Changes between Initial Version and Version 1 of PS1_Photometric_System

Timestamp:

Oct 29, 2010, 9:11:10 AM (16 years ago)

Author:

jt

Comment:

--

PS1_Photometric_System

@@ +1 @@
+= PS1 Photometric System =
+
+This is a compendium of all the information for Pan-STARRS 1 bandpasses
+assembled by John Tonry.  The version is 2010-10-28.
+
+The primary sources of information are rebinned onto a uniform 1nm
+table, which is then integrated against various SEDs to get color
+terms and zero points.
+
+== Derivation of bandpasses and throughputs ==
+
+The program "homogenize" creates two files that assemble all of the
+various pieces of information onto the uniform, 1nm wavelength scale.
+
+{{{
+ ps1.phot     - transmissions of system and filters
+ ps1.zp       - sensitivities in each filter
+ ps1.bandpass - net throughput in each bandpass relative to unobscured 1.8m
+ ps1.oob      - filter reflection and out-of-band transmission (not very useful)
+}}}
+
+The columns in ps1.phot are
+
+{{{
+ wave     - wavelength [nm]
+ scatt    - atmospheric scattering loss per airmass, probably KPNO
+ molec    - atmospheric absorption loss per airmass, probably CTIO
+ airglow  - dark sky airglow [ph/sec/m^2/nm/arcsec^2]
+ alum     - reflectivity of PS1 aluminum, per surface (2 total)
+ AR       - transmission of PS1 AR coatings, per surface (6 total)
+ QE-80    - PS1 OTA QE, set to -80C
+ CCD_AR   - PS1 OTA reflectivity
+ Tput     - PS1 system total throughput, measured by laser, no filter, no atm
+ Barr:    - Average transmissions measured by Barr for 6 filters
+ laser:   - Average transmissions measured in situ by laser for 6 filters
+}}}
+
+where the laser throughput is the ratio of the signal measured when 
+filter was in place to the signal when it was out of the beam.
+
+Given this, we should expect to see (and do find)
+
+{{{
+ Tput ~ alum^2 * AR^6 * QE
+}}}
+
+The laser measurement did not have any means to put an absolute scale
+on throughput, so "Tput" is adjusted to match this product.  It is
+reassuring that this simultaneously gives zeropoints for the entire
+system that match SDSS-derived zero points quite well.
+
+[[Image(ps1_throughput.png, 50%)]]
+
+There are distinct differences between the laser-derived filter
+bandpasses and those measured by Barr: the laser red side is a bit
+brighter and the laser throughputs are higher.  Inasmuch as the
+throughputs are simply laser through filter divided by laser without
+filter, we are at a loss to understand why this difference occurs, but
+we worry about things like extra scattered light that is far out of
+focus and therefore does not contribute to a star-based sensitivity,
+or possibly a non-filled pupil that traverses the AR coating at a
+non-representative set of angles.
+
+[[Image(ps1_filt.png, 50%)]]
+
+On the other hand, we find that the SDSS-based zero points have an 0.1
+mag trend from g to y when we use (alum^2 * AR^6 * QE) and it is not
+present for Tput.  Here we worry about the accuracy of the aluminum
+and AR coating traces.  (For example the wiggles measured in the Al
+come from the overcoat, but do not appear in Tput, suggesting that the
+overcoat may not have a uniform thickness over the full aperture.)
+
+Therefore our best guess at the system response is (Tput*Barr_filter),
+and that is what is used for ps1.zp and ps1.bandpass.
+
+[[Image(ps1_trans.png, 50%)]]
+
+The photons collected from a source of AB=0 in a bandpass dln_nu should be
+
+{{{
+  ZP = 5.48e6 ph/cm^2/sec/ln_nu * Aeff * dln_nu * (scatt*molec)**secz * Tput * Barr_filter
+}}}
+where 
+
+{{{
+  Aeff = (1-VIGNETTE) * pi/4 * (180cm)^2,    VIGNETTE ~ 0.35
+}}}
+
+The zeropoints derived for PS1 listed in ps1.phot are (secz = 1.3):
+
+{{{
+ Filt   weff    FWHM   sigma    Q       ZP   ZP-SDSS
+   g     483      99  0.0870  0.0769   24.61   -0.01
+   r     619      98  0.0672  0.0965   24.85    0.01
+   i     752      90  0.0509  0.0912   24.79    0.01
+   z     866      72  0.0352  0.0592   24.32   -0.01
