Update for 2011A: Proposals for the nuller will
be accepted, but
availability of this instrument in future semesters is not
guaranteed.
The sensitivities given below are the current performance
levels. These sensitivity values are based on past interferometer
performance in good weather. Note that the interferometer performance
degrades in poor seeing.
In the Nuller mode the main data product is the coherent null
leakage
for a given source. The null leakage is related to the visibility as
N = (1-V)/(1+V) which in the limit of small N, approximates to V2 = 1 -
4N for the squared
visibility (the normal KI visibility measurement).
Integrations on the target source and calibrators
are interleaved in time. Each integration includes all local
(detector, background) calibrations. See the
KI nulling description for more details on the nulling observations
and how these observations can be used for measuring visibilities.
Observations with the nuller mode can only be combined with the
K/L split pupil mode within a single night. See the
configuration
page for
more details.
- Sensitivity numbers updated: August 3, 2009
- AO (adaptive optics) sensitivity:
- R < 10.0 (somewhat dependent on source color, less
sensitive than single telescope AO due to the dichroic which sends
light to interferometer)
- See the Keck Observatory
AO page for more details.
- Note that the image rotators are used in vertical angle
mode.
- Off-axis observing is NOT available in nulling mode.
- Angle tracking: For nulling, the preferred angle tracking band is
H,
although J band can be used in average or better
seeing conditions.
- H < 4.3 mag (80 Hz)
- J < 7.5 mag (80 Hz)
- 40 Hz tracking: 1 magnitude fainter limits.
- Fringe tracking:
For nulling, the fringe tracker operates in K band
with a 4 msec frame time.
- K < 4.6 - X mag, where X is based upon the expected
angular
size of the target at K. This K band sensitivity level is for the mode
in which an iris is used to filter the fringe tracker input and is the
mode in which the nulling performance limits have been demonstrated.
Additional fringe tracker sensitivity (to K < 6.0 - X mag) can be
obtained under good seeing conditions by running without this iris, but
the system has not been well characterized in this mode.
- Note that the targets CAN NOT be larger than 4.5 mas at K.
| X |
diameter (mas) |
| 0.00 |
< 1 |
| 0.25 |
1.6 |
| 0.50 |
2.2 |
| 0.75 |
2.7 |
| 1.00 |
3.1 |
| 1.25 |
3.4 |
| 1.50 |
3.7 |
| 1.75 |
3.9 |
| 2.00 |
4.2 |
- An 8 msec clock is available for the fringe tracker which
can be
used under good seeing conditions to increase the K-band limiting
magnitude
by 0.75 magnitudes over the values above at the expense of null
leakage. The nuller has not been characterized with this clock;
however, we would expect at least a factor of 2 degradation in the
null leakage uncertainty given below. We would recommend this clock
only for targets with a large expected excess, and it must be used for
both target and calibrator.
- Nuller: N band flux > 2.4*F Jy, where F is based upon the
expected angular size of the target at 9.5 microns. Note that targets
CAN NOT be larger than 20 mas at N band. Fainter objects (down to
1.7 Jy for sources less than 5 mas) can be observed at lower
nulling performance (see below).
-
| F |
theta (mas) |
| 1.0 |
< 5 |
| 1.1 |
7 |
| 1.2 |
9 |
| 1.5 |
13 |
| 2.0 |
16 |
| 2.5 |
18 |
| 3.0 |
20 |
- Spectral dispersion: in order to obtain best SNR, the default
data reduction bins the spectral data as follows:
- "wideband" channel: 8 - 9 microns.
- 10 narrowband channels across the N-band (8-13 microns).
- Null leakage uncertainty: The uncertainty in the null leakage
from
interleaved integrations of target/calibrator pairs is 0.25% (1 sigma)
over 2.5 to 3 hours in moderately good
weather when the source flux meets all stated sensitivity limits.
For fainter N-band fluxes (1.7 to 2.4 Jy), the uncertainty increases
to 0.5% in 3 hours.
More detail on calibration is provided in nulling
basics.
- Sky coverage:
- Delay Range:
- continuous (fast delay lines): +/- 13 m
- quasi-static (long delay lines): +/- 65m
The total delay is calculated by summing the FDL and LDL
values.
Zenith pointing occurs at a delay of approx. -40 meters, which means
that targets at high and low declinations can not be
observed at all hour angles. We recommend
using getCal
to
see the detailed coverage of specific objects.
Up to different 3 long delay line positions can be used in a
single
night. The long delay lines take 15 minutes to re-position and align
and should be changed only between groups of targets and calibrators.
The length of the FDLs typically allows a given star to be tracked for
approximately 2-3 hours for a given LDL position; the precise track
length depends on the LDL position and the source declination. The
timing gui in getCal
plots the detailed coverage for a given LDL position.
- Zenith Range:
- maximum zenith angle: 50 deg
- 3 deg radius around zenith excluded; degraded performance
is
sometimes seen within 5 deg of zenith.
- Observing efficiency: For sources where all magnitudes are
> 1 magnitude above the sensitivity limits, KI in nulling mode can
achieve 2 scans
per hour under good weather and seeing conditions. A scan includes
the fringe tracking data and detector calibrations. A separate scan on
a
nearby calibrator is required to determine the system visibility and
nulling calibration. These scans are typically grouped into 2-3 hour
blocks (depending on the FDL coverage and the observing goals) to
provide 2-3 calibrated scans on the target. Pointing optimization
on bright 10 micron sources is required between these observing blocks.
These pointing observations will require 20-30 minutes each.
For sources at the sensitivity limit of one of the sub-systems,
the efficiency is generally closer to 1.5 scans per hour.
The first 30 minutes after the domes open may be necessary for other
interferometer set-up and no science observations are
guaranteed during this time.
- Field-of-view: ~0.6 arcsec FWHM for the nuller (limited
by the pinhole at the 10 micron detector focus); the FOV is somewhat
elliptical given the use of the split aperture.
- Keck-Keck baseline is 85 meters and roughly 38 degrees East of
North. The KI planning tool getCal can be used to find source delay
line and u-v coverage.
getCal web
interface
K-band u-v Coverage (as a function of declination):
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