Changes between Initial Version and Version 1 of Calibration_Concept_Discussion

Timestamp:

Jun 9, 2010, 10:10:32 AM (16 years ago)

Author:

eugene

Comment:

--

Calibration_Concept_Discussion

@@ +1 @@
+== Calibration of the 3pi Survey == 
+
+'''The discussion below was part of a conversation with Eric Bell about the utility of the calibration fields'''
+
+The calibration fields are effectively internal standard star fields.
+Consider the goal to re-calibrating the survey after all data have
+been taken.  Let's say we have a target photometric accuracy for the
+calibrated catalog of dM (the current goal is 1%), where dM is not the
+stability within a field for relative photometry, but the accuracy
+across the sky of any random patch.
+
+How well can we achieve this goal, and what are the drivers?
+
+First, there are 2 classes of observations : those that have been
+taken in "photometric weather" and those which have not.  For our
+purposes, "photometric weather" means that the sky transparency (dF)
+is stable to < dM, both for long periods of time and for large spatial
+scales.  Let's defer the definition of "long periods of time" for now,
+but accept that "large spatial scales" means >> GPC1 FOV.  Also, note
+the it is acceptable for dF to have coherent trends in both spacial
+and temporal scales smaller than the above; it is merely necessary
+that dF(x,t) not be decoupled from dF(x+dx,t+dt).
+
+Exposures which have NOT been taken in such conditions cannot be used
+to constrain the calibration of the catalog and must be (a) identified
+and (b) excluded from the initial analysis.  The remaining exposures
+can then be used to determine an internal photometric system.
+
+Now imagine the full sky tiled with a grid of these photometric
+exposures.  For most of the sky, the coverage is sparse.  We don't
+know the fraction of time which will be photometric on Haleakala, but
+on Mauna Kea, depending on your choice of dM, the fraction is probably
+something in the range of 50-75% of good weather conditions.  If that
+holds, that means there will be on average something like 6 - 8
+exposures per field that are photometric.  In reality the distribution
+will be complex and non-Gaussian because of choices at the observatory
+and the correlated nature of weather.
+
+There are two ways we can pin down the full system.  Observations of
+fields with external standards provide one pin.  Obviously, the SDSS
+area is a big help in this regard, especially the Stripe 82 region
+which is better characterized than the rest of the survey (with only a
+single visit per filter).  The 2 downsides of this calibration are (a)
+the internal to external color terms (especially for z and y in our
+case), and (b) more than 1/2 the sky is very far from SDSS (or the few
+other fields with high-accuracy photometric calibrators).
+
+The other way to pin down the system is to use fields with many
+observations and determine the photometric data from the internal
+consistency of the zero points.   If you examine a histogram of the
+zero points in a field which has been observed many times, the
+photometric data is seen as a well-defined peak in that distribution.
+The MD fields and the other calibration fields provide this
+measurement and act as a set of hard points on the sky.
+
+To tie together the full system, and to calibrate both spatial and
+temporal variations in the transparency, we can use both the spatial
+overlaps of neighboring images and the temporal information in
+sequences of observations.  Since the hard points are the calibration
+fields, the quality of the calibration is partly determined by the
+distance (number of overlaps) to the calibration fields.  The other
+determining factor is the time-scale between visits to a calibration
+field.  It is the timescale of these visits (and to a lesser extent
+the spatial distance between the calibration fields) which sets the
+necessary constraints on the definition of "photometric weather"
+above.
+
+The choice of calibration field re-visit timescale was set by
+observations from Mauna Kea of the temporal coherence of the sky
+transparency in photometric weather, which seems to be about 45
+minutes for 1%.  To be sure, this is not a very well determined
+number: it could be too large for Haleakala (with probably worse
+weather); it could be overly conservative if we can better detrend the
+transparency variations than in that analysis.
+
+Note that bad fields are not part of this calculus.  The calibration
+fields are used to constrain only the true zero points of the
+photometric observations.  If all of the exposures for a field are bad
+and the field cannot be calibrated, more or less calibration
+information will not help.