**Deep Impact - Limitations of the HRI-IR Instrument Calibration**

**By O. Groussin and K. Klaasen (April 7, 2006)**

2006-11-30 DI:McLaughlin Revised for version 2.0 of the reduced HRI-IR
PDS data set

The calibration of the Deep Impact HRI-IR instrument data currently in
the Planetary Data System (PDS) has several limitations that must be
taken into account when analyzing the data. The current DI data set
included here represents the best understanding and implementation of
the calibrations as of August 2006.  Version 1.0 of the data were
processed with earlier calibrations in November 2005 and submitted to
the PDS in fulfillment of the requirement of having the data included in
a permanent archive within 6 months of the encounter.  Calibrations were
improved during 2006 and version 2.0 of the reduced HRI-IR data set was
delivered to PDS in December 2006. However, there are known shortcomings
in the calibration process.

The calibration applied in versions 1.0 and 2.0 of the PDS deliveries
includes decompression (if necessary), linearization, dark current
subtraction and absolute spectral-radiometric calibration. All of these
aspects of the calibration have some limitations. In addition, there are
some aspects of the data for which calibrations have not yet been
adequately derived (flat field, scattered light, correction for bad
pixels, and cosmic ray removal). We summarize here those limitations,
and one should refer to Klaasen *et al.* "Deep Impact Instrument
Calibration" (2007) for a full discussion on the calibration process.

Compression of the data typically decreases the pixel-to-pixel
variations because one compressed DN (8 bits) usually represents a
larger signal range than one un- or de-compressed DN (14 bits). This
effect becomes important for cases where it is of interest to measure
low levels of contrast at low signal levels. For example, if an
uncompressed image of the coma has a mean signal of 100 DN~14~ above
background and a pixel-to-pixel variation of 4 DN~14~, a compressed
image of the same scene with LUT 0 or 1 will have a pixel-to-pixel
variation of 0 DN after decompression -- all contrast will be lost.
However, when we have high signal like on the nucleus itself, this
problem is less important since the signal is large (\>1000 DN~14~) and
so is its contrast (for example \>40 DN~14~) compared to the size of a
compressed DN (\~20 DN~14~). In conclusion, one should be careful when
interpreting low-signal-level data (\<100 DN~14~) that have been
compressed.

Correction of the non-linear response function of HRI-IR is quite
accurate and reliable in most cases. However, the correction is based on
quadrant average response functions. A given pixel might have a somewhat
different non-linearity; we have insufficient data to really know if
this is true or not. We also have seen some variation in the quadrant
average non-linearity functions over the span of the ground and inflight
calibrations, although changes during the inflight cruise and encounter
periods appear to have been minimal. We also see effects of using
subframe modes on the response nonlinearity. While these effects have
been taken into account to first order in our calibrations, not all of
the second-order effects may be understood or characterized.

The dark frame background level is strongly dependent on the exposure
time, temperature of the instrument (bench, electronics, and detector),
mode, and the recent history of detector resets and readouts. A careful
analysis of the dark level was performed to correct for these effects,
but the result is still only good to within ±10 DN in the best case.
Correction may be significantly poorer than this, especially for the
first frame in any set of HRI-IR readouts. The first frame has an
elevated background level compared to subsequent frames, and the degree
of elevation is a complex function of many variables. No correction for
this is attempted in the calibration pipeline. While the residual dark
level error is not normally an issue in the 2.0-4.8 μm wavelength range
where there is a high sensitivity of the detector, it becomes a real
issue at short (\<2.0 μm) and long (\>4.8 μm) wavelengths where the
sensitivity of the detector drops rapidly. As a consequence, one should
be extremely cautious when interpreting any data below 2.0 μm or above
4.8 μm.

The absolute and relative spectral sensitivity calibrations are limited
to the \~10% level. This uncertainty is even more pronounced below 1.5
μm, where there is clear evidence for beamsplitter effects that are less
accurately corrected in the current version of the calibration. In
addition, the effect of the anti-saturation filter introduces
uncertainties in the radiometric calibration. This filter reduces the
signal at longer wavelengths so that the thermal contribution of the
nucleus does not saturate the detector. As a result, the sensitivity of
the detector above 4.3 μm in the anti-saturation zone drops very rapidly
to zero at 4.6 μm, and one should be very cautious when interpreting
data in this region. Around 2.7 μm the transmission of the
anti-saturation filter changes rapidly from \~90% to \~40%. Small
residual uncertainties in the exact wavelength (and column) of this
sharp transition sometimes lead to calibration anomalies (artificially
high or low computed radiance), and one should be careful in any
interpretation of the data in this particular wavelength region that
fall behind the anti-saturation filter.

Flat-field correction of HRI-IR data is not yet reliable. The bad-pixel
criteria allow pixels having response rates up to 2x different than the
mean response rates in their column to be considered "good". Without a
flat-field correction, these variations remain uncalibrated. Work is
proceeding on deriving flat-field corrections for pixel-to-pixel
response rate differences, and a preliminary correction has been applied
to the unbinned data only in the December 2005 PDS delivery. However,
correcting for any spatial variations in optical throughput across the
detector appears impossible with the calibration data we have. Such
optical throughput variations appear to be no greater than about 5%.

The HRI-IR exhibits scattered light effects that are not corrected in
the calibration pipeline. Response at the 1-2% level was seen 10 pixels
off the bright limb of the Moon. And a ghost image at the 3-4% level is
seen about 35 slit widths away from the primary image in the cross-slit
direction.

Our bad pixel map criteria allow pixels to pass that have residuals to a
linear fit of up to 2% rms and up to 15% maximum. We have observed some
variation in the locations of bad pixels during flight; as of February
2006 (version 2.0), we are now using 4 different bad pixel maps for four
flight phases. For the December 2005 PDS delivery (version 1.0), a
time-independent bad pixel map with fewer bad pixels was used. There may
be some uncharacterized bad pixels that have changed between
calibrations. No bad-pixel reclamation has been performed on pipeline
calibrated data to date.

Detection and correction of cosmic ray signatures in HRI-IR frames is
not yet reliable. Such signatures are best detected by differencing
pairs of successive frames. But no pipeline process is currently being
applied to detect or correct cosmic rays in individual frames.