Planning Observations with Subaru AO188

Please read section 1 carefully before submitting the proposal.

If you wish to observe with AO188, you have to find a bright natural guide star (NGS). You can use a target it self as a guide star if your target is a bright stellar objects. If your target is optically faint (e.g., distant galaxies, brown dwarfs) or extended sources (e.g., nearby galaxies), you have to find a bright natural guide star close to your target. In case there is no sufficiently bright NGS in the FOV (1 arcmin in diameter), laser guide star (LGS) will be also available. For the LGS mode, a natural guide star (TT-NGS) is also required for measuring tip and tilt but the limiting magnitude of the guide star should be fainter than that required in the NGS mode. In addition, since the isoplanatic angle for the tip and tilt modes is larger than the higher order modes, the sky coverage should be larger than the NGS mode.

Read the following instructions carefully if you wish to apply for observing time with AO188. If technical details described in your proposal are incomplete, it may be rejected even if your science case is great.

1. Information you have to describe in your proposal

The following should be clearly stated in Section 9, 10, 13, 14 of the proposal form.

(NGS mode)

  • R (or V if not available) magnitude of your AO guide stars (see section 2 for details).
  • Separation between your targets and AO guide stars (see section 2 for details).
  • Strehl ratio and/or FWHM of the point-spread function (PSF) required for your project.
  • Integration time required.
    --- Refer the page for IRCS sensitivity and the expected sensitivity gain due to the AO188 correction.
  • Whether the AO guide star is a point source or not.
    --- If you use an extended object, binary objects, or a star associated with nebulosity, you must describe a FWHM of its spatial distribution, their separation, or contrast between the star and nebulosity, respectively (see section 2 for details).
  • Backup program for poor observing conditions (see section 6 for details).
  • Schedule requirement to avoid the moon's glare which severely degrades the AO loop performance (see section 4 for details).

(LGS mode)

  • R (or V if not available) magnitude of your AO guide stars (see section 2 for details).
  • Separation between the science target and TT guide stars (see section 2 for details).
  • Strehl ratio and/or FWHM of the point-spread function (PSF) required for your project.
  • Integration time required.
    --- Refer the page for IRCS sensitivity. The preliminary results of the AO-correction performance in the LGS mode are available on this page.
  • Whether the TT guide star is a point source or not.
    --- If you use an extended object, binary objects, or a star associated with nebulosity, you must describe a FWHM of its spatial distribution, their separation, or contrast between the star and nebulosity, respectively (see section 2 for details).
  • Backup programs that can be observe with the NGS mode or without AO assistance (no-AO correction mode, see section 6 for details). The preparation of the backup program is very important for the LGS mode, since there is high possibility to cease the laser launch due to weather conditions or some other reasons.
  • Schedule requirement to avoid the moon's glare which severely degrades the AO loop performance (see section 4 for details).

2. Selecting AO guide stars

* Note: For checking and selecting the AO guide stars, "hskymon" which is a Subaru observing planning tool is available. Please see Tip-Tilt Guide Star selection .

(NGS mode)

Guide stars/objects for AO correction should be selected with the criteria listed below. The science target itself can be used as a guide star. Extended objects or non-sidereal objects can be used if they satisfy the following criteria.
  • Brightness --- For open use we recommend to use AO guide stars/objects with R=16.5 or brighter. Brighter stars provide better performances (see the performance page for details). A star with R=17 allow for AO correction in some cases, but it strongly depends on observing conditions such as seeing and sky brightness. Stars with R < 8.5 so far provide the same performance as R = 8.5 because a neutral density (ND) filter is used to prevent the wavefront sensor (WFS) from over-exposure. The brightest R magnitude acceptable is -1.
  • Location --- The image quality can significantly be degraded at larger distances (see this figure). We recommend you to find an AO guide star within 30'' of your target.
  • Morphology --- Single point sources without surrounding nebulosity are highly recommended. AO loop might be closed even using some extended sources and stars associated with nebulosity. However, use of such AO guide objects does not guarantee the performance you require. If you use an extended source as a AO guide star, make sure that its FWHM of spatial distribution should be less than seeing. If you use a star with nebulosity, make sure that the stellar magnitude must be brighter than the total background in a 2-arcsec aperture. You can use binary objects as a guide star if their separation is less than 1 arcsec. The large separation (> 1 arcsec) objects might be used in some case, but caused a significant increase of the overhead time for AO parameter tuning.

