User's Guide to Shane Adaptive Optics System


Table of Contents


Introduction
AO Observing Preparation
Hardware Overview
Software Overview
Real-time code shaneao
Real-time control GUI
Peeko
TTPeeko
Power Control saopower_ui
Motor Control saomot_gui
Field Steering fieldsteer.tcl
Laser Shutdown Monitor LSM.tcl
WFS Focus Tracking saoWFSFocusTrack.tcl
SAO System Monitor
aopococ
imageSharpen
Gallery of Aberrations
Techref

Data Archive
Mt. Hamilton Homepage

AO Observing Preparation

Preparing for a ShaneAO observing run is somewhat different for Natural Guide Star (NGS) or Laser Guide Star (LGS) operations. Because no single observing procedure works for all science programs, details about observing strategies (dithering, exposure times, etc.) will not be covered here. Basic information is provided below to help observers prepare target lists, find guide stars, PSF stars, and to be prepared for observing at the telescope. See the ShARCS manual for details about the sensitivity and characteristics of the NIR science camera.

In both NGS and LGS operation, the off-axis guide star (if there is one) is acquired first, then offsets to the science target are applied to put it in the field of view of the ShARCS camera.

Contact a Support Astronomer if you need additional information not provided in the ShaneAO or ShARCS documentation.

NGS Preparation
LGS Preparation
Other Information


NGS

NGS mode is used for targets that are either bright and compact enough to be their own guide star (GS) or that have a star nearby that is bright enough. Natural guide stars need to be brighter than R~13.5 (there is some color dependence, so blue stars might have to be brighter, but this has not yet been quantified as of Jan 2015). Natural guide stars must also be within 10 arcseconds of the science target.

Note: It is possible to do NGS with stars outside the 10 arcsecond range if willing to have only tip/tilt correction by using the LGS field steering motors. In this case, the star must be within 55 arcseconds (see field steering ellipse is shown in the LGS section below, as some directions one can go further off-axis with the guide star). Setting up for this mode usually requires going to a nearby bright star to set an appropriate static shape for the deformable mirrors.

Preparing for an AO run involves the following steps:

  1. Choose Science Targets:
    Determine if science target can be its own guide star or if an off-axis guide star is required. Guide star must be R~13.5 or brighter and within 10 arcseconds of the science target. Currently (as of Jan 2015) having a star more than 6.8 arcsec E or W (when of instrument PA=0) of the science target is difficult to acquire, so allow more time for target acquisition.
  2. Prepare Offsets if using Off-axis Guide Star:
    At the telescope, the GS will be acquired first, then do an offset to put the science target on-axis. Offsets from the GS to the science target should be calculated in arcseconds North and East from the GS.
  3. Choose PSF Calibration Stars:
    PSF calibration stars are single stars that are used to evaluate the AO system performance and structure of the point spread function.
    It is always a good idea to have at least two PSF stars ready in case the PSF or GS is actually a binary star or galaxy or some other confounding issue.
  4. Prepare Starlist File:
    Preparing a Starlist, an ascii file with the target names and coordinates, as well as other optional information is highly recommended. This file should include all science targets, guide stars, PSF stars, standard stars, etc.). This file can be in either the Lick Observatory starlist format or in Keck Observatory starlist format. This file will be used by both the observer and the telescope operator during the AO run.


LGS

LGS mode is used in the cases where the science target is too faint or extended to be used as its own guide star and there is no nearby star bright enough to be a natural guide star. However, even in these cases a natural tip/tilt (TT) star must still be used because the laser only measures higher order aberrations and not tip/tilt. The TT star must be brighter than R~18.0 (there is some color dependency which has not yet been fully calibrated as of Jan 2015) and be within about 60 arcseconds of the science target.

The laser is approximately V~12. However, during spring 2017 a new laser is scheduled to be installed, which should create a brighter guide star, and hence better correction. The laser is normally propagated between 10pm and 5am. Requests to shift these hours (e.g. 9pm to 4am) must submitted at least three weeks before the first night of the run to mh-laser@ucolick.org. We can not guarantee that the request will be granted. All laser propagation must be between evening and dawn 12 degree twilight. The limit of seven hours of propagation time is because of staffing and funding limits at Lick Observatory (see Support Lick for more details).

The maximum zenith distance for all laser observations is 45 degrees because of the heavy air traffic to the many major airports in the area. This 45 degree zenith distance limit prevents observers from looking at targets south of about -5 degrees Declination.

All laser targets must be cleared by the Laser Clearing House at US Air Force Space Command in advance. They determine when we need to stop propagating the laser to prevent damage to a satellite or injury to an astronaut. Laser target lists should be submitted to mh-laser@ucolick.org at least 5 days before the beginning of the observing run.

Please come prepared with NGS targets (either science or calibration) for times when laser propagation is not allowed or cloud cover prevents laser operation.

