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  1. Data Management
  2. DM-9046

Filter characterisation for SNIa cosmology

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    • Team:
      DM Science

      Description

      At the DESC meeting this summer (2016), Nicolas Regnault showed in the Photometric Corrections session that in order for LSST SNIa cosmology not to be systematics limited, the nominal "edge" of the filters, and the evolution thereof, needs to be known to and monitored at the 0.1nm level.

      This is a somewhat surprising result (to some at least), and imposes some very stringent requirements on the photometric calibration system.

      There is currently nothing in the SRD stating that we need knowledge of the filters at this level so this putative requirement is not being flowed down to the design in LDM-151.

      This ticket is for someone to decide if a change to the requirements is necessary as this will likely involve making hardware change requests, given the current plans for filter and photometric calibration hardware.

      A link to the slides can be found here:
      https://confluence.slac.stanford.edu/display/LSSTDESC/Photometric+Corrections

      https://confluence.slac.stanford.edu/download/attachments/211803218/sncalib-desc-oxford.pdf?version=1&modificationDate=1468915736000&api=v2

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            wmwood-vasey Michael Wood-Vasey added a comment -

            I think the root of the concern from the SN cosmology community is that it seems possible that LSST could produce standardized fluxes of stars to better precision that LSST could tell you the per-wavelength transmission function.

            This can be the case merely implicitly because one never quoted a transmission function, e.g., in the traditional ("legacy") model of calculating color terms between standard systems and your observation, one doesn't explicit calculate the transmission function, even though the terms one applies could be inverted to do so. But it also is likely to be the case from an available information perspective, as the range of color-color-color-color space occupied by main-sequence stars is a subset of the overall color space, so perhaps there are degeneracies possible in the effective transmission function that are not resolved solely by calibrating stars.

            One potential action would be to clarify if we have an explicit per-epoch requirement on the knowledge of the transmission function. I think the answer is no, and I'm leery of trying to writing one down (the easy thing to write down isn't constraining enough, and the naive desirement one would like to write down is too constraining).

            Show
            wmwood-vasey Michael Wood-Vasey added a comment - I think the root of the concern from the SN cosmology community is that it seems possible that LSST could produce standardized fluxes of stars to better precision that LSST could tell you the per-wavelength transmission function. This can be the case merely implicitly because one never quoted a transmission function, e.g., in the traditional ("legacy") model of calculating color terms between standard systems and your observation, one doesn't explicit calculate the transmission function, even though the terms one applies could be inverted to do so. But it also is likely to be the case from an available information perspective, as the range of color-color-color-color space occupied by main-sequence stars is a subset of the overall color space, so perhaps there are degeneracies possible in the effective transmission function that are not resolved solely by calibrating stars. One potential action would be to clarify if we have an explicit per-epoch requirement on the knowledge of the transmission function. I think the answer is no, and I'm leery of trying to writing one down (the easy thing to write down isn't constraining enough, and the naive desirement one would like to write down is too constraining).
            Hide
            wmwood-vasey Michael Wood-Vasey added a comment -

            I read the document (finally). It makes two key statements

            • Photometric calibration (in relative color) must be know to sub mmag level.
            • The average effective filter mean wavelength needs to be known to better than 0.1 nm.

            I agree with the conclusion that DESC SN Cosmology requires sub mmag precision for SN Ia lightcurves to reach the full potential of LSST SNeIa. Figuring out how to do that is within DESC's remit and not the Project or Operations. It's also possible, although challenging.

            The 0.1 nm specification from the document is a requirement on the knowledge of the mean system bandpass for that filter (i.e., the "effective wavelength of the filter"). Conceptually this is about the issue of when we say that the precision in "g" band is 3~mmag, what did we mean by "g"?

            It is not a requirement on determining the edges of the filter response to 0.1 nm on a per-visit basis, which would lie somewhere between "not possible" and "crazy".

            While the paper presents its analysis as being optimistic, I think they're missing an opportunity for further self-calibration. In particular, the analysis presented in the paper ignores the fact that SNeIa at a range of redshifts will be being observed simultaneously in each image. So an effective wavelength variation would affect a set of supernovae, and you would know which ones those were.

            Show
            wmwood-vasey Michael Wood-Vasey added a comment - I read the document (finally). It makes two key statements Photometric calibration (in relative color) must be know to sub mmag level. The average effective filter mean wavelength needs to be known to better than 0.1 nm. I agree with the conclusion that DESC SN Cosmology requires sub mmag precision for SN Ia lightcurves to reach the full potential of LSST SNeIa. Figuring out how to do that is within DESC's remit and not the Project or Operations. It's also possible, although challenging. The 0.1 nm specification from the document is a requirement on the knowledge of the mean system bandpass for that filter (i.e., the "effective wavelength of the filter"). Conceptually this is about the issue of when we say that the precision in "g" band is 3~mmag, what did we mean by "g"? It is not a requirement on determining the edges of the filter response to 0.1 nm on a per-visit basis, which would lie somewhere between "not possible" and "crazy". While the paper presents its analysis as being optimistic, I think they're missing an opportunity for further self-calibration. In particular, the analysis presented in the paper ignores the fact that SNeIa at a range of redshifts will be being observed simultaneously in each image. So an effective wavelength variation would affect a set of supernovae, and you would know which ones those were.
            Hide
            ctslater Colin Slater added a comment -

            The SST discussed this and would like to understand what these constraints would translate to if they were framed in terms of DM's requirements, e.g. on single-epoch measurement of source color. Without that, it's hard to know what DM can do about this (or whether in practice this is the dominant systematic uncertainty).

            Show
            ctslater Colin Slater added a comment - The SST discussed this and would like to understand what these constraints would translate to if they were framed in terms of DM's requirements, e.g. on single-epoch measurement of source color. Without that, it's hard to know what DM can do about this (or whether in practice this is the dominant systematic uncertainty).
            Hide
            wmwood-vasey Michael Wood-Vasey added a comment - - edited

            I've attached the relevant DESC Note by Hazenberg, Betoule, Bongard, Le Guillou, Regnault, Gris for DESC
            calibration_impact_forecast.pdf

            Show
            wmwood-vasey Michael Wood-Vasey added a comment - - edited I've attached the relevant DESC Note by Hazenberg, Betoule, Bongard, Le Guillou, Regnault, Gris for DESC calibration_impact_forecast.pdf
            Hide
            lguy Leanne Guy added a comment -

            Robert Lupton What is the status?

            Show
            lguy Leanne Guy added a comment - Robert Lupton What is the status?

              People

              • Assignee:
                rhl Robert Lupton
                Reporter:
                mfisherlevine Merlin Fisher-Levine
                Watchers:
                Colin Slater, Johann Cohen-Tanugi, John Swinbank, Leanne Guy, Merlin Fisher-Levine, Michael Wood-Vasey
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