + | |
| - | |r| 0.009| 0.020| | + | |
| - | |i| TBC| TBC| | + | |
| - | Table 3: Residual magnitudes in Sloan u,g,r,i filters left after local illumination correction. Input data are the 32 dithered observations on SA fields. The data are flatfielded using a flatfield that is a combination of a twilight flat and a domeflat. The large scale variations are set by the twilight, the small scales by the domeflats. We define an internal and external rms in residuals. External is with respect to the stellar reference magnitude in the standard star catalog (SDSS DR7 stellar photometry). Internal is wrt the mean of its observed magnitudes. | + | |
| - | </box> | + | |
| - | + | ||
| - | == Dependence on rotator angle | + | |
| - | + | ||
| - | *The OmegaCAM commissioning reports give the rotator angle dependency of the illumination variation. The straylight patterns are not fully circularly symmetric and can change more irregularly than a simple rotation. | + | |
| - | + | ||
| - | * Domeflats are non-circurlarly symmetric (amplitude ~5%) but the pattern simply co-rotates with rotator angle. | + | |
| - | + | ||
| - | * Twilight: variations wrt average: expect 2% or smaller | + | |
| - | * dependence on angle wrt dome, i.e., rotator angle (ABSROT) | + | |
| - | * NOT on angle wrt sky POSANG | + | |
| - | + | ||
| - | + | ||
| - | We have explored | + | |
| - | * Twilight flat are also non-cicularly symmetric (amplitude ~10%). This pattern does not simply co-rotate with rotator angle. | + | |
| - | + | ||
| - | + | ||
| - | + | ||
| - | Flat fields and illumination corrections, the latter being an correction on the flat field, must be derived in an internally consistent manner. Thus new weekly twilight flatfields and dome flatfields have to be observed at all times at the same position angle wrt to the sky / dome screen as those used in deriving the illumination correction. This makes the current approach a bit cumbersome in terms of calibration data acquisition and calibration procedure. | + | |
| - | + | ||
| - | dataset & \textbf{rms internal} & \textbf{rms external} & comments (all models derived from SA113 data-set)\\ | + | |
| - | & \textbf{(mag)} & \textbf{(mag)} &\\ | + | |
| - | \hline | + | |
| - | r & 0.011 & 0.024 & polynome model applied on same data-set (i.e. SA113)\\ | + | |
| - | r & 0.009 & 0.020 & binning model applied on same data-set (i.e. SA113)\\ | + | |
| - | r & 0.011 & 0.023 & binning model applied on SA95 data-set, SA95 reduced with same ff\\ | + | |
| - | r & 0.011 & 0.024 & binning model applied on SA95 reduced with tw. flat Delta(ROTANG)=70\\ | + | |
| - | + | ||
| - | *In Section 13 we report the rotator angle dependency of the sky concentration. The straylight patterns are not fully rotationally symmetric and change more irregularly than a simple rotation. Flat fields and illumination corrections, the latter being an correction on the flat field, must be derived in an internally consistent manner. Thus new weekly twilight flatfields and dome flatfields have to be observed at all times at the same position angle wrt to the sky / dome screen as those used in deriving the illumination correction. This makes the current approach a bit cumbersome in terms of calibration data acquisition and calibration procedure. | + | |
| - | + | ||
| - | *To address the ”cumbersomeness” of the current approach the consortium is investigating alternative routes that take into account the irregular behavior. This investigation also tackles correcting the systematic residuals near CCD edges. The investigations will be reported in a future report (due 31-12-2011). | + | |
| - | + | ||
| - | + | ||
| - | == Tackling non-rotationally-symmetric illumination variation | + | |
| - | + | ||
| - | * Binned approach | + | |
| - | *local excursions: corrected for, robustly (see application to SA95) | + | |
| - | * CCD-edge effects are ~0.06mag | + | |
| - | * other-local-excursions: ~0.03mag local excursions | + | |
| - | *effect of rotator angle: local excursions of ~0.03mag (for delta(rot)=70deg of twilight) | + | |
| - | + | ||
| - | == Tackling rotator-angle dependence of illumination variation | + | |
| - | * try with domeflat (always same rotator angle by design) | + | |
| - | + | ||
| - | == Domeflat | + | |
