Hi Chris,
> I have one rather basic question: what exactly do you mean by "zeropoint"
> for an instrument whose colour terms vary over the field? Put another
> way, the photometric grids flatten the photometry only for stars of
> one colour. I think what you have to do is adjust the flats to refer
> to stars of zero colour, rather than to the mean colour of grid stars.
> Was this done? Maybe I'm missing something here, or perhaps this is
> a small effect ...
What we have here is an averaging effect, though most stars fall within
the central 20x20sq' of MegaCam's FOV where the color effect is not so
pronounced. The radial color correction has not been applied to the
Q97 catalogs when extracting the ZPs in consequence. When we'll swap
to tertiary standards over the whole FOV, this will be a must do.
> I agree with your procedure of homogenizing the drifts for riz and ug
> zeropoints separately. It would be nice to understand why this difference
> exists!
Indeed, and once again the Deep fields study will point things out a
lot more accurately I bet.
Jean-Charles.
> Not so sure about the comment about SAC. I asked 2 SAC members, and they
> had never heard of the MAPC program, nor of any program to do this kind
> of work.
>
> Chris
>
> Ray Carlberg wrote:
> > Anchoring to SDSS is crucial. As Nicolas and others can explain, the
> > Landolt system is not all that well defined in terms of a flux
> > calibration. It is the single largest systematic error in SNLS. This
> > means that it will be abandoned as better systems come along--in
> > particular SDSS.
> >
> > Amazingly the SAC seems to view the MAPC program to do this calibration
> > as a waste of engineering time. We need a coherent explanation of this
> > to gain SAC support.
> >
> >
> >
> > Jean-Charles Cuillandre wrote:
> >> Hi Chris,
> >>
> >> FYI, Chris had provided this plot at the time we were starting
> >> to worry about the decrease of ZPs (turned out to be the optics
> >> contamination). You will see the similar trend for r', i' and z'
> >> and a slightly different shape for g' while u* is quite different
> >> in 03B (interestingly this correlates the Elixir ZPs derived then).
> >>
> >> That plot along with mine prompted me to anchor r, i and z ZPs to
> >> the average behavior of r&i. And since u* was so poorly calibrated,
> >> I opted to anchor its run to run behavior to g' - sounds reasonable
> >> to you? Matching to Sloan ought to bring some answers soon!
> >>
> >> Jean-Charles.
> >>
> >>
> >>> Hi,
> >>>
> >>> Here's a short summary of the "Q97" analysis: the Smith et al. SDSS
> >>> standards located in the Landolt fields gathered from early 2003 up
> >>> to today. Even though we are working on using tertiary standards from
> >>> the Deep fields, for timing reasons and values of comparison for
> >>> gradual evolutions in the processing chain, it was decided to stick
> >>> to the Smith et al. standards for T0005. This new analysis of Q97
> >>> consists of 7000 standards exposures!
> >>>
> >>> The new Elixir flats for the analysis are called "B4". Previous
> >>> generations that have been distributed at CADC since 2003 are B1,
> >>> B2, and B3 - corresponding to various iterations of the photometric
> >>> grids as we were attempting to address the photometric flatness
> >>> issues. The B4 version is the dual-color derived grid by Nicolas
> >>> Regnault for SNLS. We adopted 4 grids for the lifetime of the camera,
> >>> matching the time when an instrument change caused a significant change on
> >>> the illumination pattern of the camera (grids are typically captured
> >>> after any instrument evolution, and sometimes we see no change, such
> >>> as the light baffle which oddly enough did not have any impact).
> >>>
> >>> Grid Range (Runs) Event
> >>> ---------------------------------------------------------
> >>> 03B 03Am01 -> 04Bm04 Initial setup
> >>> 04B 04Bm05 -> 05Am06 L3 lens flip
> >>> 05B 05Am07 -> 06Am03 Lift of WFC
> >>> 06B 06Am04 -> 06Bm05 WFC contamination
> >>> 05B 06Bm06 -> present WFC cleaning
> >>>
> >>> For matching the dates or QSO runs, please consult the following page:
> >>> http://www.cfht.hawaii.edu/Instruments/Imaging/MegaPrime/megaprimeschedule.html
> >>>
> >>> The situation with T0004 is that it had a mix of data generated with
> >>> flats of various generations (B1/2/3). The idea for T0005 is that all
> >>> LS data are processed following the exact same recipe for the photometric
> >>> grid for all epochs.
