Preparing for a Lick AO Run
Now that the Lick AO system is available for use by observers from any UC campus or laboratory, there are many observers wanting to use Lick AO that have never done adaptive optics observing before. Potential AO users are best off consulting experienced AO observers at their home institution, but I'll put a few guidelines here so that observers can best prepare for using our AO system (in either NGS or LGS mode). Because no single observing procedure works for all science programs, I will not go into detail about observing strategies (dithering, exposure times, etc.), but just the basic information to set up observing programs and to be prepared for observing at the telescope. See the IRCAL manual for information on the sensitivity of the NIR science camera and some calibration procedures.
Contact Elinor Gates if you need additional information not provided in the Lick AO documentation.
NGS Preparation
LGS Preparation
Other Information
NGS
- Choose Science Targets:
NGS AO science targets require a guide star (GS) of R~13.5 or brighter (see graphs of WFS sensitivity for details) within 55" of the science target. This magnitude limit depends on seeing, transparency, and color of the star (the WFS camera is more sensitive to red stars). Occasionally it is possible to close loops on fainter stars, but more often you are limited to stars brighter than R~13.0 because of observing conditions. - Choose PSF Stars:
- If your science target is also the GS: You need to find a single star nearby of similar brightness and color to the GS so that AO performance is similar.
- If your GS is off-axis from your science target: You need to find a pair of stars with the similar separation and position angle as your science target - GS pair. The PSF GS should be the similar in brightness and color to your science target GS so that AO performance is comparable. The PSF-GS pair also should be as close in the sky as possible to your science target (ideally within a few degrees). Check out Roy's PSF Pair Finder if you don't have your own scheme for finding PSF-GS pairs. The magnitude of the PSF star isn't as important as the magnitude and color of the GS, and will often be fainter, but should be bright enough to get a reasonable signal-to-noise ratio.
- Prepare Offsets if using Off-axis GS:
At the telescope we will acquire the GS first then offset to the science target. Offsets to the science target should be calculated in arcseconds North/South and East/West from the GS. - Prepare Starlist:
It is recommended that you prepare an Object Coordinate List ascii file (see starlist for the format of the file) with all your targets (science, PSF, standard stars, etc.) in advance. This file will be referred to by the telescope operator during your observing run.
LGS
- Choose Science Targets:
LGS AO science targets require a natural guide star for Tip/Tilt that is V=16.0 or brighter (see graphs of sensitivity for details) within 55". Sometimes guide stars that are very red as faint V~17.0 can be used, bluer stars may have a limiting magnitude of V=15.5. Note that seeing, transparency, and moonlight can drastically affect the ability to close the tip/tilt loop on fainter stars.The laser can only be propagated between 10pm and 5am when the San Jose airport is closed to most large aircraft traffic. Please come prepared with NGS targets (either science or calibration) for times when laser propagation is not allowed.
We also have a 45 degree zenith distance limit because of air traffic to the major airports in the area. This 45 degree zenith distance limit essentially prevents observers from looking at objects south of about -5 degrees Declination.
The laser clearance cone from Space Command is 10 arcminutes in diameter. If you have multiple objects within that area, you need only clear the center of the region instead of listing each object separately.
- Choose PSF Stars:
- If your science target is also the GS: You need to find a single star nearby (within a few degrees) of similar brightness and color to the GS so that AO performance is similar.
- If your GS is off-axis from your science target: You need to find a pair of stars with similar separation and position angle as your science target - GS pair. The PSF GS should be the similar in brightness and color to your science target GS so that AO performance is comparable. The PSF-GS pair also should be as close in the sky as possible to your science target (ideally within a few degrees). Check out Roy's PSF Pair Finder if you don't have your own scheme for finding PSF-GS pairs. The magnitude of the PSF star isn't as important as the magnitude and color of the GS, and will often be fainter, but should be bright enough to get a reasonable signal-to-noise ratio.
- Prepare list of 9th Magnitude Stars:
Because of flexure in the AO system, we require a R~9 magnitude star close to (within 10 degrees) the science target for alignment. This star is also used to check the focus of the WFS on the sodium layer when the laser is propagated. It is recommended to have at least two 9th magnitude stars picked in advance in case one turns out to be fainter than expected, binary, or a galaxy, etc.. - Prepare Target List for Space Command:
The sodium laser power is ~15W, so all LGS science targets, PSFs, and 9th magnitude stars must be submitted to US Space Command in advance for clearance so we can avoid hitting the space shuttle, ISS, or sensitive cameras on satellites.Format of the target list should be
Target_Name RA Dec Epoch
For example:
NGC_1239 03 08 22.3 -02 44 25 2000
or
NGC_1239 03:08:22.3 -02:44:25 2000There should be no spaces in the target name. RA and Dec can be either space or colon separated.
Submit the target list to Elinor Gates (egates@ucolick.org) at least one week prior to the first night of your observing run.
- Prepare Offsets if using Off-axis GS:
At the telescope we will acquire the GS first then offset to the science target. Offsets to the science target should be calculated in arcseconds North/South and East/West from the GS. - Prepare Starlist:
It is recommended that you prepare an Object Coordinate List ascii file (see starlist for the format of the file) with all your targets (science, PSF, standard stars, etc.) in advance. Note that the format of the LGS target list above is the same as the most basic starlist format. This file will be referred to by the telescope operator during your observing run.
Other Information
Under the best conditions (r0>18cm) and using a bright guide star (V<7), the NGS AO system can achieve Strehls of ~70%. It is not unusual during the summer (Lick Observatory's best seeing season) 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 ~8th 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 50 Hz for V~14.5 and fainter, limiting the 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 can not only causes the turbulence to change on timescales faster than the AO system can correct, but can also cause the spot-size on the WFS to increase, which causes 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 250 Hz or 200 Hz at zenith and 100Hz or 50 Hz at
airmass greater than about 1.25):
- 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, especially for the tip/tilt sensor. 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 stars and PSFs 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).
Elinor Gates Last modified: Mon May 26 18:52:29 PDT 2008