# Presentation: 016078

## The Photometric Calibration of the DES

- Douglas L. Tucker
- (FNAL)
- Joint DOE/NSF Review of DES
- 29-31 January 2008

## Basic Observing Strategy

- Survey Area

- Overlap with
- South Pole
- Telescope
- Survey
- (4000 sq deg)

- Overlap with SDSS equatorial
- Stripe 82 for calibration (200 sq deg)

- Connector
- region
- (800 sq deg)

- Observing Strategy
- 100 sec exposures
- 2 filters per pointing (typically)
*gr* in dark time
*iZ* in bright time
- Y filter
- Multiple tilings/overlaps to optimize photometric calibrations
- 2 survey tilings/filter/year
- All-sky photometric accuracy
- Requirement:  2%
- Goal: 1%

- J. Annis

- Total Area:  5000 sq deg

## Spectro-

- photometric
- standard
- stars

## Photometric Monitoring

- 10 micron All-Sky Camera
- Provides real-time estimates of sky conditions for survey strategy
- Provides a measure of the photometric quality of an image for off-line processing
- Detects even light cirrus under a full range of moon phases (no moon to full moon)

- APO 10 micron all-sky camera

-      **For more details, see the presentation on the Sky Camera during the SISPI Breakout Session. **

## Nightly Absolute Calibration

- (Evolving) Standard Star Observation Strategy:
- Observe 3 standard star fields, each at a different airmass (*X*=1-2), between nautical (12°) and astronomical (18°) twilight (evening and morning).
- Observe up to 3 more standard fields (at various airmasses) throughout the night
- Also can observe standard star fields when sky is photometric but seeing is too poor for science imaging (seeing > 1.1 arcsec)
- Use fields with multiple standard stars
- Keep an eye on the photometricity monitors
- Nightly Absolute Calibration Strategy:
- Calibrate to the DES *griZY* “natural” system
- In theory, no system response color terms in the photometric equations
- In practice, there may be small (1-2%) color terms over the focal plane and over time
- Avoids coupling science images obtained in different filters (to first order)
- Use *u’g’r’i’z’* and *ugriz* standards transformed to the DES *griZ* “natural” system
- SDSS *g’r’i’(z’) *and *gri(z) *are similar to DES *griZ*, so transformations should be well behaved
- Use or create Y band standards
- UKIDSS observations in SDSS Stripe 82?  VHS Y-band standards?
- Synthetic magnitudes of hot white dwarfs in SDSS Stripe 82?

## 	Already part of the DES survey strategy.

	Readily observable at a range of airmasses throughout most nights during the DES program.

	2.5° wide (compares favorably with DECam's FOV (≈2.2°).

	Value-added catalogue of tertiary standards is being made
Area of Stripe 82 has been observed by SDSS > 10x under photometric conditions
~ 1 million tertiary SDSS ugriz standards (r = 14.5 - 21)!
~ 4000 per sq deg (on average)
See Ivezić et al. (2007)

- 	Readily observable at a range of airmasses throughout most nights during the DES program.
- 	2.5° wide (compares favorably with DECam's FOV (≈2.2°).
- 	Value-added catalogue of tertiary standards is being made
- Area of Stripe 82 has been observed by SDSS > 10x under photometric conditions
- ~ 1 million tertiary SDSS *ugriz* standards (*r* = 14.5 - 21)!
- ~ 4000 per sq deg (on average)
- See Ivezić et al. (2007)

- Nightly Absolute Calibration:SDSS Stripe 82 *ugriz* Standards

## Nightly Absolute Calibration:Southern u’g’r’i’z’ Standards

- Smith, Allam, Tucker, Stute, Rodgers, Stoughton
- 13.5' x 13.5' fields, typically tens of stds. per field
*r* = 9 - 18, ~60 fields, ~16,000 standards

- stars as bright as r≈13 can likely be observed by DECam with 5+ second exposures under conditions of poor seeing or with de-focusing (FWHM=1.5”).

- http://www-star.fnal.gov/Southern_ugriz/

- (Others:  SkyMapper standards?  VST OmegaCam standards?  Stars from PanSTARRS-1 3 survey?)

