The "mini-scan map" mode is a particular case of the scan mapping mode,
where the the scanning is done along the 2 diagonal of the bolometer detector.
We advocate to use this mode for point-sources,
in a so-called "mini-scan map" configuration, with short (3 arcmin) scan legs.
The advantages for the mini--scan map mode over the original chop-nod point source mode are:
better point-source sensitivity in all bands as a high-pass filter can be used to remove 1/f
noise up to higher frequencies.
it provides a better characterisation of the close vicinity of the target and larger scale
structures in the background
also targets with positional uncertainties of 10 arcsec or more are still perfectly covered
the final map has a much larger area of homogeneous coverage (about 50 arcsec in diameter)
depending on observation configuration.
more pixels see the target, the impact of noisy, variable and dead pixels
is less problematic
no negative beam in final map.
In case of using the scan map mode for point-sources and very small fields we propose the following configuration:
- medium scan speed (20 arcsec/s)
scan angle in array coordinates along the two array diagonal directions: 110 and 70 degrees
scan length: 3.0, (the array diagonal has about 4 arcmin).
The source is on-array
during satellite constant-speed parts if the observer follows
the mini scan-map recommendations, but during satellite turn-arounds
and acceleration and deceleration phases the source is outside
the array. In order to have the source always on-array during all
phases one should select a scan-leg length of 2.5 arcmin.
Note that scan-legs have to be multiples of 5 arcsec.
small and even number of scans: 4, 6, 8, 10, ...
for minimisation of satellite movements and a match to the array diagonal.
small leg separation: 2...5 arcsec with the smaller separation for a larger number
of scan legs and vice versa (to have the source on-array in all legs).
Examples: 8 scan legs of 3 arcmin length and 4 arcsec separation.
This map would then match a sky region with the width of about 3xFWHM in the red band
with very high coverage.
- repetition factor: as needed to reach the required sensitivity
- cross-scan maps: it allows to apply all kinds of map-making techniques
and not just the high-pass filtering.
The cross-scans are also useful to obtain higher photometric accuracy for faint sources
and better spatial characterisation in the near source vicinity.
It is recommended to group/concatenate the 2 cross-scan AORs to minimise slew overheads.
Each AOR will have its own 30 s calibration block.
Do not use the homogeneous coverage in mini-scan map mode !
Coverage map for 2 mini scan-maps at array angles of 70 and 110 degrees.
The homogeneous, high-coverage area (circle) is about 50 arcsec in diameter.
The 1 hour point-source sensitivities that can be achieved with this mode are reported
The sensitivity in miniscan map mode is returned in the column "central area point-source sensitivity"
in the "instrument performance summary" window.
Otherwise the sensitivity in the central area of the mini-scan map mode can also be directly estimated
using the formula in Section 5.1.3
and computing T (on source integration time) as follows:
T = (scan_leg_length/20) * number_of legs * map_repetition_factor