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  1. Applying for Time
  2. Adaptive Optics
  3. Other Technical Questions
  4. Data Reduction
  5. 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?
There is further information on the Lyon web pages.
  • 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.

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Contact:  (OASIS Instrument Specialist)
Last modified: 08 November 2012