## Nightly Absolute Calibration:The Photometric Standards Module (PSM)

- The PSM is basically a big least squares solver, fitting the observed magnitudes of a set of standard stars to their “true” magnitudes via a simple model  (photometric equation); e.g.:
*m**inst** **-** m**std** *=* **a**n** *+* **k**X  **	**	**	**	**	**	**	**	**	**	**	**	**(1)*
*m**inst* is the instrumental magnitude, *m**inst** = -2.5log(counts/sec) *(input)* *
*m**std* is the standard (“true”) magnitude of the standard star (input)
*a**n* is the photometric zeropoint for CCD *n* (*n* = 1-62) (output)
*k* is the first-order extinction (input/output)
*X* is the airmass (input)
- A refinement: add an instrumental color term for each CCD to account for small differences between the standard star system and the natural system of that CCD:
*m**inst** **-** m**std** **=** **a**n** **+** **b**n **x** **(stdColor **‒** stdColor**0**) **+** **k**X**	**	**	**	**	**	**	**(2)*
*b**n** *is the instrumental color term coefficient for CCD *n* (*n *= 1-62) (input/output)
*stdColor* is a color index, e.g., *(g-r) *(input)
*stdColor**0* is a constant (a fixed reference value for that passband) (input)
- DES calibrations will be in the DECam natural system
- Even if SDSS Stripe 82 *ugriz* and Smith et al. Southern *u’g’r’i’z’* standards are “pre-transformed” to the DES system, eq. 2 is still useful for track changes in DECam instrumental response across the focal plane and over time

## Blanco Cosmology Survey, Fixing b’s to 0(rms=0.041 mag, 2/=4.24)

## Blanco Cosmology Survey, Solving for b’s(rms=0.017 mag, 2/=0.74)

## Global Relative Calibrations:Hex-to-Hex Zeropoint Offsets

- DECam Focal Plane:
- “The Hex”

- Global Relative Calibrations:Hex-to-Hex Zeropoint Offsets

- Jim Annis
- DES Collaboration Meeting,
- May 5-7, 2005

## Global Relative Calibrations:Star Flats

- Koch et al. 2004, ESO WFI star flats based on SDSS Stripe 82 observations (2nd order polynomial fits)

- R

- V

- Due to vignetting and stray light, a detector’s response function differs for point sources and extended sources
- Standard flat fields (domes, twilights, skies) may flatten an image sky background well, but not the stellar photometry
- The solution:  star flats (Manfroid 1995)
-  offset a field (like an open cluster) multiple times and fit a spatial function to the magnitude differences for matched stars from the different exposures
-  can also just observe a well-calibrated field once (Manfroid 1996)

- mag = -0.16  +0.03

- mag = -0.14  +0.02

## Global Relative Calibrations:The Global Calibrations Module (GCM)

- GCM Zeropoint Solver Code
- Uses matrix inversion algorithm developed by Glazebrook et al. (1994) and used by MacDonald et al (2004).
- Written in Java
- Uses cern.colt.matrix
**Input:** An ASCII table of all unique star matches in the overlap regions
**Output:**  The ZP offsets to be applied to each field, the rms of the solution, and QA plots

- GCM Star Flat Code
- Basic prototype code developed in the SDSS software environment (Tcl/C)
- Future version in Java
- Currently assumes the star flat correction is a purely radial, 3rd order polynomial

**For more details, see the talk on **
**Global Photometric Calibration **
**during the Wednesday morning **
**Data Management Breakout Session**

## Global Absolute Calibration and Final Calibration

- Global Absolute Calibration
- Compare the synthetic magnitudes to the measured magnitudes of one or more spectrophotometric standard stars observed by the DECam.
- The differences are the zeropoint offsets needed to tie the DES mags to an absolute flux in physical units (e.g., ergs s-1 cm-2 Å-1).
- Absolute calibration requires accurately measured total system response for each filter passband as well as one or more well calibrated spectrophotometric standard stars.
- Final Calibration
- Apply the magnitude zeropoint offsets to all the catalog data.

