# Presentation: 476253

## SDSS-II Photometric Calibration:

- How well can we do?  (and why do
- we care?)
- Brian Yanny
- Fermilab
- Work done by:  N. Padmanabhan,
- D. Schlegel, D. Finkbeiner, H. Lin,Z. Ivezic,
- D. Tucker, D. Eisenstein, H. Newberg,
- many others!

## Types of photometric calibration:

- 1. Stars vs. Galaxies
-         Stars are easier, but crucially depends on modeling the PSF
-         accurately.   Galaxies are hard because of very extended
-         profiles (sky subtraction must be exact in outer parts).
-         Accuracy of Flat fields are an important limiting factor.
- 2. Relative vs. Absolute
-         Much easier to do relative photometry over small
-         areas than global photometry.  Easier to do relative
-         photometry than to put on a global system (i.e. the
-       Vega V=B=R=0.0 system).
- 3. ugriz standard star system vs. AB 'physical units system'.
- We are doing pretty well already....well enough to do a lot
- of good science....

## ~DR6 data sample

- F stars
- (0.2 < g-r_0 < 0.3)
- with
- 19.5 < g_0 < 20.5
- Good
- calibration
- Detection
- of several
- streams
- Several
- Dwarf
- Galaxies
- in halo
- Overall
- halo density
- asymmetries

- Sagittarius

- Monoceros

- Virgo

- Halo
- Dwarfs

## Color-Magnitude

- degeneracies:
- one in the blue:
- Blue Horizontal
- Branch vs.
- Blue Straggler/A stars
- One in the red:
- K dwarfs vs.
- K giants
- (actually runs from G-K-M)

## Distinguishing David (K-dwarfs) from Goliath (K-giant)

## Giants have narrow, weak lines (low surface gravity),

- dwarfs have strong Mg triplets (though watch out for metals)
- Note that the photometry got it backwards....

## Since the difference in absolute magnitude between

- BHB and BS stars is about 2 mags (factor of 2.5 in distance)
- and can be 3-6 magnitudes in G-K-M (factor of 4-16 in distance)
- If we wish to use stars as 'standard candles' to trace
- structure in the halo, we must aware of and
- be able to resolve these degeneracies.
- If we can do this with photometry alone, the whole SDSSstellar data set (>100 M stars) becomes available for
- tracing structure.
- Precise color photometry (< 2%, required).  The u-band especially
- important for gravity/luminosity discrimination work, to separate
- giants and dwarfs.

## Yanny et al 2000

- With no Separation....  BS or BHB?

## From Yanny et al. 2000, photometric separation of gravities

##  Blue Horizontal Branch and Blue Stragglers,halo 

-  Blue Horizontal Branch and Blue Stragglers,halo

- 80 kpc

- 45 kpc

- 12 kpc

- Sag. Trailing tail, Virgo, Leading Tail

- Monoceros

- Newberg et al
- 2006

## H. Lin, stripe 82

- co-add calibration
- comparison.

## 4 PT

- Patches
- across
- 6 2.5m
- columns

- Indicates possible issue with camcols.

## Uber cal uses crossing stripes from SEGUE and Apache Wheel to

- solve large matrix for flat fields and zeropoints across the whole sky.

## N. Padmanab.

- Difference
- between
- 'Regular
- SDSS
- Flat fields'
- and
- 'uber-cal'
- flats.
- g-band for
- the 6
- camcols,
- Note the
- range of
- +/- 2%.

- This
- is a systematic
- error
- which
- directly
- affects the
- magnitudes
- of all
- objects, and
- limits our
- current errors
- to 2%.
- However,
- it can be
- removed!!
- Resulting
- in errors
- of 0.5% or so.

## SDSS photometry is intended to be on the AB system (Oke and Gunn 1983)

- by which a magnitude=0 object has a source of F_nu = 3631 Jy.
- This is not exactly true for the SDSS, but it is close.
- Our present estimate of how close we are comes from comparing
- with STIS (HST) standards of Bohlin, Dickinson and Calzetti 2001
- and involves spectroscopy of fainter hot white dwarfs.
- u_AB = u_SDSS -0.04.
- g,r,i _ AB = g,r,i_SDSS +/- 0.01
- z_AB = z_SDSS + 0.02
- There is an uncertainty of at least 1% in these numbers, and
- more in the u band.   These conversions are important for
- SN precision work, and for converting work between ugriz and other
- filter systems such as UBVRI and physical models of stellar
- atmospheres and Galaxy population synthesis work.

- AB photometry is really hard to absolutely calibrate!

## Summary of photometric calibration status:

- 1. Stars vs. Galaxies
-         Stars are easier, but crucially depends on modeling the PSF
-         accurately.   Galaxies are hard because of very extended
-         profiles.  In the limit, one expects stellar photometry to
- 	  be better, and for galaxy photometry to approach that of
-         stars.
-         Accuracy of Flat fields are an important limiting factor,
- 	  can be improved.   Currently 2%, can get down to <<1%.
- 2. Relative vs. Absolute
-         Much easier to do relative photometry over small
-         areas than global photometry.  Easier to do relative
-         photometry than to put on a global system.  Can
-         get relative to about 0.5% with ubercal.  Absolute may
- 	  still be at about 1.5-2%, but that's vs. Vega system.
- 3. ugriz filter system vs. AB 'physical system'
-            4% offset in u, errors of 1-2%, may be hard to improve.

## Improving the calibration of the SDSS imaging from about 2%

- to about 0.5% (relative) and perhaps 1% (absolute) opens the
- door to new science:
- 1. Greatly improved Photometric parallax (distances from colors) for
-  over 100M Milky Way (and halo stars)
- 2. Improved dwarf/giant discrimination (without needing spectroscopy)
- 3. Photometric metalicity indicators
- 4. Improved quasar/stars separation via photometry
- 5.  Improved photometric redshifts for galaxies (leading to improved
- large scale structure results)
- 6.  Note that stripe 82 (equator south), with repeat co-added
- scans (50 to date and growing) offers oppurtunity for deeper,
- more accurately calibrated 300 square degree gold mine!