KI V2 Sensitivity and Observing parameters
This page contains KI sensitivity and instrument details for proposal
and observation planning for V2 observations. In the V2 mode the main
data product is the normalized visibility amplitude squared for a
given source. Integrations on the target source and calibrators are
interleaved in time, where each integration comprises fringe tracking
data and all necessary calibrations.
The V2 mode is open for proposals from all Keck users (UC, Caltech,
UH, NASA and NOAO TACs). The sensitivity values below are based on
past interferometer performance in good weather. The interferometer
performance degrades substantially in poor seeing.
Sensitivity numbers updated: August 3, 2009
New for semester 2010A
In addition to the standard H and K observing,
the following modes are available for shared-risk observing
- Self-phase referencing (V2-SPR)
- Simultaneous K/L band (split pupil mode results in a 1.2 mag loss in sensitivity for both K and L)
values given below are based on current performance for these
modes and while every effort will be made to perform
proposed science observations, no guarantees are made for
AO (adaptive optics) sensitivity:
- R < 12 (somewhat dependent on source color, less sensitive than AO alone due to the dichroic which sends light to interferometer)
- See the Keck Observatory
AO page for more details.
- Offset from a nearby AO star to the interferometry target is
supported but this mode has a lower observing efficiency. The maximum
offset is roughly 15 arcsec, but is position dependent, please contact
NExScI for details. The minimum offset is ~0.2 arcsec, but see the angle
tracking section for other options for closer binaries.
- Note that the image rotators are used in vertical angle mode.
- J-band, 80 Hz: 5.0 < J < 10.5
- H-band, 80 Hz: 5.0 < H < 10.5 (for fringe tracking in K-band only)
- H-band, 80 Hz: 5.0 < H < 9.0
- For bright sources, a neutral density filter can be used to
shift the range by up to 4.2 mags.
- The FOV is 0.6 arcsec with a plate scale of 0.04 arcsec/pixel.
- For binaries in which both components in are in the angle
tracker FOV, a single component can be selected and sent to the
fringe tracker if the separation is greater than ~0.1 arcsec. However,
note that this mode results in decreased observing efficiency.
- General notes
- There are several KI visibility modes covering different
wavelengths and spectral resolutions. Only two
modes may be run within a single night, the allowed
combinations and switching overhead times are given
(need link here).
- V2: The fringe tracker performs science observations
in EITHER K (2.2 microns) or H (1.6 microns) band. Note that the
frame rate must be the same for an entire cluster (target +
calibrators), but that it can be changed between clusters.
If an aperture is used for attenuation, it must also remain
in place for the entire cluster.
- V2-SPR: The primary fringe tracker
stabilizes the fringes in K-band for the secondary fringe tracker
which performs science observations in K-band using a longer
integration time (1Hz) to obtain high dispersion (R~1700; 330 channels).
The spectral range covers from 2.0 to almost 2.4, including the
HeI, Br gamma and at least 3 of the CO bandheads.
- L band: The L-band primary fringe tracker
performs science observations at L-band. The calibration accuracy
of this mode is not yet quantified due to the background
contribution but will be better for brighter sources.
- Simultaneous K and L-bands: In this configuration, the light from
each Keck are divided as for the nulling mode and half sent
to both the K and L-band fringe trackers. This results in
a 1.2 mag sensitivity loss compared to the values below.
The K-band camera can be set for 5, 10 or 42 channels.
The following table summarizes the best sensitivity for each
mode, see the text below for details.
||# Spectral Channels
The limits below and in the table apply to unresolved targets, see table
below for corrections for resolved sources.
- Absolute and relative calibration levels
- Visibility mode verification tests have shown a 0.03 systematic bias
in the squared visibility at H and K bands.
Users should not assume measurements of
absolute visibility to better than this level. The spectral
channel-to-channel relative visibility systematic has been measured at
0.02 in squared visibility for the 4 and 5 channels modes and 0.01 for
the 10 and 42 channels modes. See the flux bias memo for more details.
- For the V2-SPR mode (330 channels), the channel-to-channel performance is
expected to be 0.02. The expected absolute visibility-square accuracy
based on preliminary measurements is 0.05. Differential phase measurements
demonstrated to ~6 deg.
- Spatial resolution and field of view
- Nominal fringe spacing (wavelength/baseline) for the 85 m Keck-Keck baseline:
- Fringe tracker field-of-view: ~50 mas FWHM at K' band, scales with
wavelength (set by a single-mode fiber matched to
the diffraction limit of a single telescope)
- Keck-Keck baseline is 85 meters and oriented approx. 38 degrees East of North. Plots of the u-v Coverage (as a function of declination for a 50 degree zenith angle range):
- Delay Range:
- continuous (fast delay lines - FDL ): +/- 15 m
- quasi-static (long delay lines - LDL): +/- 70 m
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 4 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 timing gui in getCal plots the detailed
coverage for a given LDL position.
- Zenith Range:
- Good performance for zenith angles < 50 deg.
- Observations can be made at somewhat higher zenith angles, but
the performance degrades.
- The hard limits depend on the azimuth and
telescope, and are included in the getCal calculations.
- 3 deg radius around zenith excluded, degraded performance is
sometimes seen within 5 degrees of zenith
For sources where all magnitudes (V,J,K) are
> 1 magnitude above the sensitivity limits, KI can achieve 6 scans
per hour under good weather and seeing conditions. A scan includes
the fringe tracking data and calibrations. A separate scan on a
nearby calibrator is required to determine the system visibility.
This means that for a source/calibrator pair, there will be 3
integrations on each in an hour.
For sources at the sensitivity limit of one of the sub-systems,
the efficiency is generally closer to 4 scans per hour.
The first 30 minutes after the domes open may be necessary for
interferometer set-up and no science observations are
guaranteed during this time.
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