| - | + | ||
| - | * non-rotationally symmetric component 5%: see OCAM3-DFS req566 (normalization Fig15 OCAM3-DFS to 400 (rebinning)) | + | |
| - | + | ||
| - | == Twilight flat non-rotationally-symmetric component | + | |
| - | + | ||
| - | * 10%: see OCAM3-DFS req566 again. | + | |
| - | + | ||
| - | == Binned results | + | |
| - | + | ||
| - | * Paste in See Gert's req548_new.tex results | + | |
| - | + | ||
| - | == Illumination Variation in Domeflats | + | |
| - | + | ||
| - | + | ||
| - | == Conclusions (based on polynomial results: GVK: replace with new conclusiosn) | + | |
| - | * We have 0.04/0.03 magnitudes (rms) photometric precision in u /(g,r,i) using SDSS DR7 as reference catalog. The dedicated OmegaCAM secondary standards catalog is expected to improve on this photometric precision. | + | |
| - | *The goal to determine the illumination correction better than 1% for the amplitude over a single CCD is met over almost the whole mosaic. Exception are thin areas near CCD edges. Investigations show that correction for this effect is possible as will reported in the coming report on illumination variations. | + | |
| - | * The irregular behavior of straylight in twilights in general make the approach cumbersome in terms of calibration data acquisition and calibration procedure. Alternative more simple approaches to illumination correction have promising first results: also in report that will be submitted by 31 Dec 2011. For example using only domeflats which appear to have an amplitude of ~5% or less in the amplitude of the sky concentration. | + | |
| - | + | ||
| - | == Next steps | + | |
| - | + | ||
| - | ^Action item^Responsible^Deadline^Comment^ | + | |
| - | |Characterize sky concentration in domeflats | GVK | ASAP |ra,dec,rotangle wrt instrument, wrt dome wrt sky. If negligible the way to go might be domeflats-only calibration. If not rotationally symmetric it will show up in ratio image of rotated domeflats.| | + | |
| - | |Read http://arxiv.org/abs/1111.2058 | GVK | ASAP | | | + | |
| - | + | ||
| - | == Scratch notes: | + | |
| - | + | ||
| - | (see e.g. Section 2.8.4. of McFarland et al. 2011, arXiv:1110.2509). | + | |
| - | This is likely due to vignetting. Similar results are derived by comparing illumination corrections based on observations of different SA fields (see Figure 6) | + | |
| - | + | ||
| - | + | ||
| - | Dietrich's comments on illum section in OCAM3-DFS report: | + | |
| - | Sect. 3 | + | |
| - | o Have ALT/AZ been the same for all sky flats? | + | |
| - | o On what basis can one safely assume that the difference in ALT/AZ between dome and sky FFs can be neglected? | + | |
| - | o Have all dome flats been taken at the default position and PA? | + | |
| - | o Please send us the paper you've submitted to Astronomy Express and any referee reports. | + | |
| - | o I don't understand the applicability of the Fabry-Perot angle-of-incidence criterion: The filter plane is parallel to the detector plane so that there should be point (rotational) symmetry? What would break this symmetry? | + | |
| - | o Gain: Please use standard nomenclature. Gain is a term used in electronics, and its the same for each amplifier chain. Probably, you mean sensitivity or QE. | + | |
| - | o Do distinguish between effects arising within the filter and light scattered off some other structure. It should be useful to analyze pairs of images with ABSROT = + n deg / POSANG = -n deg and ABSROT = -n deg / POSANG = +n deg. Has this been done? Can it still be done? | + | |
| - | o Figs. 5 and 6: The labels of the color bar/scale on the right do need seem to agree with the caption describing the data as residuals. - What sense should one make of the patterns? | + | |
| - | o Fig. 6: Which polynomial terms were used? Was the origin kept at 0,0? | + | |
| - | o Fig. 7: Please expand the caption. | + | |
| - | o Figs. 8-10: Explain the color patterns (shields over bond wires?). | + | |
| - | o Figs. 9 and 10: Why should one believe that scattered light is gone but large-scale chip variations remain? I don't see any chip-to-chip patterns. The requirements seem to be missed by some factor. | + | |
| + | [[projects:omegacam:illuminationcorrection|Internal page on Illumination variations and correction for OmegaCAM]] | ||