+   y     971      89  0.0391  0.0248   23.38   -0.01
+   w     609     267  0.1866  0.2940   26.06    0.35 (bogus, see below)
+   o     652     379  0.2471  0.4035   26.41    0.25 (bogus, see below)
+}}}
+
+The "ZP-SDSS" column here comes from a comparison of guide star fluxes
+with magnitudes from 2MASS on the SDSS system.  The w and o magnitudes
+were simply taken to be i band, incurring an error that we can now
+correct.  Note that this is calculated for a CCD temperature of -80C.
+If the CCDs were warmed to -60C the y band ZP would increase by 0.1
+magnitude to 23.48.
+
+We follow Fukugita 1996, 
+
+{{{
+  S(nu)  = 5.48e6 [ph/cm^2/sec/ln_nu] * Aeff *
+                            (scatt*molec)**secz * Tput * Barr_filter
+  weff  = exp[ int(dln_nu S(nu) lnlam) / int(dln_nu S(nu)) ]
+  sigma = sqrt[ int(dln_nu S(nu) (lnlam/weff)^2) / int(dln_nu S(nu)) ]
+  Q     = int(dln_nu S(nu))
+  ZP    = 2.5 log[ Q ]  + 27.40
+        = 2.5 log[ int(dln_nu 5.48e6 ph/cm^2/sec/ln_nu * Aeff *
+                            (scatt*molec)**secz * Tput * Barr_filter) ]
+
+    NB: 48.60 = 2.5log(h[cgs]*5.48e6) 
+}}}
+
+Note that these are dimensionless bandpasses, so AB magnitudes are
+obtained for a source of flux f_nu [cgs] as
+
+{{{
+   m    = -2.5 log[ int(dln_nu S(nu) f_nu) / int(dln_nu S(nu)) ] - 48.60
+}}}
+
+Please do *not* confuse with bandpasses traditionally presented as
+a weighting function for an energy integral.  The "starphot" program
+uses the energy weighting only for the BVRI magnitudes.
+
+As with SDSS, Pan-STARRS defines its filter system at a standard
+airmass of 1.3.  
+
+== Synthetic photometry and colors ==
+
+The program "starphot" assembles all sorts of filter bandpasses
+(Bessel BVRI and synthetic JHKs, SDSS, and Pan-STARRS1 as described
+above), all sorts of stellar SEDs (Gunn&Stryker, STScI calspec
+standards, and SPEX MLT dwarf standards), and calculates magnitudes.
+
+The result in starphot.out lists 
+
+{{{
+  Star     - running index
+  Name     - star name
+  Sptype   - ascii spectral type
+  Type     - spectral type: O0 = 1.0 through T9 = 9.9 (-1 for no type)
+  L        - luminosity class, 1-5 and 10 for white dwarf
+  V        - cataloged apparent V magnitude (except Sun is absolute)
+  H        - cataloged apparent H magnitude
+}}}
+  
+Then columns for magnitudes and uncertainties:
+
+{{{
+  Johnson/Bessel:  B, V, R_KC, I_KC, J, H, Ks
+
+  SDSS:            u_SDSS, g_SDSS, r_SDSS, i_SDSS, z_SDSS
+
+  PS1:             g_PS1, r_PS1, i_PS1, z_PS1, y_PS1, w_PS1, o_PS1
+}}}
+
+(o_PS1 is "open", i.e. no filter).  Note that 99.99 is used for a
+non-calculated magnitude (SED didn't overlap bandpass) and 9.99 is
+used for a completely uncertain magnitude (ditto).
+
+From this output file, we derive two equations for w_PS1 and o_PS1
+magnitudes that can be used for assigning guide star magnitudes
+(and therefore extinctions when these filters are in):
+
+{{{
+  (w-r) = 0.05 + 0.18 (r-i) - 0.47 (r-i)^2
+
+  (o-r) = 0.08 - 0.08 (r-i) - 0.96 (r-i)^2
+}}}
+
+Prior to this the w_PS1 and o_PS1 guide star magnitudes were simply
+being set to the i magnitude, therefore incurring the "ZP-SDSS"
+magnitude errors above from a typical star color of (r-i)=0.25.
+
+As an example of what can be gleaned from starphot.out, here is a plot
+that shows the comparison between PS1 and SDSS magnitudes as a function of (r-i).
+Note that the PS1 g filter is distinctly redder than that of SDSS (partially
+because the SDSS CCDs have better 400nm QE than PS1 and partly because
+PS1 did not feel the need to exclude the 5577 and 5460 sky lines), the PS1
+z filter is distinctly bluer than SDSS (because it is cut off on the red
+side and picked up by the y filter), and the PS1 r and i filters are very
+close to SDSS.
+
+[[Image(sdss_cmp.png, 50%)]]
+
+
+== Detailed source directories ==
+
+The full directory of all information is found in PS1_PHOT.tar.bz2.
+Within it the directory Etc has typical atmospheric extinction functions
