IRCS observations with NIR WFS
IRCS observations can be done with the near-infrared wavefront sensor (NIR WFS) using AO3k-mode with shared-risk policy. The NIR WFS is the wavefront sensor designed for infrared wavelengths (Y-, J-, and H- bands). It will allow to use AO with targets that are faint in visible but bright in near-infrared.
The deformable mirror (DM) in Subaru Telescope's facility AO system (AO188) has been upgraded from a 188-element bimorph mirror to a 3228-actuator magnetic mirror in May 2024.
Before preparing the proposal, please carefully read the information below and the AO3k + NIR WFS website (especially the "Planning Observations" part).
Observation modes of with NIR WFS
The IRCS observation modes that can use NIR WFS are below.
- 20 mas imaging (Y- to K-band)
- Grism spectroscopy (zJ- to K-band)
- Echelle mode (zJ- to K-band)
- 20 mas imaging polarimetry (Y- to K-band)
- Grism spectropolarimetry (NIR settings)
L- and M- band observations and 52 mas imaging are NOT available due to the limitation of the NIR WFS (vignetting, accuracy of the steering mirror movement, throughput of the dichroic, etc.).
Sensitivities
In order to conduct the AO correction in near-infrared, a pick-off dichroic mirror has to be used. The sensitivity of the IRCS observations will depend on which dichroic mirror is selected. Sensitivities shown below are for 5 sigma detections in 1 hour of on-source integration.
| Dichroic mirror | NGS brightness (mag)(1) |
Sensitivities (mag) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 20 mas Imaging | Echelle (0.14" slit) | |||||||||
| Y | J | H | K' | K | zJ | J | H | K | ||
| K- band dichroic | H < 12.8 | NA | NA | NA | 21.9 | 21.6 | NA | NA | NA | 14.1 |
| YJH 50 dichroic | H < 12.1 | 23.4 | 22.4 | 21.9 | 21.9 | 21.6 | 14.5 | 14.3 | 13.8 | 14.1 |
| YJH 90 dichroic | H < 10.3 | 24.1 | 23.1 | 22.6 | 21.9 | 21.6 | 15.2 | 15.0 | 14.5 | 14.1 |
| Vis-HK dichroic | J < 12.2 | NA | NA | 22.7 | 21.9 | 21.6 | NA | NA | 14.6 | 14.1 |
Notes
(1)This is a rough estimate, and assume the use of broadband filter of the NIR WFS. Please see here for the limiting magnitudes using other NIR WFS filters. The limiting brightness for the AO correction depends on the color and the extinction of the star.
Limitations
The limitations described below must be taken into account during the observation planning, and explained clearly in the "Technical Justification" section of the proposal.
Natural guide star (NGS)
The NGS should be placed within a separation of < 10 arcsec from the center of the IRCS FoV.
- The target itself has to be the NGS in most of the cases, unless there is an appropriate nearby star for NGS.
- For grism mode, offset guide stars with larger separation can be used when the guide star is located at the center and the target is placed at the slits at the outer region of the IRCS FoV (0.225", 0.3", 0.6", and 0.9" slits).
- However, the target itself must be the NGS for grism polarimetry, since it will be extremely difficult to keep the image rotator angle to an appropriate angle when an offset guide star is used. For this reason, the grism polarimetry mode will be limited to 0.1" and 0.15" slit only.
For more details (using extended objects and binaries as a guide star, AO with low elevation, and limitation with moon distance), see here.
Dithering width
Because of the movement capability of the steering mirror, the maximum dithering width is 10 arcsec.
Target acquisition overheads
An extra overhead time (5-10 min per target) is required for field acquisition, in addition to the normal IRCS observations.
Switching dichroic mirrors
Only one dichroic mirror can be used on the same night. If two or more nights are allocated, then the dichroic mirror can be switched in the daytime.
Telescope tracking mode
Non-sidereal tracking is NOT available for NIR WFS observations.