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Overheads

All Observers


The overhead for slew and set-up is usually 20 minutes. Acquisition of the target consists of images, offsets and instrument configuration changes, all of which are done manually by the observer on the dedicated Phoenix computer.


The following overheads must also be included in the time requested for each scientific observation.


  • Observing efficiency is 80% in JHK, and 75% in LM when nodding on source (object always in the slit), and 70% in all bands when nodding to sky.
  • Phoenix' flexure coupled with its narrow slits means that targets need to be re-centered in the slit approximately once per 90 minutes. It is generally advisable to use Phoenix with the slit oriented east-west in order that most of the drift caused by flexure is along the slit. Recentering typically takes 10 minutes. This overhead should be included for observations that require more than an hour of total exposure time.


Classical Observers


There are additional overheads that typically do not count against the program time for queue observers but MUST be included in a classical request for time. These include:


 


  • Flat field exposures must be obtained every time the grating is moved. To obtain an average level of ~ 2500ADU, the exposure times are ~1 sec in H and K, and ~10 sec in M. The instrument overhead for a series of 10 exposure is ~5 minutes (26 seconds per image). Therefore it takes about 15min to obtain a combined flat of S/N ~ 500, and the corresponding darks.

 


If the proposal requires several different grating positions, overheads can add up very quickly. Let's assume a proposal with 3 grating positions per object.


  • Example 1: exposure time per grating setting is short enough that all settings can be observed within an acceptable range in airmass
    • Acquisition overhead for target: 20min
    • First grating setting exposure = actual on-source time/efficiency
    • Flats for setting #1: 15min
    • Second grating setting exposure = actual on-source time/efficiency
    • Flats for setting #2: 15min
    • Third grating setting exposure = actual on-source time/efficiency
    • Flats for setting #3: 15min (charged to baseline)
    • Acquisition overhead for telluric standard star from bright star list: 20min
    • ... same as above, adding to flats overhead for the standard: 45min
  • Total: 2.2hr + target exposure times + standard exposure times
  • Example 2: exposure time per grating setting is long or background variations can be significant (thermal IR observations)
    • Acquisition overhead for target: 20min
    • First grating setting exposure = actual on-source time/efficiency
    • Acquisition overhead for standard: 20 min
    • Standard star observation
    • Flats for setting #1: 15 min
    • ... and repeat for settings #2 and #3:
  • Total: 2.8hr + target exposure time + telluric