Doc:
Issue
Date
Page
VLT-MAN-ESO-14650-4942
P96
24.06.2015
23 of 161
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
2.2.1.4 The Acquisition and Guiding Camera
The A&G camera allows to visually detecting and center objects from the U- to the z-band.
This unit consists in:
a filter wheel equipped with a full UBVRI Johnson filter set and a full Sloan Digital
Sky Survey (SDDS) filter set. Transmission curves are provided in Section 4.
a Pelletier cooled, 13 µm pixel, 512
512 E2V broad band coated Technical CCD57-
10 onto which the focal plane is re-imaged at f/1.91 through a focal reducer. This
setup provides a plate scale of 0.173”/pix and a field of view of 1.47’
1.47’. The QE
curve of the detector is provided in Section 4.
This acquisition device –that can also be used to record images of the target field through
different filters– provides a good enough sampling to centroid targets to <0.1” accuracy in all
seeing conditions.
The noise of the technical CCD is currently of RON of 4.1e-.
The limiting magnitudes for a direct acquisition were measured for different filters under
relatively bad conditions (thin cirrus, full Moon, seeing about 0.7”), see Table 3.
Table 3: Limiting magnitudes for a direct acquisition
U
B
V
R
I
22
22
22.5
22.5
22.5
30s
30s
20s
20s
20s
We still have to measure their limiting magnitudes under clear conditions and in dark time.
However, in case of worse weather the limiting magnitudes are smaller.
We still recommend to use blind offsets in case the object is fainter than 22-22.5, especially
if the weather constraints are selected for thin/thick transparency and seeing worse than
0.7”. In case of blind offsets, we recommend to select an acquisition star with a magnitude
about 19 or brighter to ensure a good centering before the offsets are done.
For other SDSS filters, we recommend to keep a limiting magnitude of 20 for a direct
acquisition in I’ and z’ but to go up to 21 in other SDSS filters. The exact limiting magnitudes
for those filters will be determined during P93.
Examples of recommended exposure times for the acquisition CCD:
Vmag=6 integration time=0.001s
Vmag=7 integration time=0.005s
Vmag=16-20 integration time=1 to 5s
V, R mag=23 integration time=60-120s
V,R mag>=24 integration time
180s
These integration times should suffice for doing a direct acquisition in case of clear
conditions, darktime and usual seeing. However, in case of very faint objects, the blind offset
could be the best solution as it could shorten the acquisition overheads.
See Sect 4 about the imaging mode that provides updated information about the AGCCD
and the imaging mode facility.