JWST Calibration Error Budget
Jerry Kriss
JWST Flux & Wavelength
Calibration Requirements

SR-20: JWST shall be capable of achieving data
calibration into physical units with absolute accuracies
shown in the following table.
Table 8-2. Required Calibration Accuracies
Absolute
Calibration
Accuracy
Flux (%):
Imagery
Flux (%):
Coronagraphic
Imagery
Flux (%):
Spectroscopy
Wavelength (%
resolution element):
Spectroscopy
NIRCam
5
5
N/A
N/A
NIRSpec
N/A
N/A
10
12.5
MIRI
5
15
15
10
FGS-TFI
5
10
NA
10
There are no requirements on absolute flux calibration for the FGS-Guider.
15 March 2007
2/14
JWST Astrometric Calibration
Requirements
3.7.1.5.4 Field Distortion Uncertainty
MR-120 After calibration, the uncertainty in the
Observatory field distortion correction within any SI
and the guider shall not exceed 0.005 arcsec RMS.
(There are similar ISIM requirements on knowledge
of relative placements of instruments in the focal
plane.)
These requirements enable the precise target
acquisition required for multi-object
spectroscopy with NIRSpec and coronagraphy
15 with
March 2007NIRCam, MIRI, and FGS-TFI.
3/14
Required Error Budgets

NIRCam



NIRSpec







All tied to OTE/PSF Stability
Imaging: Flux
Coronagraphy: Flux
Both tied to Pointing
Low-resolution Spectroscopy: Flux and Wavelength Error Budget
Medium-resolution Spectroscopy (IFU): Flux and Wavelength
FGS-TFI



All tied to OTE/PSF Stability.
MSA Spectroscopy: Flux and Wavelength
Fixed Slit Spectroscopy: Flux and Wavelength MSA & slits tied to Pointing Error
Budget.
IFU Spectroscopy: Flux and Wavelength
MIRI


Imaging: Flux
Both tied to OTE/PSF Stability
Coronagraphy: Flux
Imaging: Flux and Wavelength
Coronagraphy: Flux and Wavelength
Flux for both tied to
OTE/PSF Stability
Astrometric Calibration
15 March 2007
4/14
Generic Error Budget
Error in Final
Result
Statistical
Error
Absolute Flux
Error
Routine
Calibration
Errors
Pointing
Errors
15 March 2007
Background
Subtraction
Errors
Errors due to
Detector
Effects
Function of
Source
Brightness
Stability
Function
of Source
Brightness
Data
Processing
Errors
OTE/PSF
Stability
5/14
Elements of the Error Budgets (1)

Routine radiometric calibration

Ultimately limited by quality of other calibration
observations.


For NIRCam, since the current known standards are too bright, a
second tier of calibration standards will be required, which
introduces more errors.
Can tie into other error budgets. E.g., pointing errors affect
corrections for slit transmission.
15 March 2007
6/14
raw file from 1 SCA;
1 reset of detector;
M MULTIACCUM’s
Flags set as FITS
header keywords
CalNIRCamA
Reference files
used by pipeline
MASKCORR
Apply Mask
MASKFILE
REFCORR
REFCORRTYPE
Ref Pixel Correct
NOISCALC
Calculate Noise
NOISFILE
BDCORR
Sub Bias/Dark
BIASDARKFILE
NLINCORR
Non-lin Correct
NLINFILE
CRID
CR Identify
SLOPEFIT
Slope Fit
FLATCORR
Flat Correct
FLATFILE
PHOTCALC
Calculate Phot
PHOTTAB
Calibrated
Image set
Basic JWST Image Processing
Radiometric Flux Calibration
Count rate in spectral bin i
The flux in spectral bin i is
1
1
ri
f  I  

s LFi i
Inverse Sensitivity
Width of spectral bin i
Slit transmission
Low-order flat correction
Non-linearity
Raw counts
Corrected counts in pixel j
rj 
15 March 2007
nj
texp
Exposure time
Bias Level
Ref. Pix. Corr.
G( N j  B j R) 1 1

texp
FFj PFj
Fringe Flat
Pixel Flat
8/14
Radiometric Error Propagation
Standard propagation of uncorrelated errors then gives the
fractional error in the absolute flux as
 2f 
f 2
15 March 2007
2
2
2
 I2  s2  LF
 1 
 2   PF
 FF

 2  2 
 2  


2
2
2 
s LF
  PF
FF  n pix 
 I 
9/14
Elements of the Error Budgets (2)

Background subtraction

More important for fainter sources. Fractional errors scale as
2
 B 2   2



1
    PF2  PF2  
FF  nB 
 f    PF



Dominated by systematic errors for faint sources
Several potential sources:





15 March 2007
“Sky” (includes scattered light from outside the instrument)
Internal scattered light
Overlapping spectra
Overlapping orders
Light leaking through closed shutters (similar to optical ghosts)
10/14
Elements of the Error Budgets (3)

Detector Effects

Some can be characterized and corrected in routine processing:



Others that are more unpredictable:



Non-linearity
Intrapixel sensitivity
Persistence
Electronic ghosts & EMI pattern noise
Can depend on source brightness:


15 March 2007
External signals can swamp source photons
Bright sources cause their own problems in non-linearity and persistence
11/14
Elements of the Error Budgets (4)

Stability

May affect all aspects of routine calibration:




Flat fields
Flux calibration
Biggest external tie in is to the OTE and PSF
Data Processing Errors


These are expected to be small, but we must manage them.
Examples: Approximations, round-off/truncation errors
15 March 2007
12/14
Example: Error Tree for NIRSpec Flux
Calibration
15 March 2007
13/14
Conclusions


JWST calibration requirements, although “lax” by
HST standards, will be difficult to meet.
The challenges:

For flux calibration, key concerns are:





Current standards are too bright for NIRCam.
PSF stability will have impacts at the few percent level.
Detector effects need to be characterized and quantified better.
Scattered light and backgrounds will be limitations for spectra,
long-wavelength MIRI observations, and coronagraphy.
For wavelength calibration, knowledge of target placement
is crucial. Relying on target acquisition is not enough.
15 March 2007
14/14