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INGRID: Planning Observations & Observing Requirements
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
If you are applying for laser-assisted AO, note that for safety reasons
the laser may not be used at a zenith distance which exceeds 65 degrees.
In practice this limits the southerly declination of targets
to > ~-30 degrees. This declination complies with the laser's pointing
constraints
for up to ~4 hours per night.
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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.
- The guide star should be bright
(V<14).
- The guide star should be pointlike
(<1.5 arcsec). (NAOMI has
successfully worked with the nuclei
of galaxies.)
- The guide star should be as close
as possible to the science target (d<40").
- There should be no stars of comparable
magnitude within ~5" of the guide star (to avoid confusing the
wavefront sensor).
- Guide stars with V<6 require a neutral density filter in
front of the wavefront sensor to avoid saturating it
Performance of the AO system with GLAS is expected to be comparable to that
with a bright, natural guide star. However, laser-assisted AO still requires a natural guide
star to perform tip-tilt correction.
- The laser-assisted tip-tilt guide star should be brighter than V=17,
and for optimal performance should be located within 1 arcmin of the target.
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.
- The guide star should have a magnitude V<17.
- 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.
- The guide star should be within 1.5 arcmin of the science target.
- 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.
- 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.
PSF Calibration - Do I need to do this and if so
how?
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:
- A similar separation to that between the guide star and the
science target (to within ~5").
- A similar position angle to that between the guide star and
science target (to within ~30°).
- 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).
- 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.
Overheads should be included in the request for telescope time. There
are a number of overheads specifically related to (i) adaptive optics
systems and
(ii) infrared observing, which are discussed below. Don't forget to
also include other standard overheads, such as filter and pupil stop
changes. Overheads when observing with the laser system are not yet
fully quantified, and you do not need to address them explicitly.
- Acquisition: 5-10 minutes
per target (~20 minutes with OSCA)
- AO settle time: Negligible
- Dithering: ~7s to change
dither point
- Rotational dithering (with OSCA):
~5-10 minutes per position angle
- INGRID readout time: ~1.5s
- INGRID filter/pupil-top changes: ~60s
- PSF calibrations: These
overheads should be included if required
- Offset sky exposures:
These overheads should be included if required
Acquisition overheads
involve (i) slewing the telescope, (ii) acquisition of the science
target on the detector, (iii) acquisition of the guide star (if
required) on the wavefront sensor, and (iv) acquisition of the image behind the OSCA mask (if required).
AO settle time is the
time taken to settle into the AO correction after closing the feedback
loops on the guide star.
Dithering is required
for infrared imaging. It entails (i) opening the AO loop,
(ii) moving the telescope and the guide star pickoff probe
simultaneously, and (iii) closing the AO loop. Note that standard 5 and
9 point dithers are available.
Rotational Dithering is encouraged for infrared imaging with the coronograph mask in OSCA deployed. It involves changing the PA of the derotator closed-loop, and accurately recentering the target behind the mask.
INGRID's readout time
becomes significant when taking short exposures to avoid saturation
from the sky background. For example, if you are performing a coaverage
of 40 individual 1s exposures, the total readout time will be 60s (ie.
a 150% overhead).
PSF calibrations, if
needed, can introduce significant overheads. See above for more
discussion.
Offset sky exposures
are required for infrared observations if observing extended targets. One must observe
for a similar amount of time "off" the science target (looking at blank
sky) as on it, in order to do proper flat fielding and sky subtraction.
This thus doubles the total exposure time. Such exposures are not
required for point sources because a dithering pattern is sufficient.
To calculate these overheads it is essential for you to plan how you
will perform the observations. Your proposal should specify how the
observations will be broken down ie. (i) the integration time per
exposure, (ii) the number of exposures coaveraged to give one image,
(iii) the number of images at each dither point, and (iv) the size of
the dither etc.
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