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. See the data overview for more details on the data.
Updates for 2011A call for proposals:
- The L-band, dual field phase referencing (DFPR), and simultaneous K/L visibility modes are available for shared-risk proposals. See below for sensitivity details. Backup observing plans using standard V2 modes must be included when proposing for these modes.
- L-band: During the shared-risk period, up to 1 hour of testing or
calibration time may be used by the KI team during a science
night. The timing of these tests
will be coordinated with the PI to minimize disruption to the observing program.
- See the DFPR page for senstivity details and calibration requirements for that mode.
- The sensitivity values given for shared-risk modes 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 shared-risk observing.
Sensitivity numbers updated: August 1, 2010
The sensitivity values below are based on past interferometer performance in good weather. The interferometer performance degrades substantially in poor seeing.
- 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 dependant, 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.
- Angle tracking:
- V2:
- 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)
- V2-SPR:
- H-band, 80 Hz: 5.0 < H < 9.0
- 40 Hz tracking also available: 1 magnitude fainter limits.
- 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 31.1 milliarcsec/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.
- V2:
- Fringe tracking:
- 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).
- L band: The L-band primary fringe tracker performs science
observations at L-band. The L-band camera has been used
for science observations on several nights. However, the calibration accuracy of this mode is not yet quantified due to the background contribution (although it will be better for brighter sources) and the operations are not fully automated. This mode will continued to be offered as shared-risk until those tasks are complete. A backup program in a non-shared-risk mode is required.
- DFPR: See the DFPR page for senstivity details and calibration requirements for that mode.
- Details on which modes can be combined within a night and the reconfiguration times are given here.
- The availability of the infrasil windows is not guaranteed for 1/2 night allocations, but the transmission difference is less than 0.1 mag in the K band.
- Visibility accuracy and spectral precision:
- Absolute visibility accuracy for H and K bands is 0.03 in squared visibility.
- 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
- SPR mode visibility accuracy is 0.05 based on
preliminary
measurements and
0.02 channel-to-channel. Over narrow spectral features, differential
phase measurements have been demonstrated to about 1 deg rms (or
equivalently, for unresolved objects, 16 microarcsec sky offset), per
single visit (120 sec of data), and for stars near the sensitivity
limit. However, differential phase slopes or curvatures across the band
are dominated by instrument effects (mainly the continuously changing
differential optical paths in air), and therefore these broad features
are not likely to contain astrophysical information.
- L-band accuracy is not yet measured.
- See here for a longer description of the accuracy and precision measurements.
- Sensitivity
-
The limits below apply to unresolved targets. See Table 1 below for the
reductions due to resolved sources.
- Low resolution modes:
- 5 pixels across K'
- 200 Hz: 6.2 < K' < 9.9
- 100 Hz(**): 6.6 < K' < 10.3
- 10 pixels across K'
- 200 Hz: 5.4 < K' < 9.1
- 100 Hz(**): 5.8 < K' < 9.5
- 4 pixels across H
- 200 Hz: 5.3 < H < 9.0
- 10 pixels across L
- 100 Hz: 2.5 < L < 6
- Medium resolution mode (see additional notes)
- 42 pixels across K'
- 200 Hz: 3.5 < K' < 7.2
- 100 Hz(**): 3.9 < K' < 7.6
- K/L split pupil mode:
- 10 pixels across L
- 100 Hz: 1.3 < L < 4.8
- 5, 10 or 42 pixels across K
- 5 pixels: 5.0 < K' < 8.7
- 10 pixels: 4.2 < K' < 7.9
- 42 pixels: 2.3 < K' < 6.0
- 10 pixels across L
- High resolution mode (V2-SPR):
- Primary fringe tracker (OPD stabilization)
- 5 pixels across K', fringe stabilization
- 200Hz: 4.45 < K' < 7.0
- Secondary fringe tracker
- 330 pixels across K', long integrations
- 1Hz: limits set by primary fringe tracker
- Primary fringe tracker (OPD stabilization)
- (**) NOTE: The spectral channel relative visibility calibration of 0.02 has only been validated at 200 Hz. Although this is considered low risk, the 100 Hz mode may have higher relative flux errors.
- For bright sources, an aperture can be used to shift the magnitudes by ~2.4 mags.
- The DN-to-electron conversion is 0.26.
- Low resolution modes:
- Nominal fringe spacing (wavelength/baseline) for the 85 m
Keck-Keck baseline:
- K: 5.2 mas
- H: 3.8 mas
As KI is a fringe-tracking interferometers, all targets and calibrators must exceed a threshold visibility to be observable. The limits given above are appropriate for unresolved (i.e. point-like) sources. If the target is resolved, the instrument sensitivity decreases as shown in the table below. Targets can not be larger than 4.5 mas diameter at K band or 3.3 mas diameter at H band. Please contact NExScI if you have questions about the observability of potential targets.
Table 1: The reduction in sensitivity (X given in magnitudes) for a given source diameter.
X K_mag Diameter (mas) X H_mag Diameter (mas) 0.00 < 1 0.00 < 0.7 0.25 1.6 0.25 1.2 0.50 2.2 0.50 1.6 0.75 2.7 0.75 2.0 1.00 3.1 1.00 2.2 1.25 3.4 1.25 2.5 1.50 3.7 1.50 2.7 1.75 3.9 1.75 2.8 2.00 4.2 2.00 3.0 - Sky coverage:
- Delay Range:
- continuous (fast delay lines - FDL ): +/- 15 m
- quasi-static (long delay lines - LDL): +/- 70 m
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:
- maximum zenith angle: 50 deg
- 3 deg radius around zenith excluded, degraded performance is sometimes seen within 5 degrees of zenith
- Delay Range:
- Observing efficiency: 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.
- Fringe tracker field-of-view: ~50 mas FWHM at K' band; ~35 mas at H band; ~80 mas at L band. The FOV 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
- See the getCal
planning tool to
check coverage for any object.
K-band u-v Coverage (as a function of declination):
- Coverage given for 50 deg zenith angle range and u,v axis are in Mlambda
- 50 deg dec
- 35 deg dec
- 25 deg dec
- 15 deg dec
- 5 deg dec
- 0 deg dec
- -5 deg dec
- -15 deg dec
- -25 deg dec
