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Cal-R1 » History » Version 3

Gary Bernstein, 08/28/2012 09:18 AM

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h1. Cal-R1
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A photometric model (i.e. flat fields and sky-­‐subtraction methodology) is in place which produces relative magnitudes between bright stars that are reproducible to <0.02 mag RMS on different exposures taken (a) at different times during a cloudless night (b) on different cloudless nights up to 3 weeks apart. [This is essentially a test of whether we can construct a flat field that describes system response to light and remains stable. DECAM TG-­‐12. DECam TO-­‐8 covers pupil ghosts, TD-­‐10 is QE gradients]
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Cal-G4 is same but cuts the tolerance in half to 0.01 mag RMS.
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h2. Prerequisites
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* Observation of rich stellar density field during photometric conditions for model construction.  Dither pattern?
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* Use overlapping DES survey data to test model.  Need repeated observations separated by 3 weeks.
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* Need single epoch catalogs with astrometric solution <1" (and pixel-area photometric correction image)
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* Bad-pixel maps
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h2. Procedure
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* During Commissioning or early SV
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** Use observation of dithered, dense stellar field to derive large scale variations of the stellar photometric response
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* During Science Verification
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** Test model on DES survey data with repeated observations on same night or across multiple nights.
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** Repeat derivation of star flat with dithered star field, check for variation.
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In detail: the photometric model will initially be that small-scale response is accurately given by the flats and is independent of wavelength across a given band (except for very narrow-band fringing, which will wash out for any modest bandwidth of illumination).  Large-scale photometric flat field will potentially differ from the flats (due to scattered light, ghosts, dome illumination, etc.) and will need to be derived from stellar-field observations.  This photometric flat field is the thing that should be stable, whereas large-scale behavior of flat fields may vary with time.
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*Analysis steps:
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** Create high-S/N robust combined dome flats, twilight flats, and DECal flats at blue/red ends of each filter.
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** For each filter: Take ratio of twilights and DECal flats to the dome flats.  Apply high-pass filter to these images.
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** Bin flat-ratio images on scales from 1 to ~20 pixels.  Note any locations where small-scale flat structure deviates by *>1% (TBD)*.  Deviations at bad pixels are not a problem, but >1% deviations at valid pixels may cause problems for our flat-fielding paradigm.
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** Next step is to create star flat by deriving large-scale multiplicative correction to the dome flat.  Take the dithered star-field images and flatten them with domes.
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** Get stellar instrumental mags for all dithered star-field images.  Apply pixel-area corrections.
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** Use ReMatchPhoto or other solver to derive the star-flat correction vs array position that forces agreement among dithered stellar magnitudes.  Will need to include a few reference ("standard") star magnitudes to break the linear-gradient degeneracy in solution.
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** Report RMS photometric disagreement in the dithered star-field images.  Should be *<0.02 mag*.  Plot residuals vs array position to check for anomalies.
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** Apply dome flat, star-flat correction, and pixel-area corrections to generate stellar photometry for "standard mode" DES observations of other fields, e.g. Minisurvey
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** Use ReMatchPhoto to generate zeropoints for all the Minisurvey exposures.
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** Report RMS photometric deviations for stars in Minisurvey using the starflat.  *Should be <0.02 mag.*
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*Temporal stability check:
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** Take 2nd set of flats, dithered star-field images >1 week after first.
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** Measure ratio of new flats to old: after high-pass filtering, changes should be *<<1% (TBD)*.  Maybe features from dust motes on filter, etc.  How to handle these?
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** Create new star flat.  Ratio of new (star flat) x (dome flat) to old one should be constant to *<1% (TBD)*.  How would we handle dust motes etc given that we do not expect large-scale illumination of flats to be stable over time?
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h2. Verdict
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* Relative photometry of bright stars <  0.02 mag RMS
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h2. Consequences
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If the requirement is not met we may need to refine model or reexamine flat-fielding and sky subtraction methods.