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.
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)…
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 science data.
“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.