Shane Adaptive Optics System User's Manual: AO Observing Preparation

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). Natural guide stars must also be within the field steering limits the science target listed below and depicted in the Figure 1:
~20 arcsec radius circle, offset 2" South.

Figure 1: Field steering region for a natural guide star (NGS) with instrument at position angle = 0 degrees. Red circle is approximately the ShARCS camera field of view (10" radius). The purple ellipse is the NGS field steering range, which will rotate accordingly as the instrument is rotated to a different position angle on the sky.

Note: It is possible to do NGS with stars outside the NGS field steering 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:

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 the NGS field steering range of the science target.

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.

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 GS: Find a single star nearby of similar brightness and color as the science target so that AO performance is similar.
If the GS is off-axis from the science target: Find a pair of stars with similar separation and position angle as the science target-GS pair. The PSF GS should be of similar brightness and color to the science target GS so that AO performance is comparable. The PSF-GS 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 GS, but should be bright enough to get a reasonable signal-to-noise ratio. A good tool to help observers find PSF-GS 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 GS is actually a binary star or galaxy or some other confounding issue.
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~16.0 (there is some color dependency which has not yet been fully calibrated) and be within about 55 arcseconds of the science target.

NOTICE: The field steering mechanism for LGS TT stars has failed and in no longer available. Hence, TT stars must be within 5 arcseconds of the science target and a preset 2" dither script has been implemented for LGS operations.

The laser is approximately V~12. 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:

Choose Science Targets:
1. Targets must have Declinations greater than -5 degrees.
2. If the science target is R=16.0 or brighter and compact (less than 1.5 arcsec in extent, it can be its own TT star. Otherwise a TT star R=16.0 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.
3. Notice: LGS field steering is not available due to a failure of the motion mechanism and it is not scheduled for repair. Hence, the TT star must be within 5 arcseconds of the science target.
  Old information retained for reference: 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 55 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 2: 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.
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.
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.
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, 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. Additionally, targets must have minutes and seconds less 60.0.
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% in the BrG filter. 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:

Running the AO system at less than 500 Hz:
Stars fainter than ~9th magnitude require the WFS to operate at less than 500 Hz (and as slow as 50 Hz for V~12 or fainter) to collect enough light per subaperture for good wavefront measurements.
In LGS mode the tip/tilt sensor typically runs at 40 Hz for V~15 and fainter, limiting the tip/tilt correction possible.
Anisoplanatism from off-axis guide stars (NGS or LGS) is more pronounced the further off axis the guide star is from the science target.
Observing at high airmass means looking through more atmosphere, thus seeing more turbulence, which the system may or may not (depending on seeing conditions) be able to correct effectively.
Seeing/Wind/Clouds:
- Bad seeing not only causes the turbulence to change on timescales faster than the AO system can correct, but also causes the spot-size on the WFS to increase, which may cause a decrease in the signal-to-noise of the wavefront measurement.
- Clouds will decrease the apparent brightness of the guide star, decreasing signal-to-noise on the WFS and/or requiring the system to run at a slower rate.
Laser performance can be degraded by any of the following: (note that the WFS typically runs at 50 Hz for LGS operations):
- Mediocre to poor seeing affects the laser much the same way it does for a natural guide star, but more so because the laser passes through the atmosphere twice.
- Cirrus clouds not only block some of the laser light from reaching the sodium layer, creating brighter Rayleigh scatter, but also attenuates the guide star itself.
- At higher airmass the laser passes through more atmosphere, creating more Rayleigh scatter and hence a fainter spot. The sodium layer is also further away, leading to an intensity decrease due to distance.
- Sodium abundance can vary from night to night and from one area of the sky to another. If there is less sodium, the guide star will be fainter, necessitating running the WFS at a slower rate.
Moonlight can contaminate the WFS and tip/tilt sensor, making it more difficult to find or close the loop on faint stars. While the AO operator has some techniques to mitigate this effect, LGS observers with faint guide stars will do best to avoid observing within 15 degrees of the Moon (and better to be further away).
Background nebulosity can decrease the contrast of a tip/tilt star with respect to the sky brightness in the region and make it difficult to close the loop.

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: Thu Jun 6 08:51:52 PDT 2024

User's Guide to Shane Adaptive Optics System

Table of Contents

AO Observing Preparation

NGS

LGS

Other Information