MAMBO 117 sky-noise flatfields
- The sensitivity of the array for sky-fluctuations (sky-noise) is different
from the sensitivity for astronomical sources (see
plot). This has to be taken into account in the proper removal of the
sky-noise during data reduction. In opsi the sky-noise flatfield (SNFF)
is usually calculated during the sky-noise reduction of the actual data
set. In low sky-noise conditions this might cause problems as the SNFF
determination might get very inaccurate then. It might be useful to use
a predefined SNFF instead.
- We used the data from all maps taken in Dec. 2002, Jan. 2003 and Feb
2003 to determine a standard SNFF,
checking carefully for temporal variations (see
plot) as well as variations of the SNFFs with elevation (see
plot). There seem to be no significant variations with time (except
for a few receivers, see note); there is a
clear dependence of the SNFF on elevation, but the effect is much smaller
than the inaccuracies in the usual SNFF determination and might thus also
be neglected.
- For the proper usage of a predefined standard SNFF see map.MOPSI
 
MAMBO-2 sky-noise flatfield (top) versus "astronomical" flat field
(bottom) |
  
Evolution of the sky-noise flat-field with time; the plots show the
deviation (in percent) from the median SN gain for each channel as a function
of JD. The only receivers showing significant,
systematic deviations from a median gain are 18/65/94/106/113 in a time
span between the scans 20030204_0209 and 20030207_0632 (i.e., starting
after the maintenance on Feb. 4, and ending after switching back from SIS
to bolo); the channels are all all on the same edge of the array; this
temporal offset in the gain also is responsible for the strange appearance
of these receivers in the SNFF vs. elevation plot below. |
Deviations from median SNFF plotted over elevation |
   
Median SNFF (1st plot), "mean SNFF" as derived from linear fits to
the SNFF vs. elevation plots (2nd plot), slope of the SNFF vs. elevation
distribution (3d plot), and rms of the SNFFs after correcting for the elevation
dependency. |
last modified: 21-02-2003 by TS