(LGS mode)

For the LGS mode operation, a natural guide star (TTGS) is still required to correct for the tip-tilt motion that cannot be sensed by the LGS. The science target itself can be used as the TTGS. Extended objects or non-sidereal objects might be used as the TTGS if they satisfy the following criteria.
  • Brightness --- For open use we recommend to use AO guide stars/objects with R=18.0 or brighter. Brighter stars provide better performances. You might be able to get small correction with 18< R <19 guide star, but its only in dark night.
  • Location --- We recommend to use a TTGS located at the separation of less than 60 arcsec from the science targets, although the large separation up to 90 arcsec might be acceptable in some case. The science target can be used as the TTGS. The image quality would become worse as increasing the separation between the TTGS and the science target.
  • Morphology --- Single point sources without surrounding nebulosity are highly recommended. AO loop might be closed even using some extended sources and stars associated with nebulosity. However, use of such AO guide objects does not guarantee the performance you require. If you use an extended source as a AO guide star, make sure that its FWHM of spatial distribution should be less than seeing. If you use a star with nebulosity, make sure that the stellar magnitude must be brighter than the total background in a 2-arcsec aperture. You can use binary objects can be used as a guide star if their separation is less than 1 arcsec. The large separation (> 1 arcsec) might be used in some case, but causes a significant increase of the overhead time for AO parameter tuning.

3. Selecting PSF reference stars

(NGS and LGS modes)

You may observe a PSF reference star if the stars the science camera FOV cannot be used as a reference. The PSF reference stars should be observed at a condition as similar as possible to that of the science targets, i.e., the same instrumental configuration, at a similar airmass and just before/after observing your target (as the PSF can vary with time). For this reason, you may select a PSF reference star as close as possible to your target.

To let PSFs similar to each other, the R-band flux to the high-order (NGS mode) or low-order (LGS mode) wavefront sensor should also be nearly the same between the AO guide star/object and PSF reference star. To adjust the flux of the PSF reference star, a number of neutral density (ND) filters are installed in WFS. The table below shows a list of ND filters available.

Filter Density (mag.)
ND 0.01% 10.0
ND 0.03% 8.8
ND 0.1% 7.5
ND 0.3% 6.3
ND 1% 5.0
ND 3% 3.8
ND 10% 2.5
ND 30% 1.3

4. Planning dates of observations

Evaluate possible dates of your observations based on the following tips:

(NGS mode)

  • Elevation --- We recommend you to observe targets at an elevation of 45o or larger. Strehl ratios and/or FWHMs can significantly be degraded at lower elevations. Please not that atmospheric dispersion correctors (ADC) installed at the science path, high-order wavefront sensor, or low-order wavefront sensor do not guarantee the correction at the elevation less than 30o.
  • Distance to the moon --- We cannot perform AO correction if the sky is very bright. We then recommend you to observe the target at least 30o apart from the moon. For the same reason you may avoid observing targets close to Venus, Mars, Jupiter, Saturn etc.

(LGS mode)

  • Elevation --- We recommend you to observe targets at an elevation of 45o or larger. Strehl ratios and/or FWHMs can significantly be degraded at lower elevations. Please note that the performance of the LGS mode would get drastically worse at less than 30 degrees in elevation because Rayleigh scattered light from the laser beam start to leak from the telescope secondary shadow at around 30 degrees. We cannot launch the laser at less than 20 degrees in elevation.
  • Moon phase --- Since AO188 uses the optical wavelength for wavefront sensing, you have to care about the moon phase especially for using faint TT guide star in the LGS mode. We recomment you to request dark nights for R>17 TT guide star, gray nights or fainter for 16 Distance to the moon --- We cannot perform AO correction if the sky is very bright. We then recommend you to observe the target at least 30o apart from the moon. For the same reason you may avoid observing targets close to Venus, Mars, Jupiter, Saturn etc. This distance limit should increase as the brightness of the TTGS become fainter.

5. Overheads

(NGS mode)

We usually need 10-15 minutes for overheads of each target. These include acquisition of the AO guide star, optimization of AO, and acquisition of the target. You may omit the optimization step if the guide star magnitude and separation are similar to that used just before your observations and the weather condition has not been changed. Below is a summary of the overheads for the guide star acquisition and the performance optimization. In addition to these, we need up to 12 minutes to slew the telescope to your target.

Guide star Acquisition Optimization
Bright star (R < 14) 3.0 min 5-10 min
Faint star (R > 14) 3.0 min 10-15 min
Extended source 3.0 min 10-15 min
Bright star located > 30 arcsec away 6.0 min 5-10 min

Here are some tips for reducing overheads.
  • Any large telescope requires a significant overhead to slew to the target. It takes 6 minutes to slew Subaru to the opposite azimuthal angle. Plan your observations carefully to avoid slewing the telescope from east to west, north to south etc. An overhead to change the elevation is far less crucial.
  • If you perform imaging observations, we recommend you to let both the target and AO guide star/object located in the same field view (FOV). Otherwise, you may need another 3-5 minutes for overheads of each target. See the IRCS page for their FOVs.
  • We usually perform dithering within 7 to 10 arcsec box for point source targets. It will take 6 to 9 seconds for dithering between two position within the box. Larger dithering width can be performed with a slight increase in the overhead.