+and OH emission, the Bessel directory contains the BVRI bandpasses from
+Bessel (199), the SDSS directory has the SDSS bandpasses, and the Std_star directory 
+has spectral energy distributions from Gunn&Stryker (augmented to the IR by 
+Bruzual and Persson), the STScI Calspec standards, and the SPEX cool dwarf
+standards.
+
+{{{
+****************************************************************
+These are the various filter curves measured by Barr, 
+in the PS1_trans directory
+
+  Note: y2 is the original 975-1025 bandpass filter
+        y3 (or y) is the new 925 long-pass filter
+
+PS_filter.g              - Extract from Barr spreadsheet of g transmission
+PS_filter.r              - Extract from Barr spreadsheet of r transmission
+PS_filter.i              - Extract from Barr spreadsheet of i transmission
+PS_filter.z              - Extract from Barr spreadsheet of z transmission
+PS_filter.y              - Extract from Barr spreadsheet of y3 transmission
+PS_filter.w              - Extract from Barr spreadsheet of w transmission 
+PS_filter.y2             - Extract from Barr spreadsheet of y2 transmission
+
+PS_filter_oob.g          - Extract from Barr spreadsheet of g out of band transmission 
+PS_filter_oob.r          - Extract from Barr spreadsheet of r out of band transmission 
+PS_filter_oob.i          - Extract from Barr spreadsheet of i out of band transmission 
+PS_filter_oob.z          - Extract from Barr spreadsheet of z out of band transmission 
+PS_filter_oob.y          - Extract from Barr spreadsheet of y3 out of band transmission
+PS_filter_oob.w          - Extract from Barr spreadsheet of w out of band transmission 
+PS_filter_oob.y2         - Extract from Barr spreadsheet of y2 out of band transmission
+
+PS_filter_refl.g         - Extract from Barr spreadsheet of g second surface reflection 
+PS_filter_refl.r         - Extract from Barr spreadsheet of r second surface reflection 
+PS_filter_refl.i         - Extract from Barr spreadsheet of i second surface reflection 
+PS_filter_refl.z         - Extract from Barr spreadsheet of z second surface reflection 
+PS_filter_refl.y         - Extract from Barr spreadsheet of y3 second surface reflection
+PS_filter_refl.w         - Extract from Barr spreadsheet of w second surface reflection 
+PS_filter_refl.y2        - Extract from Barr spreadsheet of y2 second surface reflection
+
+In the XLS directory:
+---------------------
+gbandRAWDATA.xls.bz2     - Spreadsheet from Barr with g filter transmission, etc
+rbandRAWDATA.xls.bz2     - Spreadsheet from Barr with r filter transmission, etc
+ibandRAWDATA.xls.bz2     - Spreadsheet from Barr with i filter transmission, etc
+zbandRAWDATA.xls.bz2     - Spreadsheet from Barr with z filter transmission, etc
+y3bandRAWDATA.xls.bz2    - Spreadsheet from Barr with y3 filter transmission, etc
+wbandRAWDATA.xls.bz2     - Spreadsheet from Barr with w filter transmission, etc
+y2bandRAWDATA.xls.bz2    - Spreadsheet from Barr with y2 filter transmission, etc
+****************************************************************
+}}}
+
+
+{{{
+****************************************************************
+This is the AR coating on the lenses (6 surfaces total) from Infinite Optics
+in the PS1_trans directory
+
+PS_lens.ar               - reflectivities of PS1 lens AR coatings, per surface.
+****************************************************************
+}}}
+
+
+{{{
+****************************************************************
+These are the reflectivity of the M1 and M2 coatings (provenance uncertain)
+in the PS1_trans directory
+
+  Note: M2 originally had a silver coating that degraded quickly and badly so
+        it was replaced with aluminum, presumably the same as M1.
+
+PS_m1_al.xls.bz2         - vendor supplied spreadsheet of M1 aluminum reflectivities
+PS_m2_ag.xls.bz2         - vendor supplied spreadsheet of M2 silver reflectivities
+
+PS_m1.al                 - extract of aluminum of M1 from spreadsheet