Preparing for an LGS AO run involves the following steps:

  1. Choose Science Targets:
    1. Targets must have Declinations greater than -5 degrees.
    2. If the science target is R=18.0 or brighter and compact (less than 1.5 arcsec in extent, it can be its own TT star. Otherwise a TT star R=18.0 or brighter must be identified within the LGS field steering field of view (see below). Please note that color of the star will affect the faintest magnitudes detectable on the TT sensor, but the color dependence has not yet been characterized (as of Jan 2015).
    3. If the science target is not bright or compact enough to be its own TT star, it must have a natural TT star within about 60 arcsec. The region for guide stars is actually elliptical and shown below. If target will be observed at a position angle other than 0 degrees, the field for guide stars will rotate accordingly. Tools to help identify potential TT guide stars for ShaneAO have not been written, however, Keck Observatory has tools that may be helpful at Keck LGSAO Software.


      Figure 1: Field steering region for a TT star with instrument at position angle=0 degrees. Red circle is approximately the ShARCS camera field of view. Ellipse will rotate accordingly as the instrument is rotated to a different position angle on the sky.

  2. Prepare Offsets if Off-axis TT star:
    At the telescope we will acquire the GS first then offset to the science target to place it in the ShARCS camera field of view. Offsets to the science target should be in arcseconds North and East from the TT star.
  3. Choose PSF Calibration Stars:
    PSF calibration stars are single stars that are used to evaluate the AO system performance and structure of the point spread function.
    • If the science target is its own TT star: Find a single star nearby of similar brightness and color as the science target so that AO performance is similar.
    • If the TT star is off-axis from the science target: Find a pair of stars with similar separation and position angle as the science target-TT star pair. The PSF TT star should be of similar brightness and color to the science target GS so that AO performance is comparable. The PSF-TT pair should also be as close in the sky as possible to the science target (ideally within a few degrees). The magnitude of the PSF star isn't as important and the magnitude and color of the TT star, but should be bright enough to get a reasonable signal-to-noise ratio. A good tool to help observers find PSF-TT star pairs is to use Roy's PSF Pair Finder. Keck Observatory also has some good tools for finding guide stars and PSF stars at their LGSAO tools web site.
    It is always a good idea to have at least two PSF stars ready in case the PSF or TT star is actually a binary star or galaxy or some other confounding issue.
  4. Prepare list of 10th Magnitude Stars:
    An R~10 star near (within 5 degrees) the science target is required for alignment purposes. This star is used to establish proper wavefront sensor focus for the laser guide star and to tune the system to maximize Strehl. A single 10th mag star can be used for multiple science targets if they are close enough together.
  5. Prepare LGS Target List:
    All targets at which the laser will be propagated need to be cleared with Space Command. These targets include the TT stars, 10th magnitude stars, PSF calibration TT stars, and any other calibration targets (e.g. standard stars, etc.). You may use the Keck AO starlist format for LGS targets.

    If you choose not to use the Keck starlist format, the basic, minimum information required and format of this file is

    Target_Name RA Dec Epoch lgs=1
    
    For example:
    
    NGC_1239 03 08 22.3 -02 44 25 2000 lgs=1
        or
    NGC_1239 03:08:22.3 -02:44:25 2000 lgs=1
        
    Where Target_Name is the object name with no spaces, RA is the Right Ascension in HH MM SS.SS or HH:MM:SS.SS format, and Dec is the Declination in (-)DD MM SS.S or (-)DD:MM:SS.S format. Epoch is in YYYY.YY format and lgs=1 for laser targets and lgs=0 for NGS targets. Only targets with lgs=1 will be submitted to US Space Command for clearance. Also, all targets with lgs=1 must be in either 1950 or 2000 coordinates.

    Observers may submit a maximum of 350 targets to US Space Command for clearance each night of their run. If a half night of ShaneAO LGS time has been allocated, the maximum number of submitted targets is 175.

    The target list must be submitted to mh-laser@ucolick.org at least five days prior to the first night of your run. Failure to submit a target list by the deadline will result in the targets not getting cleared by US Space Command.


Other Information

Under the best conditions and using a bright guide star (V<9), the NGS AO system can achieve Strehls of ~80%. It is not unusual during good seeing to get Strehls better than 50% on brighter guide stars. Performance of the AO system will degrade for any of the following reasons:

In preparation for your AO observations, it is often a good idea to check the magnitude and color of your guide stars in multiple catalogs, if possible. Many catalogs have errors or the stars are variable over long time scales. Some catalogs are also noted for listing galaxies as stars, and these galaxies might be too extended to use as a tip/tilt star. Some galaxies have compact cores that could potentially be used as a tip/tilt star, but the magnitudes for the galaxies are usually the integrated magnitudes, not the core magnitude. Spending a night at a telescope (e.g. the Nickel 40") to do some quick B, V, R photometry on your guide and PSF stars in preparation for an AO observing run can save you time and effort at the Shane 3m and confirm that the guide star is bright enough for the WFS or tip/tilt sensor and that your PSF star is more likely to be suitable.

You might notice that photometric standards are not listed specifically in the NGS or LGS sections above. Photometric standards are usually observed with the AO loop open (meaning you don't need the laser, and hence don't need US Space Command clearance) because you must use large apertures in any case to collect all the flux from star (remember, the typical well corrected image is a diffraction or near diffraction limited core on top of an uncorrected seeing disk).

Any questions about using AO or preparing for run, please contact a support astronomer at sa@ucolick.org.


sa@ucolick.org
Last modified: Sun Nov 6 21:01:51 PST 2016