Planning Observations with Subaru AO188

Note: In the semester S08B, AO188 is opened only for natural guide star (NGS) mode in combination with the IRCS. Laser guide star mode will be available from S09A.

If you wish to observe with the AO188, you have to find a bright natural guide star (NGS). If your target is a bright stellar objects, the AO guide star should be a target itself. If you wish to observe optically faint targets (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.

If there is no sufficiently bright NGS in the FOV, laser guide star (LGS) will be also available from the semester S09A. For the LGS mode, a natural guide star (TT-NGS) is also required for measuring the tip and tilt but the limiting magnitude of the required natural guide star is fainter. In addition, since the isoplanatic angle for the tip and tilt modes is larger than the higher order modes, the sky coverage is larger.

If you wish to observe diffuse nebulosity associated with a very bright star (e.g., a circumstellar disk), you may want to observe a PSF reference star to subtract contaminated emission from the star. Also, we need some overheads to optimize AO for each target, and we cannot achieve good AO correction if you targets are at low elevations or close to moon or bright planets.

Read the following instructions carefully if you apply for observing time with the 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
• 2. Selecting AO guide stars
• 3. Selecting PSF reference stars
• 4. Planning dates of observations
• 5. Overheads
• 6. Flexible change between AO/no-AO modes
• 7. Backup program
• 8. Further information

1. Information you have to describe in your proposal

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

(NGS mode)

• R (or V if not available) magnitude of your AO guide stars.
• Separation between your targets and AO guide stars.
• 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 or a star associated with nebulosity, you must describe a FWHM of its spatial distribution or contrast between the star and nebulosity.
• Backup program for poor observing conditions

(LGS mode)

2. Selecting AO guide stars

(NGS mode)

• Brightness --- A star with R=18 allow for AO correction in some cases, but it strongly depends on observing conditions such as seeing and sky brightness. For open use we recommend to use AO guide stars/objects with R=18 or brighter. Brighter stars provide better performances (see the performance page for details). Stars with R < 10 so far provide the same performance as R = 10 because a neutral density (ND) filter is used to prevent the wavefront sensor (WFS) from over-exposure. The minimum 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 --- 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 these 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. Note that binarity does not affect AO correction.

(LGS mode)

3. Selecting PSF reference stars

(NGS mode)

You may observe a PSF reference star if the AO guide star or the other stars in the FOV cannot be used as a reference. The PSF reference stars should be observed at a condition as similar as possible to that of your 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 wavefront sensor (WFS) 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. Using one of these filters, you can use a PSF guide star with a R-magnitude of 0-11 brighter than your target.

Filter No.	Density (mag.)
1	11.3
2	10.0
3	8.8
4	7.5
5	6.3
6	5.0
7	3.8
8	2.5
9	1.3

(LGS mode)

4. Planning dates of observations

(NGS mode)

• Elevation --- We recommend you to observe targets at an elevation of 45^o or larger. Strehl ratios and/or FWHMs can significantly be degraded at lower elevations.
• 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 30^o apart from the moon. For the same reason you may avoid observing targets close to Venus, Mars, Jupiter, Saturn etc.

(LGS mode)

5. Overheads

(NGS mode)

Note: The amount of overheads is estimated based on our AO36 experiences. The overheads could increase for the AO188 observations.

• 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.
• Dithering can be performed within a 9''×9'' square without a significant increase in overhead, by using the tip/tilt mount of the deformable mirror (Note: we have not checked whether or not 9''×9'' square dithering can be performed without stopping AO loop. To decrease the overheads, 3''×3'' square dithering width is recommended). Larger dithering steps by nodding the telescope will increase the overhead between dither positions by 20 seconds for imaging. Telescope dithering is not used for spectroscopy.

(LGS mode)

6. Inflexible change between AO/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 IRCS and cannot be easily moved from the fixed position.

If the AO correction does not work at all because of a poor seeing condition, you may stop the AO closed loop and perform the observations without AO correction (no-AO correction mode). 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.

7. Backup Program

Subaru is a visitor-mode telescope. Satisfactory AO correction is not achieved under a poor seeing condition, say, > 1''. The observers should thus prepare a backup program for such an occasion.

You may perform the no-AO correction mode as a backup program (see above). Since the telescope auto guider attached at the Nasmyth focus cannot be used with the AO188, if the AO closed loop are stopped, tracking accuracy become worse and then long exposures should become impossible. 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. However, we have not tested this mode yet. At this moment, we cannot guarantee whether it works well or not. We then highly recommend you not to perform long exposures with the no-AO correction mode especially for a high-resolution spectroscopy.

8. Further information

Further information and questions regarding this page should be directed to Yutaka Hayano.

6 February 2008