+PS_m2.ag                 - extract of silver of M2 from spreadsheet
+PS_m12.refl              - extracts of M1 aluminum, original M2 silver combined
+PS_m12_refl.gif          - vendor supplied curve of Al/Ag, as measured?
+PS_m12_asbuilt.refl      - basically same as PS_m12.refl, as measured?
+PS_m12_refl.xv           - JT's tabulation from digitization of PS_m12_refl.gif
+
+Unclear what are the best reflectivities to use, probably the aluminum
+from PS_m12_asbuilt.refl, squared for M1 and M2.  Note that there are a 
+lot of wiggles red of 800nm that probably come from the overcoat, and
+these do not appear in the laser-derived Tput, probably because they
+are not consistent across the full aperture of M1 and M2.  Also, note that
+the aluminum coatings are well below bare aluminum red of 800nm and are
+a suspect in the droop of Tput measured response relative to the prediction.
+****************************************************************
+}}}
+
+
+{{{
+****************************************************************
+On 2009-12-13 Doherty, Stubbs, Keith from NIST, and Tonry used a
+tunable laser from NIST and a calibrated photodiode to measure the
+full response of the PS1 system, from entrance aperture to detector.
+We did not have a means to get an absolute zeropoint, but we think
+that the relative measurements are accurate to a few percent.  Each
+point is therefore a relative response of the full system less
+atmosphere (think of an effective aperture) that should be consistent
+for all filters (and no filter, named "o" for open).  The r filter was
+remeasured at interleaved wavelengths on the blue side of the bandpass
+several times to get a sense of the repeatability.  The throughput
+was measured in 7 different annuli with outer radius 3200*r pixels.
+In the PS1_trans directory:
+
+laser_091213.throughput  - PS1 effective aperture (arbitrary normalization)
+
+Note that the quotient of filter to open is usually a very good match
+to transmissions measured by Barr, but there are definitely some
+disagreements, for example Barr shows a much larger change of r filter
+response as a function of radius than the laser measurement.
+
+The OTA temperature on that day was about -79C.
+****************************************************************
+}}}
+
+
+{{{
+****************************************************************
+In the OTA_QE are the QE measurements for each of the 60 OTAs in 
+GPC1; GPC1_ota.081219 shows the layout.  The *.qe files list
+measurements for each cell, the *.qetemp files list the temperature
+at the time of measurement.  Since the gain is not easy to measure
+to high accuracy on a cell-by-cell basis, there is some variation
+in measured QE, so OTA.qe provides a median QE for each OTA, where
+all cells are normalized to a QE of 0.97 at 750nm.
+
+Although the OTAs are quite similar, note that the red QE depends on
+temperature, and since the OTAs were measured at different
+temperatures the red QE's differ a bit.  Also, the Lot 3 OTAs
+apparently had a better IILA than Lots 1 and 2, and so have distinctly
+higher QE at 400-550nm.
+
+The dependence of red QE on temperature is roughly
+
+  A1 = MAX(0, MIN( 0.001-3.2e-6*(w-950nm), 1e-5*(w-850nm) ) )
+  QE = A0 + A1*(T+60C)
+
+In the OTA_QE directory:
+
+OTA.aveqe                - mean OTA QE at -60C (in PS_PRIMARY_PHOT)
+OTA_realteal.ar          - reflectivity of OTA surface
+GPC1_ota.081219          - layout of OTAs in GPC1 focal plane
+58-Lot-WaferChip.qe      - cell by cell QE measured for OTA
+58-Lot-WaferChip.qetemp  - test temperatures
+blueqe.png               - dependence of blue QE on Lot number
+red.png                  - dependence of red QE on temperature
+qe.png                   - tested QE curves
+qe-60.png                - tested QE curves, adjusted to -60C
+OTA.qe                   - QE measured for each of 60 OTAs in GPC1
+****************************************************************
+}}}