Changing Filters, or between Imaging and Spectroscopy etc.
Any change of LIRIS configuration takes about 1 minute on average, for
filter changes or changes from spectroscopy to imaging etc. The LIRIS
wheel movements are made in parallel in
order to minimise the overall overhead, and all the mechanisms can be
moved while the telescope moves to the next target.
The filter change
overheads
are not symmetrical, i.e. changing from
e.g. z to H takes significantly more time than changing from H to z.
Thus, if you observe many targets in several filters and with short
integration times, the filter change overhead becomes an important
factor which can be optimised, and you can easily save half an hour per
night by selecting the best filter order. For example, observing in the
order H-J-Ks is optimal as it takes only 82s for the filter change,
whereas J-H-Ks is worst with 126s. Doing H-J-Ks-brg-heI takes 132s, as
compared to 296s for J-brg-heI-H-Ks.
The following table lists a few representative overheads for filter
changes etc. The average time is about a minute. If you use other
filters not listed
here and plan frequent filter changes, then it is worth making a few
timing measurements in the afternoon before your run and working out
the optimal strategy.
| Filter Change Overheads (s) |
|
to
from |
z |
J |
H |
Ks |
| z |
0 |
18
|
63 |
47 |
| J |
80 |
0 |
58 |
43 |
| H |
34 |
39 |
0 |
68 |
| Ks |
54 |
54 |
55 |
0 |
| Mode Change Overheads (s) |
|
to
from |
image (J) |
spec (zJ) |
spec (hk) |
lipol (J) |
lcor (J) |
| image (J) |
0 |
78 |
45 |
71 |
79 |
| spec (zJ) |
43 |
0 |
69 |
- |
- |
| spec (hk) |
78 |
56 |
0 |
- |
- |
| lipol (J) |
45 |
- |
- |
0 |
53 |
| lcor (J) |
47 |
- |
- |
74 |
0 |
Observing overheads
| Action |
Overhead |
| 12" dithering offset
(imaging, tracking) |
8 s |
| 24" dithering offset
(imaging, tracking) |
11 s |
| 12" nodding offset
(spectroscopy, tracking) |
12 s |
| 12" nodding offset
(spectroscopy, guiding) |
33 s |
| Rotator angle change by
90 degrees (in either direction) |
120 s (i.e. no. of
degrees + 30s) |
| Acquisition of a guide
star |
0 (usually, as done
during slewing) |
| Acquisition of object
onto slit for spectroscopy |
5-15 min (depends on
magnitude) |
| Acquisition of a
multi-object spectroscopy mask |
10-15 min |
Exposure related overheads
| Task |
Overhead |
| Clear-reads (default
N=3. Only before first exposure at each new
position.) |
Nx0.8 |
| Imaging read-out time,
mndr 1 1 (default) |
2 |
| Imaging read-out time,
mndr 2 1 |
3 |
| Spectroscopy read-out
time, mndr 4 1 (default) |
5 |
| Spectroscopy read-out
time, mndr 8 1 |
11 |
| Time until image is
displayed after readout |
4 |
| Time until image is
saved after readout |
4 |
| Time until you get the
prompt back after readout |
2 |
Note that while dithering or nodding, after the last exposure is taken
at a given position, the chip is readout while moving to the next
position, so only the larger overhead (movement or readout) applies.
