This page describes some notes for Observers and Support
Astronomers to bear in mind regarding operation of AO with NIRC2.
Guide Star Offset On-Line Planning Tool can
The AO Natural Guide Star (NGS) system requires a guide star of suitable
brightness (recommended brighter than V = 13m ) to correct for the atmospheric
distortion. The NGS can be offset from the object of interest but the quality
of correction degrades as the offset is increased. This quality varies
from night to night with the isoplanatic angle. There are also hard limitations
of the AO field steering mirror (FSM) system used to offset the
guide star. The figure below shows the FSM limits relative to the NIRC2 wide field
FSM control limits shown relative to the NIRC2 wide field of
view (40 arcsec on a side). The figure depicts the case of the object being
10.0 arcsec away from the guide star at a position angle of
It is necessary to determine an optimum position angle (PA) setting
for the rotator in order to allow for maximum guide star offset. The optimum
PA should take into account the need to dither the object on the science
detector. The NGS position needs to allow for this movement in FSM
space. For NIRC2 the following simple formula is recommended for determining
the PA :
PA = Guide Star to Object PA
The figure above shows that the guide star is in a position that would allow
for box dither pattern of 5 arcsec offsets.
Dome Open/Close Criteria
The Keck domes are typically closed when the sun is up. However, we recognize
that this could prevent AO observers from opening in time to acquire twilight
flats. Hence, the following modified dome opening policies apply for AO
In order to permit the acquisition of suitable twilight flats, observers
may request that the dome be opened 15 minutes prior to sunset.
Only if the dome was not opened early due to problems with
the instrument, telescope, or weather, then observers may
request that the dome remain open until 15 minutes past
sunrise. It is not allowed to have both an early open and a
AO User Tools
From the pull down menu in the VNC session, select K2 AO User
The following options are available:
AO Simbad tool: Query to Simbad to determine the star colors.
AO Monitor: Shows the general status of the AO system.
TSS Widget: Shows a DSS image centered in the current
target with the AO vignetting regions.
LBWFS Seeing: Shows the average FWHM of the PSF in the
WFS Intensity Display: Shows the light distribution in
the wawefront sensor.
LUI Shows the status of all AO components.
Note that when Guider Eavesdrop is launched, you will be viewing
the ACAM guider.
data in coordination with AO
There are many NIRC2 commands designed for doing telescope offsets
and acquiring data with dithering techniques (see
manual for a complete list of commands). These commands can be used
with or without the AO system the loops closed.
To make sure the AO loops are closed for NIRC2 data acquisition
type wait4ao on.This command sets a switch in
the control software that checks the AO loops prior to taking
a picture. wait4ao off will allow NIRC2
integrations to occur without checking the AO loops.
All of the NIRC2 commands that acquire data are dependent on
the wait4ao switch including goi
Position Angle (PA)
FACSUM should correctly report on the AO rotator mode and the AO
rotator position angle.
When observing, the instrument compass rose should correctly display the
sky PA relative to the NIRC2 detector for all rotator modes.
There is a 0.7° offset between the AO and NIRC2 detector. When
the AO rotator is at SKYPA=0°, the columns of NIRC2 are at
different by a position angle of 0.7° as projected onto the sky.
The Compass Rose will display the correct NIRC2 angle, in
this case -0.7°.
- For more information on NIRC2 angles see
on Keck coordinate systems.
- For positive angles, the compass rose move in what might be considered the
opposite direction; that is as PA increases the N vector moves CW. The telescope
rotation is using the normal sign convention N through E so the objects in
the image appear to move the opposite way and thus compass rose
- To recover the PA (that is the sky position angle on the NIRC2 detector)
from a NIRC2 fits header:
- for ROTMODE = POSANG, nirc2PA = ROTPOSN - INSTANGL
where INSTANGL = 0.7° (±0.1°)
- for ROTMODE = VERTANG, nirc2PA = PARANG +
ROTPOSN - INSTANGL
Differential Atmospheric Refraction (DAR)
Observers should be aware that differential atmospheric refraction (DAR)
is a much more serious concern with AO observing than non-AO, because the
target can move relative to NIRC2 within just a couple of minutes. Henry
Roe (UCB) has put together an
of the consideration for differential refraction and AO.
Users must provide AO guide star colors to the AO operator in the form
of B - V when DAR corrections are enabled. DAR is a
strong function of the wavefront sensor effective wavelength
which changes significantly as function of guide star color. The
wavefront sensor is calibrated for a range of B - V values
from 0.0 to 2.0. Do not use values beyond this range and use
either 0.0 or 2.0 if the guide star color exceeds this range.
Plot showing effects of DAR when using
an A0 guide star (B-V = 0.0) and observing at K band.
NOTE: The AO DAR correction software does not currently
handle changes in rotator position angle (PA). Please warn
the AO operator when a PA change is desired to help avoid
Imaging flat field frames can be acquired with dome
flats or with twilight sky flats. Spectroscopy lamps will be
installed on the AO bench at a later date.
Observing Overheads during an NGS-AO NIRC2
Total elapsed observing time(sec) = 6*(ndither+1) + 12*ndither*nframes
+ ndither*nframes*coadds*(itime+ tread(nread-1))
- nidther = number of dither position
- nframes = number of frames per dither position
- coadds = number of coadds
- itime = individual integration
- tread = nirc2 readout time (0.18sec for full array 1024x1024 ; 0.05
for sub-array 512x512)
- nread = number of reads (n=2 for CDS, n=n for
- 6 sec is the AO overhead during dither (open loops, move
telescope, reposition Field Steering Mirrors (FSMs), reposition
WFS focus (FCS), close TT looop, close DM loop)
- 12 sec is the time to read and write a NIRC2 image
including FITS information
- an additional overhead of a few seconds (<5-10 secs) may
occur, maybe once every 20 dither moves, when repositioning the
FCS or the FSMs.
Typical sequence in Ms with array 1024x1024 (tread=0.18sec)
given the short inetgration time for Lp and Ms, nreads = 2
- ndither = 4
- nframes = 3
- coadd = 50
- itime = 2
- On-source integration time: 120 sec
- Total observing time: 402 sec
- Efficiency is 30% over the observing sequence
Typical sequence in H with array 1024x1024 (tread=0.18sec)
given the longer integration time, we will run in MCDS mode
nread = 16
- ndither = 9
- nframes = 2
- coadd = 3
- itime = 10
- nread = 16
- On-source integration time: 540 sec
- Total observing time: 961 sec
- Efficiency is 56%
Typical sequence in spectroscopic mode:
- ndither = 3
- nframes = 2
- coadd = 4
- itime = 30
- nread = 16
- On-source integration time: 720 sec
- Total observing time: 880 sec
- Efficiency is 82%
- changing filters takes 16 to 18 seconds
- changing camera (wide/medium/narrow) takes
18 to 22 seconds
- Setting integration time and checking saturation level
- Positioning the target on slit (~ 1-2min)
- Additioonal spectroscopic setting : ~ 1 min
- Positioning the target behind the focal coronagraphic mask
(~1-2min) once an optimal position has been found, it can
be set as a preference for the next acquisition.