NOTES ON THE HERSCHEL DATA REDUCION OF THE HELGA DATASET 
(GT1_jfritz_1)
Papers from the HELGA Collaboration:
Fritz J. et al. (2012) A&A, 546, 34. (HELGA I)
Smith M. W. L. et al. (2012) ApJ 756, 40 (HELGA II)
Ford G. P. et al. (2013) ApJ 769, 55 (HELGA III)
Viaene S. et al. (2014) A&A 567, 71 (HELGA IV)
Mattsson L. et al. (2014) MNRAS 444, 797 (HELGA V)
Kirk J. et al. (2014) arXiv1306.2913 (HELGA VI)


General note: due to a difference in the star-trackers, the data from both
instruments suffer from an astrometric offset: 
RA: 14.16" 
DEC: 6.91"
which was hence accounted for in the data reduction process.

The observations were performed in parallel mode at fast scan speed.
The OBS_IDs are the following:
1342211294
1342211309
1342211319
1342213207

PACS DATA REDUCTION (By Jacopo Fritz: j.fritz@crya.unam.mx)

The PACS data were processed up to Level-1 with HIPE version 10.0.0, and 
then converted into format readable by SCANAMORPHOS (Roussel, 2013) with
the script convertL1ToScanam.py in HIPE. SCANAMORPHOS v23 was then used to
build the maps from the four scans. No deglitching was applied in HIPE, as 
the deglitcher in SCANAMORPHOS was adopted. Furthermore, the "jumps_pacs" 
was adopted, to get rid of brightness discontinuities. The chosen 
pixelsize is 1.70" and 2.85" for the 100 and 160 μm maps, respectively.

Both maps at 100 and 160 μm were split into separate planes in order to
reduce the size of the single files. The four planes are identified by
the following suffixes:
_img: it is the science frame;
_err: is the error map;
_wei: weight map
_drf: drift map
(see the SCANAMORPHOS paper and user manual for more information about
the meaning of these).
 
SPIRE DATA REDUCTION (By Matthew Smith: matthew.smith@astro.cf.ac.uk)
NOTE: two SPIRE data sets are made available: one optimised for point-
source photometry, and the other for extended-source photometry.

The SPIRE data were processed up to Level-1 (i.e. to the level where the
pointed photometer time-lines have been derived) with a custom driven
pipeline script adapted from the official pipeline (POF5_pipeline.py) as
provided by the SPIRE Instrument Control Centre (ICC).

This Jython script was run in HIPE with the continuous integration build
number 11.0.1200. However, we manually patched in the new v12
calibration product into the pipeline. The differences from the standard
pipeline are that we used the sigmaKappaDeglitcher task instead of the
ICC-default waveletDeglitcher and we did not run the default
temperatureDriftCorrection and the residual, median baseline
subtraction. Instead we use a custom method called BriGAdE (Smith et
al., in prep.), to remove the temperature drift and bring all bolometers
to the same level (equivalent to baseline removal). BriGAdE removes the
temperature drift by fitting the thermistor timeline to the bolometer
signal timeline assuming a linear relationship to find the baseline to
subtract (a similar process is presented in Pascale et al. 2010). In
addition to BriGAdE’s standard source masking we also manually mask
Andromeda from the timeline fitting and adjust the length of the linear
fit to every second scan-leg (including turn-around) to avoid issues
interpolating the thermistor fitting process for the two observations
where the satellite turns-around while pointing at the centre of
Andromeda. For the two northern observations a gradient in the cirrus
was found (as seen in previous IRAS observations) in the north-south
direction, however due to baseline subtraction this is not seen in the
east-west observation. To compensate for this we use the IRAS map to
identify a roughly constant level of cirrus across the two observations
and use these regions to perform our thermistor fitting. The northern
and southern observations are then normalised to have the same zero
level and mosaicked to create the final map. We have found this method
improves the baseline subtraction significantly, especially in cases
where there are strong temperature variations during the observation.

Our final maps were created using the naive mapper provided in the
standard pipeline using pixel sizes of 6, 8, and 12 at 250, 350, and
500μm, respectively. The non-standard pixel sizes were chosen in order
for the pixels to have approximatively the size of 1/3 of the beam. The
FWHM of the SPIRE beams vary as a function of the pixel size and are
18.2", 24.5", and 36.0" at 250, 350, and 500μm, for the pixel scale
we have used (Swinyard et al. 2010). In addition the 350μm measurements
are multiplied by 1.0067 to update our flux densities to the latest v7
calibration product. The calibration uncertainties are taken to be 5%
correlated error between bands with a 2% random uncertainty, however it
is recommended that these values are added linearly instead of in
quadrature (see also SPIRE user manual 5).