> >>>
> >>> The first plot (ZPoffsets_Data.B3.png) presents the old collection
> >>> of zero points from Elixir. I plotted various known events that did
> >>> have an impact (optics cleaning, mirror coating) and some less (mirror
> >>> washing). There are many more jumps visible, and we can see the u*
> >>> calibration is poor (a result mainly of Elixir getting lost on attempting
> >>> an astrometry solution on single CCDs with only 2 stars visible on those
> >>> defocused 2sec exposures, not so surprising).
> >>>
> >>> The second plot shows the same study, this time using the new collection
> >>> of flat fields (ZPoffsets_Data.B4.png). There are still many jumps but
> >>> the overall envelope has shrunk in amplitude and g' tends to be more
> >>> decoupled that riz. Also the strange increase of zero point detected on
> >>> in 03B/04A on the old processing is almost gone (we had no explanation
> >>> for that one at the time). The u* band is still rather dreadful, but at
> >>> least more balanced (Terapix had reported serious issues indeed with the
> >>> B3 version in 06A pointing to an analysis issue by Elixir).
> >>>
> >>> All things considered, the final set of zero points was set from
> >>> deriving the median value over the whole time period for each filter
> >>> (which match to a great precision the ZPs of 05Bm05 - so I anchored
> >>> all data set on that run which had a ton of Q97 observations, stats
> >>> are great on that run). I then measured the offset to that median
> >>> value for the g, r, and i, applied the g offset to the u band, and
> >>> applied the average of the offset of r and i to r, i, and z. The idea
> >>> here is to reject some noise (based on the previous plot, you can see
> >>> indeed that riz behave pretty much the same, while g is quite off, hence
> >>> it was better not to mix everything). I recreated the whole photometric
> >>> Elixir database and plotted again the offsets to the 05Bm05 ZPs, and
> >>> this is the third plot attached (ZPoffsets_Data.B4.OffsetTo05B_Final.png).
> >>> As you can see, u and g are on top of each other (u invisible) and same
> >>> for r, i and z. Looking at this plot, I wonder if I was given the correct
> >>> dates for the mirror washing - they all seem to correlate by one run offset...
> >>> Note that the signature of the calendar of B4 grids provided above is not
> >>> detectable on this data set: it is a good sign that photometric continuity
> >>> is ensured throughout the life of the camera despite instrument/recipe
> >>> changes.
> >>>
> >>> The last plot is simply the absolute value of all 5 ZPs plotted
> >>> together, with again the ug and riz having the same behavior from
> >>> run to run (ZP_Data.B4.png).
> >>>
> >>> The global CFHTLS reprocessing we are about to start will make use of
> >>> these zero points. Terapix will conduct a general photometric analysis
> >>> and we'll see how much better we are over T004 for fields that overlap
> >>> with Sloan patches.
> >>>
> >>> The future sits within the four Deep fields: the next top priorities for
> >>> Elixir (for T0006) is to integrate their hundreds of stars as tertiary
> >>> standards (and recalibrate all MegaCam data since 2003). Since last summer,
> >>> the Q97 program includes medium exposures (~2mn) on Deep fields. Even when
> >>> SNLS is over, we'll keep going back to these fields over the next years:
> >>> they will be the reference field for the lifetime of the camera. And now
> >>> that flats are (presumed) final, a focus on the defringing recipe is needed
> >>> as well.
> >>>
> >>> Time to release the data and confront them to some analysis, please let
> >>> me know if you have any comment/ideas on this!
> >>>
> >>> Thank you so much to Nicolas Regnault for his contribution on this effort!
> >>>
> >>> Jean-Charles.
> >>>
> >>> ps: for the record, Elixir photometric equations for megacam are matched
> >>> to the SNLS ones:
> >>> u* = uSDSS - 0.214 * (u-g)SDSS
> >>> g' = gSDSS - 0.156 * (g-r)SDSS
> >>> r' = rSDSS - 0.000 * (g-r)SDSS
> >>> i' = iSDSS - 0.094 * (r-i)SDSS
> >>> z' = zSDSS + 0.050 * (i-z)SDSS
> >>>
> >>
> >>
> >>
> >>
> >>
> >>
>
>
> --
> -----------------------------------------------------------------
> Chris Pritchet pritchet at uvic.ca
> Dept of Physics & Astronomy http://www.astro.uvic.ca/~pritchet
> University of Victoria
> P.O. Box 3055 250-721-7744 (office)
> Victoria, BC V8W 3P6 250-884-5047 (cell)
> CANADA 250-721-7715 (FAX)
> -----------------------------------------------------------------
Received on Fri Feb 15 2008 - 16:32:33 HST