Notes for the support astronomer
Before an OSCA night
- With OSCA in the beam, and a well sized mask (say the 1.0"),
take a dome flat
- Identify the mask center and calculate the offset from the standard
aquisition position (pinhole)
- Start the simplex tool on INGRID and locate the quadrant with the mask
- It should not be necessary to run a dedicated simplex for OSCA;
it is definitely not recommended to simplex with the Lyot stop in
the beam, since this will mask of large parts of the pupil.
- Check from pinhole images with/without OSCA that
abberations are small (eg, compare strehl values and faint structures in
these images); be aware that OSCA mirrors the images. You need to use
the naomi-clr pupil stop to mask of unwanted light. OSCA images
of the pinhole with no pupil stop will appear elongated
- Check the pupil image to determine the OSCA Lyot stop offset angle:
- With the telescope primary mirror covers open, looking at the
illuminated dome, and with the INGRID pupil imager into the beam;
check the alignment of the OSCA Lyot stop relative to the M2 vanes.
If the vanes are not completely
covered, adjust the Lyot stop Offset angle until you have redetermined
the optimum offset angle. Note that this requires the TCS to be active,
in order to get updates of the rotation angle, and that the Lyot Stop
Enabled button and the TCS-Lyot Update button are pressed
- Check also that the central obscuration of the Lyot stop is centered
over the central four segments of the DM. To judge what segments
are the central ones, use TopGui to tweak the tilt of one segment
next to the central four until its edges becomes obvious
- The images may appear double, with one image offset slightly to the
left on the RTD display. This is due to light bypassing the small
OSCA mirrors, so that you actually see part of the direct pupil
on top of the OSCA pupil. Placing a piece of paper between the two
OSCA flat mirrors will remove the ghost image
- An even sharper pupil image can be obtained by going on a very bright
(V=2-3) star in pupil imaging mode
- Check also that the INGRID pupil-stops match up with the Lyot stop
and primary mirror.
Nighttime observing
- Aquire the star on the WFS as normal with OSCA out of the beam
- Move OSCA into the beam and select a mask
- Start the simplex tool, using continous display
- The distance between the acquisition and mask position is usually
only a few arcsec, therefore select a window size in the simplex
gui which covers both positions (using the above offsets)
- The TO can then move the star close or even onto the mask
- Move the WFS pickoff probe via TopGui accordingly until the
star appears again on the WFS (or just use the previously computed offsets
and center the star on the WFS via telescope movements)
- Close the AO loop
- To center the star precisely behind the mask, move the WFS pickoff probe
in small steps (about 0.1 arcsec) until the star appears to be centered.
- If the seeing is less than perfect, or if your target is relatively
faint, it can be difficult to judge when the star is well centered
behind the mask. To check the final alignment, it can be useful to
take a longer exposure either on the simplex tool or with the observing
system. Check that the halo around the mask is
symmetrical around the mask and that (if visible) the core of the star
light is in the center of the mask
- Remember to take the Simplex tool of Continuos Display before taking
frames on the observing system
Changing the sky PA (rotational dithers)
- Keep the AO loop closed while the TO changes the PA. Since this is
a slow process (several minutes), the AO and AutoGuider should
keep the star centered on the WFS if the WFS integration time is
at its minimum. This will work well for small dithers, but if the
dithers are in steps of more than 20-30 degrees it can fail
If it fails, open the loop, let the TO move the star back, usually
only about 2-3" and close again
- Due to the difference in atmospheric dispersion between the optical
light on the WFS and the IR light on INGRID, the star will now
have moved off the OSCA mask center. (Unless you are observing at very
low airmass.) Use the pick-off probe (see above) to center it under
the mask again
- For the moment only this manual centering of the star behind the mask is
possible. An automated centering might be implemented in the future.
Some requirements for such an automated procedure can be found
here
Which pupil to use
- For observations in K band the NAOMI-Obs pupil stop inside INGRID is the
preferred one
- The central obscuration of OSCA-Obs does not fully cover the central four
segments
- For short wavelengths use OSCA-Clr when the Lyot stop is being used.
- For faint targets, the throughput can be doubled by removing the Lyot
stop. In this case, use the NAOMI-Clr pupil stop, not the OSCA-Clr
- Note that the OSCA-Clr/Obs pupils are heavely undersizing the primary
(corresponding to useing a ~3.5m telescope).
From simulations this should improve the suppression.
It depends on the particular science case which is the preferred pupil
choice, but usually the NAOMI-Obs/Clr pupils can be used for all standard
observations
Calibration frames
- As with any INGRID run, make sure you have darks for all exposure
times used.
- Flat-field images should preferably be obtained for all OSCA masks
and filter combinations used. If you only obtain OSCA flats with
the clear mask, you will account for flat-field errors (dust
etc.) from the optical surfaces except the coronographic mask itself.
But keep in mind that the clear mask may have dust on it as well,
that is not on the hard masks!
- The Lyot stop is in the pupil plane and has no optical surfaces,
and should not have any effect on the flat-field (except that it
steals about 50% of the light).
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