WiFeS Nod-and-Shuffle Mode
& A few remarks on cross-talk
Julia Scharwächter
Research School of Astronomy and Astrophysics
Nod-and-Shuffle (N&S) Technique
“Sky” lines: affect red part of optical spectrum
O lines, OH bands
O Na
180
160
140
counts [unitless]
120
100
80
60
40
20
0
-20
5600 5800 6000 6200 6400 6600 6800 7000
wavelength [angstroms]
Workshop on WiFeS data reduction - April 28th, 2010
Nod-and-Shuffle (N&S) Technique
Basic principle:
(1) expose on object
shuffle charge down,
(2)
nod telescope to sky
(3) expose on sky
shuffle charge up,
(4)
nod telescope to object
(5) expose on object
etc...
Workshop on WiFeS data reduction - April 28th, 2010
WiFeS raw frame in N&S mode
sky
object
storage
Workshop on WiFeS data reduction - April 28th, 2010
IRAF pipeline for N&S mode
“dosubshuffled = yes”
N&S image Pipeline-created
(bias-subtracted) “sky” image
shifted “down”
by 80 pixels,
optionally scaled
A (possible
modification to
B
add smoothing
of sky in y)
Workshop on WiFeS data reduction - April 28th, 2010
IRAF pipeline for N&S
A - B =
Workshop on WiFeS data reduction - April 28th, 2010
Case study: N&S with Tobj = Tsky
before sky subtraction after sky subtraction
N&S disadvantages: N&S advantages:
Increased noise (√2) Excellent sky line
removal
Less time on object
(sky integrations) Identical optical path
and pixels, no time lag
see Glazebrook & Bland-Hawthorn 2001
Workshop on WiFeS data reduction - April 28th, 2010
Case study: N&S with Tobj = Tsky
Bias subtraction
Interesting alternative: Implicit bias subtraction in N&S
(Cbias + Cdark + Csky + Cobj) - Tobj/Tsky * (Cbias + Cdark + Csky)
Generic N&S bias problem in the central slice
corresponding sky slice
one object slice falls
partly on upper and
lower amplifiers
Workshop on WiFeS data reduction - April 28th, 2010
N&S artefacts in central slice
“N&S sky-subtracted” bias frame
artefact from
central slice
Workshop on WiFeS data reduction - April 28th, 2010
A few remarks on cross-talk
Workshop on WiFeS data reduction - April 28th, 2010
Case study: cross-talk
CCD red, lower left amplifier
Workshop on WiFeS data reduction - April 28th, 2010
Case study: cross-talk
CCD red, lower left amplifier
Workshop on WiFeS data reduction - April 28th, 2010
Case study: cross-talk
Contents
red CCD
1. 1
1.1.This A2 A1
correction: 1
A3 A4
1
1.1.
(1) A3 = A3 − (A1,xy × 8 10−4 + A4,x × 3 10−4 )
flipped in x and y flipped in x
Cross-talk on the 0.001% to 0.01% level
More complicated in case of saturation!
Workshop on WiFeS data reduction - April 28th, 2010