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30
Issue 1.3
LUCIFER User Manual
Under good and smooth observing conditions, it has been calculated that offsets in active mode
(guiding and sending active optics correction) take in average ∼ 18 seconds, while only 4 seconds
when performing them in track mode.
Furthermore you have to add the time to create/save the fits file. This time is strongly related to the
number of integrations requested and the mode in which data are to be saved. The average time for
this process is ∼ 12 seconds ( between 5 - for single frames - and 20 in practice).
To those times, one has to add the preset time. This time can be only the slewing time, if one uses the
track mode. However most observations will be performed in guided mode with active optics correction
on. Therefore the guider acquisition and collimation times must be added. Over the Sept.-Oct. 2009
commissioning, over 214 succesful preset (mixed of telescope modes track & active), the average
preset time was of 70seconds. The mean time needed for collimation requests (90 measurements) was
135seconds.
A correction of the telescope pointing takes in average 7 minutes.
For spectroscopic observations one has to add the time needed to move the mask in/out of the focal
plane. To move a mask from its cabinet storage position to the focal plane, it typical takes 2.5 minutes.
Since however it is recommended to move the mask in the ‘focal plane’ position while presetting,
the overhead quoted here represents only the time to move the mask from the turnout position to
the focal plane = 45seconds.
Table 10 summarizes all types of overheads.
Example of overhead calculation (based on true examples) - without preset or acquisition time:
Imaging
Detector mode: DCR
DIT = 20 sec
NDIT = 3
NEXPO = 1
20 offsets
Total time needed
Spectroscopy
Detector mode: MER
DIT = 600 sec
NDIT = 1
NEXPO = 1
5 offsets
Total time needed
= (20.+ 2.)*3.*20. + 20.* 18. + 20. * 12.
= 1920 seconds for 1200 seconds of on-source integration
= 62.5% of shutter open time
= (600. + 10.) * 5. + 5. * 18. + 5. * 12.
= 3200 seconds for 3000 seconds of on-source integration
= 93.7% of shutter open time
6.4
Limiting magnitude & recommended integration times
6.5
Sky emissivity
Sky emissivity is an important parameter setting absolute upper limit for useable DITs in imaging
mode. Of course sky emissivity fluctuates a lot in case of clouds and is related to Moon illumination.
The bluer a filter, the stronger is the influence of the Moon in the sky background. H band sky emission
is pretty independant of the Moon illumination but however strongly affected by variable atmospheric
OH lines. Under clear weather and comparable Moon illumination, its value can fluctuate by a factor 2
on short time scale (few tens of minutes). The Mount Graham sky emissivity has been measured at