This page describes some
random notes for Observers and Support Astronomers to bear in mind regarding
operation of AO with NIRC2.
Star Offset On-Line Planning Tool
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.
Figure 1. below shows the FSM limits relative to the NIRC2 wide field
The FSM control limits shown relative to the NIRC2 wide field of view
(40" on a side). The figure depicts the case of the object being
10.0" away from the guide star at a position angle of 0.0°
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
Figure 1 shows that the guide star is in a position that would allow
for box dither pattern of 5" 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 permissible to have both
an early open and a late close.
- From the pull down menu in Openwindows, select NIRC2 Tools -> Start AO
Select AO Status .
- Select Astonomer AO Screen .
- Select Tip/Tilt Graphs.
These tools will provide the fundamental AO parameters and status of the system.
There is also the ability to set the Rotator and angle and Rotator mode from
Note that when Guider Eavesdrop is launched, you will be viewing
the ACAM guider.
Acquiring NIRC2 data in coordination with AO
There are many NIRC2 commands designed for doing telescope offsets and
acquiring data with dithering techniques (see the NIRC2
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
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, snapi and box9
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 Astronomer Screen. and Compass rose will display
the correct NIRC2 angle, in this case -0.7°
- For more information on NIRC2 angles see the PA page
- 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 do also.
- To recover the PA (that is the sky's 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 excellent synopsis
of the consideration for differential refraction and AO.
The AO software can be enabled to correct for the effects of DAR. DAR corrections
should be enabled for spectroscopy, coronagraphy and any use
of NIRC2 that requires the object to remain fixed on NIRC2 for long periods
of time. The AO operator can enable DAR correction on request. We don't
recommend enabling DAR correction when it is not necessary because it increases
overheads and reduces observing efficiency, especially when dither patterns
are being used.
User's 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's
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 complications
Anecdotal evidence suggests that using non-sidereal tracking rates with
AO on does not work.
Instead, turn off non-sidereal tracking and simply allow the AO system
to lock onto the moving target.
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
Observing Overheads during an NGS-AO NIRC2 acquistion
2 - 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
% NIRC2OBSTIME: On-source integration time: 540 sec
% NIRC2OBSTIME: Total observing time: 961 sec
--> efficiency is 56%
3 - Typical sequence in spectroscopic mode:
ndither = 3
nframes = 2
coadd = 4
itime = 30
nread = 16
% NIRC2OBSTIME: On-source integration time: 720 sec
% NIRC2OBSTIME: 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
Additional Objective Overheads:
- Setting integration time and checking saturation level (~1min)
- 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.
and of course:
- Selecting and choosing the best observing strategy
also , note NGSAO is not troublefree, and it is reasonnable to allow 20min
a night for some telescope and AO minor troubles that we have not been
able to discard totally so far like : AO/DCS communication, FSM fault,
and wavefront controller crash.