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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 the proposal.
(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.
Backup plans will be more important especially for S23A semester since the new laser observation will be conducted with shared-risk policy.
- 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.
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 the primary support astronomer (Yuhei Takagi, takagi_at_naoj.org).
Aug 05 2022
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