## Spectro-

- photometric
- standard
- stars

## Extra Slides

## Slide 17

## Title:  Engineering Tests and Initial Calibration of DECam CCDs
PI:  Darren DePoy, CoIs: Ricardo Schmidt, Douglas Tucker, Brenna Flaugher
Telescope:  CTIO SMARTS 1m telescope
Instrument:  DECam 2kx2k CCD
Filters:  DES g, r, i, z, Z, Y, and a 0.93-0.96 H20 absorption feature filter 
Goals:  on-sky tests and calibration of the DECam CCDs and DES filters, refine definition of DES z/Z and Y filter, initial search and calibration of DES z/Z and Y-band standards
Dates:  9-15 April 2008

**PI:**  Darren DePoy, **CoIs:** Ricardo Schmidt, Douglas Tucker, Brenna Flaugher
**Telescope:**  CTIO SMARTS 1m telescope
**Instrument:**  DECam 2kx2k CCD
**Filters:**  DES g, r, i, z, Z, Y, and a 0.93-0.96 H20 absorption feature filter
**Goals:**  on-sky tests and calibration of the DECam CCDs and DES filters, refine definition of DES z/Z and Y filter, initial search and calibration of DES z/Z and Y-band standards
**Dates:**  9-15 April 2008

- 2008A NOAO Proposal

## Written in Java
Uses the cern.colt.matrix Java classes
Interacts directly with the database:
Queries stars in standard star fields to match with photometric standards
Ingests solutions back to database
Solves for photometric zeropoints (“an”) and instrumental color term (“bn”) coefficients for all n CCDs simultaneously:
		 minst - mstd = a1 + ... + a62 + b1(color-color0) + ... + b62 (color-color0) + kX
Option to set k to a fixed value rather than to solve for it
Option to set all bn’s to a single fixed value rather than to solve for them
Outputs QA plots of the fits to the photometric solution. 
Tested in Data Challenges 1, 2, and 3
Is a nearly finished product
Still need to implement the option ability to fix individual photometric zeropoints (“an”) and/or individual instrumental color term (“bn”) coefficients

- Uses the cern.colt.matrix Java classes
- Interacts directly with the database:
- Queries stars in standard star fields to match with photometric standards
- Ingests solutions back to database
- Solves for photometric zeropoints (“*a**n*”) and instrumental color term (“*b**n*”) coefficients for all *n* CCDs simultaneously:
- 		 *m**inst** - m**std** = a**1** + ... + a**62** + b**1****(color-color**0**) + ... + b**62**** **(color-color**0**) + kX*
- Option to set k to a fixed value rather than to solve for it
- Option to set all *b**n*’s to a single fixed value rather than to solve for them
- Outputs QA plots of the fits to the photometric solution.
- Tested in Data Challenges 1, 2, and 3
- Is a nearly finished product
- Still need to implement the option ability to fix individual photometric zeropoints (“*a**n*”) and/or individual instrumental color term (“*b**n*”) coefficients

- Nightly Absolute Calibration:The Photometric Standards Module

## Blanco Cosmology Survey, Fixing b’s to 0(rms=0.041 mag, 2/=4.24)

## Blanco Cosmology Survey, Solving for b’s(rms=0.017 mag, 2/=0.74)

## INSTRUMENT MODEL:

A multiplicative flat field gradient of amplitude 3% from east to west

An additive scattered light pattern with a amplitude from the optical axis, 3% at the edge of the camera

An additive 3% rms scattered light per CCD

Solution:
Simultaneous least squares solution to the underlying relative photometry given the observations 

- A multiplicative flat field gradient of amplitude 3% from east to west
- An additive scattered light pattern with a amplitude from the optical axis, 3% at the edge of the camera
- An additive 3% rms scattered light per CCD
- Solution:
- Simultaneous least squares solution to the underlying relative photometry given the observations

- scaling bar is –0.20 mags to +0.20 mags

- Global Relative Calibrations: Simulation

- Jim Annis
- DES Collaboration Meeting,
- May 5-7, 2005

## Global Calibration Module :Global Relative Calibrations

- Let ij = <magi - magj>pairs    (note ij = - ji).
- Let ZPi be the floating zero-point of frame i, with ZPi = 0 if i > m.
- Let ij = 1 if frames i and j overlap or if i = j; otherwise let ij = 0.
- Minimize S =  ij (ij + ZPi - ZPj )2

- Method used by Oxford-Dartmouth Thirty Degree Survey (MacDonald et al. 2004)
- Developed by Glazebrook et al. (1994) for an imaging K-band survey

- Example:
- Frames 5 & 6 are calibrated.
- The others are uncalibrated.

