OASIS FAQs
- Applying for Time
- Adaptive Optics
- Other Technical Questions
- Data Reduction
- Background Information
Applying for Time
- How can I apply for observing
time with OASIS?
There are two avenues that one can use
to apply for observing time at the WHT.
For proposals requiring one or more nights, then investigators should
apply to the appropriate time allocation committee, in response to one
of the 'Announcements of Opportunity for Observing Time'. More details
can be found
here.
(Please note that that all OASIS runs
using adaptive optics are
performed in
service mode by ING staff astronomers, and it is not possible for
visiting observers to be present.)
The
ING service programme is
designed to allow astronomers to obtain up
to 8 hours of observing time during OASIS S/D nights. All service
programme observations are performed by ING
staff astronomers. Advantages of
this programme include its fast track nature (with monthly submission
deadlines) and the opportunity to perform pilot programmes to test
novel ideas. The OASIS application form
can be found
here.
- Is there a S/N calculator for
OASIS?
Yes, there is an
exposure
simulator that is maintained by CRAL (in Lyon). Note that (i) the
0.42" sampling is currently not available at the WHT, and (ii) the
'Atmospheric dispersion compensator' should be selected as 'OFF'. More
information can be found by clicking on 'HELP' at the bottom of the
simulator input form.
- To achieve my scientific goals I need to use more than one
spectral configuration. Is it possible to apply for more than one
configuration in my proposal?
Yes. Of the total 15 spectral
configurations, a selection of 12 can be made available on a given
night. (This is determined by which 12 of the 15 filters are placed in
the instrument's filter wheel.) However, due to the large number of
calibration frames that are required, it is not advisable to use more
than a total of three or four combinations of spectral and spatial
configurations. Note that in the 'Service Programme' application form
you are explicitly limited to five.
- I am interested in using more than one spatial sampling so that I
can combine the advantages of a larger FOV with higher spatial
sampling. Is this possible?
Yes. There are currently three
available spatial samplings (0.09", 0.14" and 0.26" per lenslet). It is
possible to use up to all three of these samplings if necessary.
However, due to the large number of calibration frames that are
required, it is not advisable to use more than a total of three or four
combinations of spectral and spatial configurations. Note that in the
'Service Programme' application form you are explicitly limited to five.
- I would like to observe a larger FOV than that provided by the
0.26"/lens sampling (10.3" x 7.4"). Is the new larger FOV available yet?
No. The new spatial enlarger, which
provides a sampling of 0.42"/lens and a FOV of 12.0" x 16.7", suffers
from vignetting and is currently undergoing engineering work. In the
meanwhile it is only possible to observe larger fields of view by
mosaicing with the 10.3" x 7.4" mode.
- I am interested in applying for time with OASIS but am concerned
about the technical feasibility of my proposed observations. What
should I do?
All proposals will be reviewed after
the submission deadline to confirm their technical feasibility.
However, ING also strongly encourages
pre-submission
enquiries.
- What is the orientation of the FOV and what sky PA should I
select for my observations?
The spectroscopic field of view is
rectangular, with the long axis running east-west at a sky PA of zero.
Therefore to align the long axis of an object with the long axis of the
instrument:
sky PA required = position angle of object + 90
- I am
preparing finding charts
for my observations. What field of view should I use and what
information do I need to include?
All finding charts should specify:
- The principal investigator's name.
- The target's name and preferably its coordinates.
- The scale and orientation of the image.
- The passband of the image. It is useful to specify the source of the
image e.g. DSS, SDSS etc.
- The proposal's reference code (if the finding chart is being prepared
after the proposal has already been submitted).
The principal finding chart for OASIS observations should be centred on
the science target and have a field of view of ~3.5 x 3.5 arcmin (to
match that of the acquisition camera). The guide star should also be
marked, along with a label giving its magnitude and distance from the
science target. If you are using a non-zero sky PA and are not
submitting a secondary finding chart (see below), you should also
indicate the required orientation of the long axis of the IFU.
If you are (i) studying an extended target, (ii) observing with a
special sky PA, and/or (iii) believe that the acquisition could be
difficult, then you may wish to submit a secondary finding chart for
the target, with
a smaller field of view and higher spatial resolution (e.g. an HST
image rather than a DSS one). In this case, you should overplot the
field of view of the IFU (with the appropriate orientation) at the
required pointing(s).
Adaptive Optics
- I want to use OASIS with AO correction from NAOMI. What are the
requirements for the natural guide star?
The guide star requirements are as
follows:
1. The guide star should be
bright
(V<14).
2. The guide star should be
pointlike
(<1.5 arcsec). (NAOMI has
successfully worked with the
nuclei
of galaxies.)
3. The guide star should be as
close
as possible to the science target (d<40").
4. There should be no stars of
comparable
magnitude within ~5" of the guide star (to avoid confusing the
wavefront sensor).
5. Guide stars with V<6 require a neutral density filter in
front of the wavefront sensor to avoid saturating it.
