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VIMOS User Manual
VLT-MAN-ESO-14610-3509
Table 3.2: VIMOS Calibration Plan: MOS Mode
Calibration
Bias
Darks
Screen Flats
Screen Arcs
Screen Arcs
Number
5
3
5
1
1
Specphot.
Standard Star
4
quadrants
Frequency When?
Daily
Day time
Monthly
Day time
Daily
Day time
Daily
Day time
Attached to science
OB with HR or LR blue
See text
Night time
Products
Master Bias
Dark current
MOS Flat
Dispersion coeff.
Dispersion coeff.
Accuracy1
RON
Comment
5%
0.3 pix
0.3 pix
All used masks
All used masks
All used masks
Response
function
10%
All grism+filter
setups used
1
Internal accuracy, not considering instrument flexures (see Sect. 2.7).
IMPORTANT NOTE The execution time for all attached night time calibrations (screen-flats and arcs) is subtracted
from the total allocated time.
3.4.10
MOS mode: Exposure Time Calculator
The approximate limiting magnitudes in MOS mode can be obtained from the exposure time calculator available on the ESO WEB pages http://www.eso.org/observing/etc/.
3.5
3.5.1
Integral Field Unit (IFU)
Acquisition accuracy
In IFU mode acquisition is “blind”, i.e., limited by the accuracy of the telescope pointing (∼ 300 accuracy).
3.5.2
Dithering/jitter pattern
With the large field of view of the VIMOS-IFU, techniques equivalent to ”shift-and-add” classically used in
imaging, are a possible mode of operations. Dithering/jitter pattern is a list of differential pointing positions
around a central position. Several short images (with exposure times set to have sky background limited
exposures) are taken at each position of a dithering pattern, which are combined to reconstruct an image with
the total required exposure time. This allows to eliminate the sky contribution, cosmic rays, the CCD fringing
(particularly strong in the red), cosmetic defects on the detectors, and to produce very accurate flat fielding for
correction of pixel to pixel variations. The dithering pattern depends on the mean size of the observed objects:
it should be larger than the mean size of the objects for the method to work efficiently.
3.5.3
IFU mode: grism + filter setup
In IFU mode, only one filter+grism combination per OB is permitted. Users who want to observe the same
targets with different filter+grism combinations are requested to submit separate OBs for different filter+grism
combinations and to consider the respective overheads.
3.5.4
IFU mode: attached night time calibrations
To better take into account the effects of instrument flexure the use of attached nighttime calibrations, arcs and
flats (see Sect. 3.5.5), is MANDATORY for all IFU OBs. This template will provide 1 or 3 flat-field images
(for HR grism: 3 screen-flats are mandatory, for the other cases, the user can choose the number) as well as 1
arc, taken at the same rotator position of the science target. The execution time for these nighttime calibrations
is always subtracted from the total allocated time.