Observing Overheads
Readout times
|
Measured readout time (s)
|
| Window size |
Fast readout mode |
Slow readout mode |
| 256x256 |
0.176 |
0.242 |
| 128x128 |
0.083 |
0.097 |
| all frame |
0.505 |
0.755 |
NOTE:
The fast readout speed has not been commisioned yet.
Single-cycle time
The single-cycle time is the time taken to complete a single
observation
of the minimum exposure time without windowing (including all the
overheads: readout, collecting headers, archiving...). This time varies
between
3.7-4.4s
(i.e. 'run ingrid 0.8' or 'run ingrid 0' takes 3.7-4.4s to give you the
prompt back).
Filter changes
The table below shows the time (in seconds) needed to change the
filters
in INGRID using the command
filter ingrid
<filter name>. Note that the filter movements in
some cases imply movements
of more than one filter wheel, since each filter movements is tied to
other filter movements and/or pupil stop (click
here to see the filter/pupil stop
combinations).
|
Final position
|
| K |
Ks |
H |
J |
blanks |
|
Initial
position
|
K |
-- |
20 |
38 |
55 |
100 |
| Ks |
not measured |
-- |
47 |
64 |
not measured |
| H |
37 |
46 |
-- |
38 |
82 |
| J |
58 |
64 |
38 |
-- |
66 |
| blank |
99 |
108 |
82 |
62 |
-- |
NAOMI observing overheads
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.
- 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.
