Planning Observations

Before applying to use INGRID you need to carefully plan your observations, particularly if you want to use INGRID with AO correction. The information that you need to put in your proposal includes:

• A guide star (if required) for each target
• PSF calibration stars (if required)
• A prediction of the AO correction that NAOMI is likely to achieve (if AO required)
  
• A prediction of the S/N for the proposed observations - see here
  
• A calculation of the approximate observing overheads

This section is divided into two sections, concerning observations with and without AO correction.

Guide Star Requirements for AO Correction

When using INGRID with AO correction, a suitable guide star is required for every science target. These guide stars should satisfy the following requirements. Note that if the science target itself satisfies them, then it can be used as the guide star.

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

You should refer to the NAOMI performance page to see how the AO correction varies as a function of (i) guide star magnitude, (ii) guide star - science target separation, (iii) the natural seeing and (iv) wavelength. To search for possible guide stars close to a list of candidates, you can use this guide star finder.

No AO Correction - Do I need a Guide Star?

When using INGRID without AO correction, you must establish whether or not you need a guide star for telescope guiding. If your individual integrations will be short (~ a few minutes) then you can simply use the telescope tracking to observe your science target. In this instance no guide star is required. (Note that there is ~1" drift in tracking over a ten minute timescale.)

For longer integrations you will need a guide star, however the requirements are much less stringent than for performing AO corrections. Note that if the science target itself satisfies these requirements, then it can be used as the guide star.

1. The guide star should have a magnitude V<19, preferably V<17.
2. The guide star should be pointlike (<1.5 arcsec). Note that this restriction can be relaxed for fainter guide stars, of V>14. Galaxy nuclei as well as stars may be used.
3. The guide star should be within 1.5 arcmin of the science target.
4. There should be no stars of comparable magnitude within ~5" of the guide star (to avoid confusing the wavefront sensor). Note that this restriction can be relaxed for fainter guide stars, of V>14.
   
5. Guide stars with V<6 require a neutral density filter in front of the wavefront sensor to avoid saturating it

Even at high galactic latitudes, there will be on average ~1.2 such stars satisfying these requirements. To search for guide stars you can use this guide star finder.

If you are using INGRID with AO correction you should consider whether you need to use PSF calibration stars. An AO-corrected point spread function (PSF) typically comprises of a near-Gaussian diffraction-limited core with a faint 'blobby' pattern extending over a disk of similar radius to the uncorrected seeing. If, for example, you are looking for faint close companions to another object you will want to know whether what appears to a companion really is one, or whether it is simply  a PSF artefact.

If you do decide to perform PSF calibrations, you must select target PSF stars.

(a) If the science target is being used as the guide star, you should locate a star within a few degrees of the target that is of comparable magnitude (generally to within ~0.2 mag, although this can be relaxed for V<10).

(b) Calibrating the PSF when the science target is different from the guide star is non-trivial in the sense that it varies strongly with both radius and position angle from the guide star. (The major axis usually points towards the guide star.) The most trivial solution is to look at the PSF of stars close to the science target - unfortunately the density of stars in the field is rarely high enough for this. A better approach is to search for separate star pairs satisfying these properties:

1. A similar separation to that between the guide star and the science target (to within ~5").
2. A similar position angle to that between the guide star and science target (to within ~30°).
3. A similar magnitude for one of the stars to that of the guide star (generally to within ~0.2 mag, although this can be relaxed for V<10).
4. A position on the sky close to the target (to within a few degrees).

The selected PSF star or PSF star pair should be observed every (~15-30) minutes, with the same dither pattern as for the science exposures, and a minimum exposure time (per dither point) of 1 minute.

To search for suitable PSF stars you can use either this PSF star finder or this PSF star-pair finder.