This page describes some random notes for Observers and Support Astronomers to bear in mind regarding operation of AO with NIRC2.



Guide Stars

  Guide 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 of view.

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 nights:

Astronomer Screen


Acquiring NIRC2 data in coordination with AO

Position Angle (PA)

Differential Atmospheric Refraction (DAR)

Non-sidereal Tracking


Observing Overheads during an NGS-AO NIRC2 acquistion sequence: ===========================================

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%


Additional Overheads:
- 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, rotator fault and wavefront controller crash.