Planning Observations with Subaru AO36

It is a bit complicated to plan observations with AO. 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 an AO guide star close to your target. If you wish to observe diffuse nebulousity 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.

Read this page carefully if you apply for observing time. Check all the information in Section 1 before submitting your proposal. If technical details described in your proposal are incomplete, it may be rejected even if your science case is great!


Information you have to describe in your proposal

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

  • 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 poind-spread function (PSF) required for your project
  • Integration time required --- refer the page for CIAO imaging/polarimetry mode.
  • Whether the AO guide star is a point source or not --- if you use an extended object or a star associated with nebulousity, you must describe a FWHM of its spatial distribution or contrast between the star and nebulousity.
  • Backup program for poor observing conditions


Selecting AO guide stars

Guide stars/objects for AO correction should be selected with the criteria listed below. The target itself or a non-sidereal object can be used if it satisfies these criteria.

  • 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=16 or less . Brighter stars provide better performances if R=10-16 (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 ---- We recommend to find an AO guide star within 30'' of your target. The image quality can significantly be degraded at larger distances (see this figure).
  • Morphology ---- Point sources without surrounding nebulousity are highly recommended. AO loop is closed even using some extended sources and stars associated with nebulousity. 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 nebulousity, make sure that the stellar magnitude must be brighter than the total background in a 2-arcsec aperture. If you have to use a guide star in an HII region, an H-alpha blocking filter is available on request prior to your observations. Note that binarity does not affect AO correction.


Selecting PSF reference stars

You may observe a PSF reference star if the AO guide star 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 11.1
2 9.8
3 8.8
4 7.1
5 6.2
6 4.5
7 3.8
8 2.3
9 1.2

Planning dates of observations

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

  • Elevation ---- We recommend to observe targets at an elevation of 45o 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 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.

Overheads

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. In addition to these, we need up to 12 minutes to slew the telescope to your target. 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 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 CIAO page for their FOVs.
  • Dithering can be performed within a 3''3'' square without a significant increase in overhead, by using the tip/tilt mirror. Larger dithering steps will increase the overhead between dither positions by 20 seconds for imaging.

    Tip/tilt dithering is not used for LM-band observations, or K-band observations with long integration. See this note for details. Telescope dithering is not used for spectroscopy.


Flexible change between AO/no-AO modes

At the telescope you may find that AO correction does not work well due to poor seeing. In this case you may want use CIAO without AO as its warm optics significantly increases thermal background (about 40 %). In another case, AO correction may not be mandatory for your project but you may want to use AO if it provides a better point-spread function or S/N. Such a change between AO/no-AO modes can be easily performed by inserting/retracting a pick-up mirror for the AO optics. The plate scale for these two modes are the same as each other.

If you want to try `flexible' AO correction in a way describe above, state this in your proposal . This is mandatory to assign an AO instrument operator and the support astronomer to your observing run.

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.

7 February 2007



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