- Global Calibration Module :Global Relative Calibrations

## Global Calibration Module :Global Relative Calibrations

| -2 | 1 | 0 | 0 | 0 | 1 |
| --- | --- | --- | --- | --- | --- |
| 1 | -2 | 0 | 0 | 0 | 1 |
| 0 | 0 | -1 | 1 | 0 | 0 |
| 0 | 0 | 1 | -2 | 1 | 0 |
| 0 | 0 | 0 | 0 | 1 | 0 |
| 0 | 0 | 0 | 0 | 0 | 1 |

| ZP1 |
| --- |
| ZP2 |
| ZP3 |
| ZP4 |
| ZP5 |
| ZP6 |

| 12 + 16  |
| --- |
| 21 + 26 |
| 34 |
| 43 + 45 |
| 0 |
| 0 |

- x

- =

- Example:
- Frames 5 & 6 are calibrated.
- The others are uncalibrated.
- (From Glazebrook et al. 1994)

- Global Calibration Module :Global Relative Calibrations

## Global Calibration Module :Global Relative Calibrations

- GCM Zeropoint Solver Code
- Uses matrix inversion algorithm developed by Glazebrook et al. (1994) and used by MacDonald et al (2004).
- Written in Java
- Uses cern.colt.matrix
**Input:** An ASCII table of all unique star matches in the overlap regions
**Output:**  The ZP offsets to be applied to each field, the rms of the solution, and QA plots
-      **For more details, see the talk on Global Photometric Calibration during the Wednesday morning Data Management Breakout Session**

## Global Relative Calibrations:Prototype Star Flat Code

- Input: 	mag = -0.040r + 0.060r2 + 0.020r3
- 	Data Points:   = 0.020 mag, npts = 1000

- Output:  	mag = -0.043r + 0.067r2 + 0.017r3

- mag

- Radial distance from center of focal plane, r [deg]

- -0.05

- +0.10

- 0.0

- 1.2

- Global Relative Calibrations:Prototype Star Flat Code

- GCM Star Flat Code
- Basic prototype code developed in the SDSS software environment (Tcl/C)
- Future version in Java
- Currently assumes the star flat correction is a purely radial, 3rd order polynomial

## Global Absolute Calibration

- Calculate the expected photon flux Fexp for each std star in each filter passband (synthetic photometry)
- Measure the magnitude for each standard star in each filter passband with the Blanco+DECam
- Calculate the zeropoint zp via the relation,
- 10**[-0.4*(mag – zp)] = Fexp

- Need:
- one or more spectrophotometric standard stars which have been calibrated (directly or indirectly) to a NIST standard source
- an accurately measured total system response for each filter passband for at least one CCD
- filter transmissions, CCD QE, optical throughput, atmospheric transmission

## ~100 Hot White Dwarfs (DA) in SDSS Stripe 82 (r=16-21)
Need to know temperature and log g for “true” SED (models)
Need high-resolution spectroscopy (ground-based) + modelling?
These set an absolute color scale

LDS 749B (DES Fundamental Calibrator?)
In SDSS Stripe 82 (RA=21:32:16.24, DEC=+00:15:14.7; r=14.8)
In HST CalSpec database (STIS observations + model)
Sets the absolute flux scale relative to Vega (i.e., Vega taken as “truth”) 

Others
E.g, G158-100, GD50, GD 71, G162-66
All are HST WD spectrophotometric standards
All are visible from CTIO
All are V> 13.0 (won’t saturate DECam at an exposure time of 5 sec (FWHM ~ 1.5”)

- Need to know temperature and log g for “true” SED (models)
- Need high-resolution spectroscopy (ground-based) + modelling?
- These set an absolute color scale
- LDS 749B (DES Fundamental Calibrator?)
- In SDSS Stripe 82 (RA=21:32:16.24, DEC=+00:15:14.7; r=14.8)
- In HST CalSpec database (STIS observations + model)
- Sets the absolute flux scale relative to Vega (i.e., Vega taken as “truth”)
- Others
- E.g, G158-100, GD50, GD 71, G162-66
- All are HST WD spectrophotometric standards
- All are visible from CTIO
- All are V> 13.0 (won’t saturate DECam at an exposure time of 5 sec (FWHM ~ 1.5”)

- Global Absolute Calibration:Spectrophotomeric Standards