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Flat field

Note: As explained in §[*], observations taken after 20100223UT include a fast flatfield ramp at their start and end. SMURF will use these to calculate the flatfield dynamically. Earlier observations have a flatfield in their headers calculated by the online system from an explicit flatfield observation taken some time prior to the observation. These flatfields are less reliable due to the variability of flatfields and the observations should be treated with caution. It is recommended that flatfield and copyflat should be used to re-calculate and re-insert the flatfield using the, now default, better ramp-fitting techniques.

The above sc2concat command applies the flatfield to the data (it does so by default). Collapsing the time series of the concatenated and flatfielded file to calculate the mean signal for each bolometer results in Fig. [*] showing the bolometer map for the s8d array used during the S2SRO: a number of dead columns can be seen as well as regions around the perimeter where the thermal gradient caused problems biasing bolometers.

Figure: Mean signal in time-series

The range of the mean value in the map is very large: from $-$28 to 30. An inspection of the cube shows that much of this is caused by differing DC levels across the bolometers. The DC term can be removed using sc2clean (mfittrend can be used as well). In general, for relatively compact sources it should be safe to remove a first order baseline to also account for a monotonic drift component in the time series.

% cat my_sc2clean_bsl.def
dcfitbox = 0
spikethresh = 0

% sc2clean sc17_con sc17_conbsl config=^my_sc2clean_bsl.def

Collapsing the time-series cube again now results in a mean in a range of $-$3.0e-13 to 2.4e-13 as shown in Fig. [*]. A histogram of the DC-removed data shows that the majority of the time-series data now are in a range of $-$0.1 to 0.1.

Figure: left: Mean signal in DC-removed time-series; Right: Histogram of values in DC-removed time series
\includegraphics[width=0.45\linewidth]{sc19_conbsl_collapse} \includegraphics[width=0.45\linewidth]{sc19_bsl_histogram}

next up previous 587
Next: Spikes and Steps
Up: Data Characteristics and Issues
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The SCUBA-2 SRO Data Reduction Cookbook
Starlink Cookbook 19
Edward Chapin, Jessica Dempsey, Tim Jenness, Douglas Scott, Holly Thomas & Remo Tilanus
3 August 2010

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