Attendees:  Jim Annis, Joe Bernstein, David Burke, Shantanu Desai, Susana Deustua,
            David Finley, David Gerdes, Steve Kuhlmann, Huan Lin, John Marriner,
            Choong Ngeow, John Peoples, J. Allyn Smith, Douglas Tucker
            (Apologies to those whose names I missed!)


Presentation:  PreCam Update:  Simulations (Douglas Tucker)
-----------------------------------------------------------

* PreCam description

  - proposed quick, bright survey of entire DES footprint
      using the University of Michigan Curtis-Schmidt

  - baseline instrument: 2x2 mosaic of DECam 2k x 2k CCDs

  - possible upgrades:
    + 2x3 mosaic
    + DECam 2k x 4k CCDs (if yield high enough)

  - For baseline instrument:
    + FOV of 1.6deg x 1.6deg (2.69 sq deg)
    + pixel scale 1.4 arcsec/pixel
    + 1860 fields to  cover 5000 sq deg
    + at 600 sec per field, it would take 47 nights to perform
      a single-pass PreCam Survey in all 5 DES filters
      - 600 sec:  60sec in g  (g <= 18.7)
                  90sec in r  (r <= 18.3)
                 120sec in i  (i <= 18.0)
                 150sec in z  (z <= 17.3)
                 180sec in Y  (Y <= 15.9)

* PreCam:  Benefits to DES

  - A baseline PreCam survey as described above would yield a
    catalog of several million bright stars calibrated in the
    DES grizY system (typically hundreds per DECam CCD).

  - Benefits to DES depend on how good the global calibrations
    of the PreCam survey are.  Global calibrations at the:
    + 5% level should be easy
    + 2% level should be do-able
    + 1% level would be challenging

* PreCam simulations setup

  - Using a completed PreCam survey as a source of calibration
    stars, how well can atmospheric extinction be measured and
    tracked during a night of DES operations using just the DES
    science fields observed that night as extinction standard
    fields?

  - Use the airmass and timestamp info for all the DES science
    fields observed in a given night of DES operations from
    Jim Annis's Survey Strategy simulations.

  - Assume each PreCam field has a has a systematic calibration error
    + The accepted magnitudes of all stars in a given PreCam field are
      all offset by some amount
    + Assume that this calibration error is random and Gaussian, with:
      - sigma=0.05mag (easy)
      - sigma=0.02mag (do-able)
      - sigma=0.01mag (challenging)

  - Assume a photometric equation of the form
       minst - maccepted = a + k*X
    where a = photometric zerpoint,
          k=first order extinction,
          X=airmass

  - Fit the above photometric equation during a DES night using all
    airmasses X from for DES science exposures for that night (as
    tabulated in Jims simulations).

  - Calculate and plot residuals and compare with the default
    method of nightly calibrations (observing 3 Stripe 82 fields
    at 3 different airmasses during evening twilight, once during
    the middle of the night, and once during morning twilight)

  - current simulations only consider completely photometric DES nights

* PreCam simulations results

  - Even with relatively poorly calibrated PreCam data (0.05mag
    calibrations), one can still get a good fit to the photometric
    equation comparable to that provided by the default nightly
    calibration method, as long as there are many DES science fields
    observed over the course of the night.

  - If the PreCam Survey has 0.02mag global calibrations, the fits
    improve, and it becomes possible to track trends in the atmospheric
    transparency throughout the night at the 0.02 mag level

  - If the PreCam Survey has 0.01mag global calibrations, the fits
    improve even more, but at this level, one might consider just
    using the PreCam Survey data as local standards and calibrate
    DES directly without the need for fitting a photometric equation.

  - If there are only a few DES science observations in a given
    filter during a night, it might be necessary to supplement
    them with the default strategy (observing Stripe 82 standards
    during evening and morning twilight).

* Questions/Comments/Suggestions

  - John Peoples asked what is the largest useful field-of-view
    available on the Curtis-Schmidt?  (No answer at this time.)

  - David Burke suggested adding extinction variations.

  - David Burke warned that it will be hard to achieve 2% calibrations
    in z band due to the water absorption feature in this bandpass.

  - Jim Annis asked if there would be any benefits to having PreCam
    after the first year or two of DES (when the previous years' DES
    data themselves could be used instead).  It is true that PreCam
    will become less useful as a source of extinction standards after
    DES year 1 or 2, but, as a bright survey, it will still be useful
    (for example) in calibrating metallicity relations in the DES grizY
    system, which is done by fitting data to the red giant branches of
    relatively bright star clusters of known metallicity.

  - Huan Lin offered his ascii table of photometric variations based
    on the Stripe 82 data set for use in these simulations.


Updates
-------

* Current calibration results from DESDM Data Challenge 4 (Choong Ngeow, Joe Mohr)
  - Reports are available on the DESDM wiki site.  Choong will give a summary
    report at next week's telecon.

* PreCam (Douglas Tucker, Darren DePoy, Steve Kuhlmann)
  - Steve Kuhlmann reports that a conceptual design is being put together
    for the dewar and should be ready in a couple weeks.

* CTIO-1m Reductions (Douglas Tucker, Brian Nord)
  - Nothing to report this week.
  - Last week, Brian Nord reported that he is investigating the use of ROTSE
    pipeline to process the CTIO-1m data.

* Calibration Milestones (Douglas Tucker)
  - Douglas Tucker has received several good comments on the proto-WBS
    he posted to the DES-calib group last week, but he has yet to
    incorporate these comments.

* Filter RFI (Darren DePoy)
  - Darren DePoy wasn't available this week, but word is that he is communicating
    with various filter vendors.

* Other
  - None

AOB
---

* Susana Deustua's talk on JDEM Calibrations has been penciled in for Feb 11.

* David Burke suggested that there be a discussion on the June DES and July LSST
  CTIO observing runs, perhaps at next week's telecon (dependent on Darren's
  schedule).