UIST Coronagraphic Imaging
The Basic Idea...
In 2006 a second imaging-polarimetry mask with occulting wires was
installed in the UIST slit wheel. Although this new mask was designed for
coronagraphic imaging polarimetry, it can also be used for normal
coronagraphic imaging (i.e. without the polarimetry prism and
waveplate in the beam).
The idea is that bright sources can be positioned behind either of
two wires, so that longer exposures can be secured without saturating
the array and without latency issues, bleeding, internal reflections,
ghosting, etc. Under normal observing conditions (0.6-0.7 arcsec
seeing, clear skies, etc.) 10-20 sec images can be obtained through
the broad-band filters on 7-8th mag stars.
Figure 1: A raw image of the bright nebulous source S106-IR
through the coronagraphic mask. S106-IR has been positioned behind the 6-pixel
wire. The image was taken with a 10sec K-band image; S106-IR has a K-band magnitude
(2MASS) of 5.9! A position angle of -90 degrees was used, giving N-left and E-up
With Coronagraphic imaging, the user may use either a
6-pixel (0.7arcsec) or an 11-pixel (1.3
arcsec) wire. These are suspended 3/4 and 1/4 of the way
along a 120"x20" rectangular aperture in the bottom half of the array.
A second, similar-sized aperture in the top half of the array is
available for sky-subtraction. Alternatively, point sources that
appear in this aperture could be used for PSF fitting. The separation
between these two apertures is ~48 arcsec (see above).
It is important to note that it is the position angle
(East of North) of the coronographic occulting wire that you have to define in
the UKIRT OT when you set up your MSBs.
The position angle of the image plane can be orientated so that
extended targets are placed orthogonal to the wire. For example, with a disk, jet or nebula orientated E-W you should use a
position angle of 0 degs; with a disk orientated N-S you should use a
position angle of -90 degs. Any angle between -90 and +90
degrees can be used, although acquisition will be easier if you use
angles between 0 and -90! . To fully understand what's happening,
see the figure below (you may need a shot of coffee first)...
| Figure 2: LEFT: A narrow-band image
of S106-IR, correctly orientated so that N is up and E is left.
CENTRE and RIGHT: raw (K-band) coronagraphic images of the same
target, but with different posn angles. Common features are marked
with an elipse and a dashed, curved arrow. |
From the above figure one can see that raw coronagraphic images
are firstly flipped about a horizontal axis. They are then
rotated by the position angle selected.
Offsets and Observing Strategy
A suggested MSB is available in the UIST imaging template library.
Since offsets must keep the source behind the occulting wire, and
because both wires are only ~20 arcsec in length, separate sky frames
will probably be needed to construct a flat-field image. Note that,
regardless of position angle, offsets in "q" will always be along the
wire. (An offset of q=+48 will put the source in the top
aperture, though this is certainly not desirable with bright targets.)
A possible sequence might consist of 11 frames, six sky frames
interleaved with five images with the target behind the occulting
wire. The frames with the source behind the wire might have p,q
offsets of 0,0; 0,+0.6; 0,-0.6; etc. (the offsets being multiples of
0.12" pixels), while the sky frames could put the bright star between
the two rectangular apertures, or well off of the array.
Imaging Acquisition: putting the bright target behind the wire
In the example MSB in the template library, a short (1sec)
exposure is used to acquire the target. The instrument is run in
"Movie mode", which means that frames are taken (though not saved)
repeatedly so that the target can be placed behind the wire. This 1
sec exposure time is subsequently updated using a "UIST Imaging
Iterator", so that longer exposures can be used for taking the actual
"Pick-object" is used with the Gaia Movie display; the posn of the
target is "saved" and the source moved behind the wire by the
telescope operator who uses UPICK. When you first slew to the target
and start Movie, if you don't immediately see the target, ask the
telescope operator to move "up 10" and/or "down 10" while you are
running Movie. When you can see the target, then use pick-object and
upick to place it behind the wire.
By default the acquisition process will put the source behind the 6-pixel (0.7
arcsec) wire. If the wider occultor is required, the telescope operator should apply
a "left 62.6 arcsec" after the slew and initial acquisition. This will move the
target from the thin wire to the thick wire; the position can be fine-tuned with
subsequent small left/right offsets (while still running Movie).
A dedicated pipeline recipe is pending.