(LGS mode)

At the beginning of each night, we have to adjust a laser beam collimation and do focusing with the LGS. It will take about 15 minutes. Second focusing might be required during a night depending on the variability of the sodium layer effective height. The expected overhead time for TTGS and LGS acquisitions are 3 minutes for each. Another 5 to 10 minutes are required for the loop parameter optimizations. Here is a summary of the expected overhead time for the LGS mode.

LGS beam collimation and focusing 15 min (once or twice per each night)
TTGS acquisition 3 min
LGS acquisition 3 min
Parameter optimization 5-10 min

6. Backup Program

Subaru is a visitor-mode telescope. Satisfactory AO correction is not achieved under a poor seeing condition, say, > 1''. Observers are responsible for preparing a backup program for such an occasion. Especially, since the LGS mode is not allowed to use even under the condition that would allow normal observations, including the NGS mode, the backup program that can perform with the NGS mode or without AO assistance is mandatory.

(NGS mode)

You may perform the no-AO correction mode as a backup program (see below). If there is an AO guide star in the FOV, you may try to perform low-order (tip/tilt) AO correction for tracking the target instead of telescope auto guider that cannot be used with AO188 optics. However, we cannot guarantee whether it works well or not since it depends on the weather condition. We then highly recommend you not to perform long exposures with the no-AO correction mode especially for a grism spectroscopy that has no slit viewer.

(LGS mode)

You may perform the NGS mode observations as a backup program. If there is no AO guide star for the backup program, you can also perform no-AO correction mode by following the instruction of the backup program for the NGS mode described above.

7. Switching between AO and no-AO modes

Flexible change between AO and no-AO (without AO optics) modes cannot be performed during a night because the AO optics are located in front of the science instrument and cannot be easily moved from the fixed position. You may observe without AO correction but including the AO optics (no AO correction mode) during the nights assigned as AO observations. In this case, the deformable mirror is used as a folding mirror. The deformable mirror should be flattened by closing an AO loop with a bright guide star or a calibration light source installed in AO188. This requires some overhead time. The sensitivity for the no AO correction mode should be worse than only IRCS (without AO optics) mode due to throughput and emissivity of warm AO optics. Since the telescope auto guider attached at the Nasmyth focus cannot be used with the AO188 optics, if the AO closed loop are stopped, tracking accuracy become worse and then long exposures should become very difficult. If you want to try no-AO correction mode as a backup program, state this in your proposal and describe the feasibility of the program. This is mandatory to assign an AO instrument operator and the support astronomer to your observing run.

8. Preperation for the LGS target list

To launch the laser on sky, the observatory must report the target coordinates to the US space command in advance. The LGS observers must submit a target list to the support astronomer at least 5 working days prior to the date of the observation. The target list must be a plain text which includes RA(J2000), Dec(J2000), and target name + additional information (one target per line) in the format as follows. RA, Dec, and target name must be separated by tab (not space) .
  HH:MM:SS.SSS	[+/-]DD:MM:SS.SS	Target name and additional information
Example for the LGS target list available at here.

9. Preperation for the AU tracking file

If the relative position between your scinence targets and AO guide star (natural guide star or tip/tilt guide star), the AO system needs to track the guide star using the acquisition unit (AU) to stop the science target on the science camera. To do this, you have to prepare for the tracking file for the acquisition unit. Below are the example of the cases that you have to prepare for the tracking file.
  • observe a non-sidereal object by guiding with a sidereal object
  • observe a sidereal object by guiding with a non-sidereal object
  • observe a non-sidereal objects by guiding with another non-sidereal object whose motion is not same as the observing non-sidereal object
    (e.g., observe Jupiter by guiding with Europa)
The AU tracking file contains the guide star coordinates and its offset from the science target as a function of the time in UTC. Providing the absolute coordinates for the science target instead of the offset is also acceptable. Below is a format for the AU tracking file.
  • 1. Relative offset
  • YYYYMMDDHHMMSS.SSS  HH:MM:SS.SSS [+/-]DD:MM:SS.SS RAoffset Decoffset
    • column 1 : observation deta and time in UTC
    • column 2 : guide star RA (J2000)
    • column 3 : guide star Dec (J2000)
    • column 4 : separation in RA between target and guide star in arcsec = {RA(gs) - RA(target)} * cos(Dec(target))
    • column 5 : separation in Dec between target and guide star in arcsec = {Dec(gs) - Dec(target)}

    Example for the relative offset tracking file is available at here.

  • 2. Absolute coordinates
  • YYYYMMDDHHMMSS.SSS  HH:MM:SS.SSS(GS) [+/-]DD:MM:SS.SS(GS) HH:MM:SS.SSS(TARGET) [+/-]DD:MM:SS.SS(TARGET)
    • column 1 : observation deta and time in UTC
    • column 2 : guide star RA (J2000)
    • column 3 : guide star Dec (J2000)
    • column 4 : target RA (J2000)
    • column 5 : target Dec (J2000)

    Example for the absolute coordinates tracking file is available at here.

10. Further information

Questions regarding this page should be directed to Joanna Bulger (jbulger_at_naoj.org).

30 July 2015



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