STIS Echelle Blaze Shift
Correction
Charles Bowers - NASA/GSFC
Don Lindler - Sigma Space Corporation
C. Bowers, NASA/GSFC Code 681
STIS Echelle Blaze Shift Correction
10/17/02
Echelle blaze shift effect
E230H
Wavelength [Œ]
Problem:
Spectra on the STIS UV MAMA
detectors are periodically shifted, by
moving the MSM, to avoid
excessive aging at selected points
on the detectors.
However this causes the energy
distribution (blaze function) across
each echelle order to change
compared to the the standard
calibration.
Can the effects of the shift on
energy distribution be predicted well
enough that the echelle calibration
doesn't need to be repeated with
each offset?
Solution:
Use the spectral wavelength
calibration to predict the change in
energy distribution. Correct and use
the standard calibration.
C. Bowers, NASA/GSFC Code 681
STIS Echelle Blaze Shift Correction
10/17/02
Cause of the effect
Detector
centerline
Nominal
Configuration
m
m+1
m+2
Blaze function
offset
Configuration
Following
MSM Motion
Any change in direction of light incident
on the echelle causes (1) the spectrum,
and (2) the blaze function to shift.
However, they shift by different amounts
depending on the grating geometry. The
standard calibration does not account for
this relative shift.
Spectrum offset
dφ grt
C. Bowers, NASA/GSFC Code 681
STIS Echelle Blaze Shift Correction
10/17/02
Expression of the effect
One can use the grating & blaze equations to predict the
relative motion of blaze/spectrum shift.
dispersion plane : db = −(
Spectrum shift
sin(a) + sin(b)
cos(a)
)da + (
) tan(v)dv (3)
cos(b)
cos(b)
cross dispersion plane : dv = −dv
(4)
Note: Since the STIS echelles are out-of-plane (φ≠ 0) a change of incident
ray angle perpendicular to the dispersion plane will still cause a spectral
shift.
Blaze shift
db 0 = −da
(6)
These equations can be combined to express the blaze shift as a function
of the measured shift of the spectrum, at the detector, in both the
dispersion (∆X) and cross dispersion planes (∆Y).
cos(b)
tan(vÂ)
Blaze shift(disp pl) = cos(a) DX − (sin(a) + sin(b))( cos(a) )DY
Table 2: Nominal values of the blaze shift terms
Mode
E140M
E230M
E140H
E230H
mx
1.23
1.14
1.6
1.6
my
0.06
0.04
0.2
0.21
The blaze shift can be expressed as function of the spectral
shift in both ∆X and ∆Y
C. Bowers, NASA/GSFC Code 681
STIS Echelle Blaze Shift Correction
10/17/02
Comparison with
measurements: E230H
Inclusion of (1) the cross dispersion term, and (2)
the hitherto unknown time dependent coefficient
permits a very good estimate of the blaze position
using the calibration lamp spectrum.
Question: What causes the time dependent shift?
C. Bowers, NASA/GSFC Code 681
STIS Echelle Blaze Shift Correction
10/17/02
What is varying with time?
E140H repeats: all identical MSM
positions over a 4.4 year period
Spectrum stability:
The X and Y positions of the echelle spectra
(E140H- 1416) vary by no more than ±2 pixels.
The entire optical system following the slits
(MSM, echelle, camera mirror, detector) is very
stable.
X offset
Y offset
Blaze stability:
the blaze position varies by ~30 pixels over the
same time period.
Conclusion - the grating blaze itself is
changing slowly with time
Mode
E140M
E140H
E230M
E230H
Blaze shift at
detector
[pixels/yr]
2.9
7.7
0.7
6.2
Blaze tilt change
[”/year]
5.9
15.4
1.5
12.5
Without the high stability of HST+STIS such a small change would
likely escape notice.
C. Bowers, NASA/GSFC Code 681
STIS Echelle Blaze Shift Correction
10/17/02
How well can we correct?
Mode 230H
Before correction
Wavelength [Œ]
determine ∆X,∆Y,∆t
coefficients
following correction,
overlap is good to
~1.5%.
After correction
Wavelength [Œ]
C. Bowers, NASA/GSFC Code 681
STIS Echelle Blaze Shift Correction
10/17/02
The Blaze Shift algorithm
For each mode, use all available science data of continuum
sources.
Spectral shifts were determined from accompanying wave
calibration images.
Blaze shifts were determined by shifting echelle ripple pattern
until overlap regions are coincident.
Fit to determine appropriate coefficients of ∆X,∆Y,∆t terms.
To apply to a spectrum, use the wave calibration image to
determine ∆X,∆Y and apply the time correction => shift the
sensitivity curve appropriately.
C. Bowers, NASA/GSFC Code 681
STIS Echelle Blaze Shift Correction
10/17/02
Conclusions/
recommendations
Sensitivity curves have not been reduced for echelle
secondary settings. Algorithm should work equally well.
Blaze angle change is linear with time over the 4.4 years
evaluated. This trend should be periodically checked in the
future to look for any deviations.
MSM offsets are no longer used in the echelle modes. Use of
the Blaze Shift algorithm will still apply the linear time
dependent correction to all future observations.
C. Bowers, NASA/GSFC Code 681
STIS Echelle Blaze Shift Correction
10/17/02