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LIRIS Overheads

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)
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)
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.

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Contact:  (LIRIS Instrument Specialist)
Last modified: 19 January 2014