Note that for bright guide stars V<12, NAOMI typically delivers a
reduction in FWHM of a few tenths of an arcsec at wavelengths 0.6 - 1.0
microns. E.g. in good seeing, 0.6 arcsecs, the FWHM is typically
reduced by a factor of two in R band. Some correction is achieved even
when the science target lies several 10s of arcsecs from the guide star.
You should refer to the
NAOMI
performance
page to see how the AO correction varies as a function of (i) guide
star magnitude, (ii) the
natural seeing and (iii) wavelength. To search for possible guide stars
close to a list of candidates, you can use this
guide star finder.
Please specify clearly in your proposal that you wish to use 'OASIS
with NAOMI'.
- There are no stars near to my science target that can be used for
full AO correction. Can a tip-tilt correction be applied?
Yes. More relaxed guide star
constraints exist for tip-tilting guiding:
1. The guide star should be V<17.
2. The guide star should be pointlike (<1.5 arcsec). (Tip tilt
guiding has been performed with the nuclei
of galaxies.)
3. The guide star should be within 80" of the science target. (In
practice the guide star probe can be sent to up to 90" from the
science target, however at radii of more than 80" there is some
vignetting. This means that only brighter guide stars should be used in
this region.)
4. There should be no stars of comparable
magnitude within ~5" of the guide star (to avoid confusing the
wavefront sensor).
Please specify clearly in your proposal that you wish to use 'OASIS
with NAOMI'.
- Is GLAS (the laser guide star adaptive optics system) available
for use with OASIS?
No, GLAS is currently not being offered
at the WHT due to issues with the output power of the laser.
- Do I still need to provide a
guide star when using GLAS? [N/A at present]
Yes. Laser-assisted adaptive optics
still requires the use of a nearby guide star in order to apply a tip
tilt correction (as well as guiding). However the constraints on this
star are much more relaxed that for high order AO with a natural guide
star system. To observe with OASIS+GLAS, the tip tilt guide star should
be a least as bright as V=17, and is best if located within ~60" of the
science target.
- What performance can I expect
when using GLAS? [N/A at
present]
Performance with GLAS is still being
characterised,
but one may reasonably
assume that it will be similar
to that obtained with a bright natural guide star (mag ~ 9.5).
(See
here for more details.)
- Are there any restrictions that
arise when observing with OASIS+GLAS? [N/A at present]
Yes, there are two important
restrictions to bear in mind:
1) When using the laser guide star, it is only possible to observe at
elevations greater than 25 degrees (due to safety reasons).
2) OASIS's usual 15 spectral
configurations are still available for use with GLAS. However, due to
the insertion of a filter (which is used to direct the returned laser
light to a wavefront sensor), part of the spectral coverage of four of
the configurations is blocked. More details can be found
here.
- Can I use OASIS without adaptive
optics?
Yes. However, it is only advisable to
use the 0.26"/lenslet spatial sampling without AO, as you will highly
over-sample with the 0.09"/lenslet and 0.14"/lenslet spatial samplings.
Please specify clearly in your proposal that you wish to use 'OASIS
without AO'.
Please note that even though you do not need an AO guide star, it is
important to provide a suitable guide star for telescope guiding (for
exposures lasting more than about a minute). This star must have
V<19.5, and be located within 80" from the science target. At high
galactic latitude there should be ~1 such star in the field. (Note that
compact galaxy nuclei may also be used.)
- Can I use my science target as
the guide star (for either telescope guiding or AO correction)?
Yes, providing the science target is
stellar (or a compact galactic nucleus) and of sufficient brightness.
Guiding and AO correction are effected using the optical wavelengths
that are not required for science, through the use of an appropriate
dichroic.
Other Technical Questions
- Can OASIS be used in conjunction
with the coronagraph OSCA?
Unfortunately OASIS cannot be used with
OSCA due to vignetting of the folded light path when OSCA is deployed.
- Can OASIS be used in imaging mode?
OASIS does also have an imaging mode
(which is used primarily for target acquisition). It has a field
diameter of 37.6 arcsec and 0.02"/pixel (unbinned) sampling. The
filters available correspond to those used for the 15
spectral
configurations.
Data Reduction
- The format of the OASIS spectra on the CCD looks complicated. Do
I need to develop or use special software to reduce the data?
OASIS comes with its own dedicated data
reduction package,
XOasis,
which was developed by the team in Lyon. Users can either use an
interactive GUI or command line scripts. Although in the past the
software has been developed on a number of platforms, we recommend
using the Linux version, which is the most up-to-date. More details on
XOasis can be found
here.
Background Information
- Where can I find out more information on the instrument?
- Where can I find out more
information about NAOMI and its performance?
General information can be found on the
NAOMI web page. Specific information
on performance (e.g. FWHM, Strehl ratios etc) can be found
here.
- What does OASIS stand for?
Optically Adaptive System for Imaging
Spectroscopy.
- What is the history of OASIS?
OASIS was built by Roland Bacon and his
team at the Observatoire de Lyon for use at CFHT. The first light was
in 1997. In 2003 it was transferred to the WHT and re-commissioned.
