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

• Guide Stars
• Dome Open/Close Criteria
• AO User Tools
• Guider GUI
• Acquiring NIRC2 data in coordination with AO
• Position Angles
• Differential Refraction
• Calibrations
• Observing Overheads

Guide Stars

The AO Guide Star Tool can be used to plan your NGS and LGS observations (see figures below).

The AO Natural Guide Star (NGS) system requires a guide star of suitable brightness (recommended brighter than V = 13mag ) 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 of view (click on the figure to enlarge).

The AO Tip-Tilt (TT) Star in Laser Guide Star (LGS) mode can be as faint as (V = 18mag). When the AO system is operated in LGS mode, the TT star can be further away from the science target than in NGS mode, because the TT star is tracked using the Tip-Tilt Stage (TSS), which has a larger range of motion than the FSMs. The figure below shows the TSS limits relative to the NIRC2 wide field of view (click on the figure to enlarge).

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
    

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:

• 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 late close.

AO User Tools

From the pull down menu in the VNC session, select K2 AO User Tools.

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 LBWFS sub-apertures.
• WFS Intensity Display: Shows the light distribution in the wawefront sensor.
• LUI Shows the status of all AO components.

Guider

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 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 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 Compass Rose will display the correct NIRC2 angle, in this case -0.7°.
• For more information on NIRC2 angles see the document 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 do also.
• 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 excellent synopsis 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 complications

Calibrations

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 acquistion sequence

• Total elapsed observing time(sec) = 6*(ndither+1) + 12*ndither*nframes + ndither*nframes*coadds*(itime+ tread(nread-1))

  where:
  
  • 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 MCDS)
  • 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%

• Additional Overheads:

  • changing filters takes 16 to 18 seconds
  • changing camera (wide/medium/narrow) takes 18 to 22 seconds
  • 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.