James Muzerolle's Talk (pdf)

24 µm Data Reduction

24µm DAT ­ mips_sloper
Input: fits file with extensions
each extension corresponds to a DCE
# planes per extension: Nreads (RAW), 2 (SUR)
units: DN/ read
Corrections: applied to slope and/or first
difference for SUR data
Uncertainties from each correction added in
quadrature, combined for final output frames

24µm DAT ­ mips_sloper
Order of processing:
droop subtraction
(rowdroop subtraction)
dark subtraction
linearization
saturated slopes replaced by first difference

24µm DAT ­ mips_sloper
Output
SUR: 7 planes
1) corrected slope w/ replacements 5) uncertainty
2) uncertainty 6) corrected 1 st
difference
3) data quality flag 7) uncertainty
4) corrected slope
units on all slope & uncertainty planes: DN/ sec

24µm DAT ­ mips_sloper
Output (cont.)
data quality flags (* important only for RAW):
1 = missing data 32 = noise spike*
2 = auto. rejected read* 64 = cosmic ray*
4 = bottom rail saturation, or 128 = no linearity correction
possible SUR 1 st diff. saturation
8 = upper rail saturation 256 = readout jump*
16 = reset (Ge only)

24µm DAT ­ mips_caler
flat field division using calibration image

on­orbit performance
Formal uncertainties after corrections & flatfielding
~ 1.5% for typical backgrounds
Droop correction working properly: no shifts in
background level apparent in maps (except where
there is large­scale saturation)
Linearity correction confirmed: no evidence for
change in calibration for bright standards

on­orbit performance

on­orbit performance
effects discovered on­orbit (primarily cosmetic!)
background­dependent gradient from "read­2" effect
"dark latents" and jailbar pattern introduced by heavy
saturation
rowdroop effect does not appear
dark spots which move with scan mirror

Read­2 effect
An offset in the second read
causes SUR slope to be too
large
Depends on array position &
background level
For typical backgrounds, total
gradient of ~1.5%
Correction applied with analytic
function subtracted from SUR
slope values
pixel in row 2
row 60

Improved reductions
mips_sloper ­t ­w (soon will be default)
mips_caler +m ­F scan_dependent_flat
for best results use same corrections for flat!!

Saturation issues
Reduction in response in
readout(s) exposed to
extremely saturated source
(>10 Jy) at about the 1­2%
level
usually reduced by
redundancy; can
sometimes be corrected by
applying column­dependent
offset

Saturation issues
Reduction in response at
pixels exposed to extremely
saturated sources (>10 Jy), at
about the 1­2% level
Dark latents can appear as a
result of prior observations!!
Removed by thermal anneal
can be eliminated by dividing
by a median of the data (e.g.,
a scan leg)

Effects on calibration
All detector/ illumination effects can be corrected to
within ~ 2% for "typical" observations (not counting redundancy!)
Saturation funnies affect a relatively small fraction of
data, and can be corrected by hand in most instances
distortion/ pixel size effects introduce a similar level of
uncertainty in photometry (see Jane & Kate's
presentations)
Final calibration uncertainties probably dominated by
systematics, such as differences in calibrator models (see
Kate)

Improved scan maps

droop subtraction
droop signal proportional to
total signal on the array

owdroop subtraction
rowdroop signal
proportional to total signal
on a given row (small!)
(will be implemented in
next DAT release)

dark subtraction
subtract calibration image;
counts are very small,
~0­3 DN/ s
typical 10s SUR dark

linearity correction
data ramps
parameterized by
quadratic fit
SUR­mode correction
uses calibration image of
ramp shape parameters