Diapositive 1

1
Spectrograph calibration
Determination of specifications Calibration
strategy
Note in http//www.astrsp-mrs/snap/spectro/calib-s
pectro.ps
2
Effect on physics

• Any systematic effect causing a dependence in
  redshift of the flux
• will affect the budget error. As example a linear
  effect as f f (1az) will imply
• to control cosmological parameters at 2 need a
  slope alt 2
• to control w at 10 , need a slope alt 2
• Calibration error can go inside this systematic
  budget error if
• spectra are used to reduce systematic with
  corrections
• there is a dependence along the wavelength range
  in flux or in wavelength
• Statistical error will not affect so much
  the error budget
• Overall scale error would have NO IMPACT and
  will not be considered

3
Specifications
Evaluations of a dependence along the full
wavelength to control an effect at 2

• Si line measurement for velocity/temperature
  measurement
• Evaluate Si line position drift on the l range
• Disentangle dust evaluation and metallicity
  effect
• Evaluate the relative flux variation on the l
  range
• Spectrophotometric calibration for cross check or
  filter monitoring
• Not affected by a linear dependence, control on
  quadratic variation
• Redshift measurement if done is affected by the
  same error type
• evaluate wavelength variation

4
SUMMARY
goal flux Wavelength (A)
Si line position 20
Dust measurement 5 10-2
Magnitude/filter monitoring Flat field Wavelength 5 10-2 2 10-2 70
redshift 25
5
Calibration strategy

• 3 steps
• Optical distortion measurement and corrections
• Wavelength calibration
• Flux calibration

6
Distortion measurement
Iso l

• Adjust parameterization of the detector plan
• with optical simulation
• Precise mapping on ground with monochromatic
  point source
• Verify after launch and correct if needed
• Not expected to change in time

spatial
spectral
7
Wavelength and flux calibration
Wavelength calibration Need lamp covering the
full wavelength range with lines Expected
precision with spectral dithering is 20 A at l
1.7 mm To be done after launch to correct mapping
if needed not expected to change a lot,
monitoring only Flux calibration Flat field
extended lamp as the imager one with broad
spectrum variation of gain can be control at 1 .
Absolute calibration Needed to correct residual
effects and the wavelength dependence at 2 Use
calibrated stars with calibrated spectra Need
shutter for short exposure time Periodicity can
follow the imager one PSF calibration stars
with narrow lines Slit effect correction need
spatial dithering
8
Future plan and studies

• Detailed simulation development will provide
• Optic studies detector mapping, distortion
  corrections, PSF shape and size, quantification
  of dithering
• Detector studies noise and dark current effects,
  gain variations, inhomogeneity estimation
• Spectral sources simulation simulate broad or
  line sources for calibration procedure

Long term Put the complete Calibration
procedure in the full simulation to evaluate
impact on physics
9
Requirements

• Extended lamp similar to imager source for flat
  fielding
• Lamps with lines which cover the full wavelength
  range
• (Can be also the imager lamp)
• Calibrated stars with calibrated spectra at 2
  in flux
• Shutter for short exposure time of bright stars (
  2)
• (or use the imager shutter..)
• Spatial dithering for slit effect
• Spectral dithering possible for wavelength
  calibration