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    IRTS/FILM [CII] 158-micron Line Channel (ch.2) Map
                              synopsis ver.2    31 January 2001
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1. Data Sets (Image map files)

  Four kinds of image maps are provided as below. Each map covers 12.8 x 12.8 
degrees with pixel size of 4 arcmin (consequently 192 x 192 pixels for a map). 
Line intensity are averaged in each pixel. Coordinate system is the Galactic 
on gnomonic projection. A interpolated image may be convenient to take a 
general view of a map.
 
  (1) ??-??-??-raw.fits    : [CII] line intensity map
  (2) ??-??-??-err.fits   : statistical error (1-sigma) map
  (3) ??-??-??-smp.fits   : map of sample number in a pixel
  (4) ??-??-??-intp.fits   : interpolated [CII] line intensity map

      ex.) film-c2-ns01-raw.fits (ns=North-Scan, ss=South-Scan)-(Map Number)

2. The FILM Instrument

  The Far-infrared Line Mapper (FILM) onboard the IRTS is the far-infrared 
spectrometer. The detector of the FILM is Ge:Ga photoconductor (Hiromoto 
et al. 1992). It observed the [CII] 158 micron line and FIR continuum (155 micron; 
see another document for ch.1). Detailed information of the instrument is 
described in other documents and references shown below.

*Instrument Summary*
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Detector        : stressed (for [CII] line) and unstressed (for continuum) 
                  Ge:Ga photoconductor
Beam Size       : 8 arcmin (dispersion direction) x 13 arcmin (FWHM)
Grating         : a varied line-space cylindrically concave grating
Resolving Power : 409 for the [CII] line channel and 130 for the continuum 
                  channel
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References
 Hiromoto, N. et al. 1992, Appl. Opt. 31, 460
 Shibai, H. et al. 1994, ApJ 428, 377
 Shibai, H. et al. 1996, proc. SPIE 2817, 267

3. Calibration

  The FILM has an internal calibrator lamp in order to calibrate and correct 
fluctuations of detector responsivity. Absolute calibration has been made 
independently from preflight experiments. 
  However, calibrated [CII] line intensity of the IRTS/FILM is different from 
results of other observations at present. We compared the independent 
observations of the IRTS/FILM and the balloon-borne telescope BICE (Nakagawa 
et al. 1998) for objects near the Galactic plane. The [CII] line intensity 
from the IRTS/FILM is about 65 % to 85 % (depending on intensity) compared to 
the BICE. It is also reported that the observation from the BICE is about 65 % 
compared to the COBE/FIRAS (Nakagawa et al. 1998; Bennett et al. 1994).

4. Responsivity

  Responsivity of the Ge:Ga detector has always fluctuated mainly owing to 
hitting of cosmic ray. Signals for the calibrator lamp have acquired at certain 
intervals during the observation and slow components of responsivity 
fluctuations have been corrected by post flight analysis with those data. 
Most of fast fluctuations components by cosmic ray have removed.

5. Zero Flux Level

  The IRTS/FILM has a cold shutter at an entrance aperture, and it was closed at 
certain intervals. The zero flux level for the [CII] line intensity was decided 
by those data. The zero flux level might be dominant uncertainty for estimation 
of high latitude [CII] line emission that is very weak. The zero flux level was 
stable on the whole. An accuracy estimated from the uncertainty in decision of 
the zero level is less than 3(-7) erg cm-2 s-1 sr-1.

6. Detector Noise Level

  The levels of detector noise (statistical) were almost steady during the 
observation. Noise equivalent line intensity (1-sigma, 1/2sec integrated) is 
about 3.5(-6) erg cm-2 s-1 sr-1.

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written by Makiuti S. (makiuti@ir.isas.ac.jp)