Issue |
A&A
Volume 414, Number 2, February I 2004
|
|
---|---|---|
Page(s) | 777 - 794 | |
Section | Astronomical instrumentation | |
DOI | https://doi.org/10.1051/0004-6361:20031489 | |
Published online | 19 January 2004 |
The effect of signal digitisation in CMB experiments
1
INAF/Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34131, Trieste, Italy
2
Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20131, Milano, Italy
3
IASF/CNR, Sezione di Bologna, Via Gobetti 101, 40129, Bologna, Italy
4
IASF/CNR, Sezione di Milano, Via Bassini 15, 20131, Milano, Italy
Corresponding author: M. Maris, maris@ts.astro.it
Received:
2
April
2003
Accepted:
15
September
2003
Signal digitisation may produce significant effects in balloon –
borne or space CMB experiments, when the limited bandwidth for
downlink of data requires loss-less data compression. In fact,
the data compressibility depends on the quantization step q
applied on board by the instrument acquisition chain. In this
paper we present a study of the impact of the quantization error
in CMB experiments using, as a working case, simulated data from
the Planck/LFI 30 and 100 GHz channels.
At TOD level, the effect of the quantization can be approximated
as a source of nearly normally distributed noise, with RMS , with deviations from normality
becoming relevant for a relatively small number of repeated
measures
. At map level, the data
quantization alters the noise distribution and the expectation of
some higher order moments. We find a constant ratio,
, between the RMS of the quantization
noise and RMS of the instrumental noise, σ over the map
(
0.14 for
),
while, for
, the bias on the expectation for
higher order moments is comparable to their sampling variances.
Finally, we find that the quantization introduces a power excess,
, that, although related to the instrument and
mission parameters, is weakly dependent on the multipole
at
middle and large
and can be quite accurately subtracted.
For
, the residual uncertainty,
, implied by this subtraction is only
1–2% of the RMS uncertainty,
,
on
reconstruction due to the noise power,
. Only for
the quantization
removal is less accurate; in fact, the
noise features,
although efficiently removed, increase
,
,
and then
;
anyway, at low multipoles
.
This work is based on Planck LFI activities.
Key words: methods: data analysis / statistical; cosmology: cosmic microwave background
© ESO, 2004
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