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Observing Overheads

Readout times

Measured readout time (s)
Window size Fast readout mode Slow readout mode
256x256 0.176 0.242
128x128 0.083 0.097
all frame 0.505 0.755

NOTE: The fast readout speed has not been commisioned yet.

Single-cycle time

The single-cycle time is the time taken to complete a single observation of the minimum exposure time without windowing (including all the overheads: readout, collecting headers, archiving...). This time varies between 3.7-4.4s (i.e. 'run ingrid 0.8' or 'run ingrid 0' takes 3.7-4.4s to give you the prompt back).

Filter changes

The table below shows the time (in seconds) needed to change the filters in INGRID using the command filter ingrid <filter name>. Note that the filter movements in some cases imply movements of more than one filter wheel, since each filter movements is tied to other filter movements and/or pupil stop (click here to see the filter/pupil stop combinations).

Final position
K Ks H J blanks
K -- 20 38 55 100
Ks not measured -- 47 64 not measured
H 37 46 -- 38 82
J 58 64 38 -- 66
blank 99 108 82 62 --

NAOMI observing overheads

Overheads should be included in the request for telescope time. There are a number of overheads specifically related to (i) adaptive optics systems and (ii) infrared observing, which are discussed below. Don't forget to also include other standard overheads, such as filter and pupil stop changes.
  • Acquisition: 5-10 minutes per target (~20 minutes with OSCA)
  • AO settle time: Negligible
  • Dithering: ~7s to change dither point
  • Rotational dithering (with OSCA): ~5-10 minutes per position angle
  • INGRID readout time: ~1.5s
  • INGRID filter/pupil-top changes: ~60s
  • PSF calibrations: These overheads should be included if required
  • Offset sky exposures: These overheads should be included if required
Acquisition overheads involve (i) slewing the telescope, (ii) acquisition of the science target on the detector, (iii) acquisition of the guide star (if required) on the wavefront sensor, and (iv) acquisition of the image behind the OSCA mask (if required).

AO settle time is the time taken to settle into the AO correction after closing the feedback loops on the guide star.

Dithering is required for infrared imaging. It entails (i) opening the AO loop, (ii) moving the telescope and the guide star pickoff probe simultaneously, and (iii) closing the AO loop. Note that standard 5 and 9 point dithers are available.

Rotational Dithering is encouraged for infrared imaging with the coronograph mask in OSCA deployed. It involves changing the PA of the derotator closed-loop, and accurately recentering the target behind the mask.

INGRID's readout time becomes significant when taking short exposures to avoid saturation from the sky background. For example, if you are performing a coaverage of 40 individual 1s exposures, the total readout time will be 60s (ie. a 150% overhead).

PSF calibrations, if needed, can introduce significant overheads. See above for more discussion.

Offset sky exposures are required for infrared observations if observing extended targets. One must observe for a similar amount of time "off" the science target (looking at blank sky) as on it, in order to do proper flat fielding and sky subtraction. This thus doubles the total exposure time. Such exposures are not required for point sources because a dithering pattern is sufficient.

To calculate these overheads it is essential for you to plan how you will perform the observations. Your proposal should specify how the observations will be broken down ie. (i) the integration time per exposure, (ii) the number of exposures coaveraged to give one image, (iii) the number of images at each dither point, and (iv) the size of the dither etc.

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Contact:  (INGRID Instrument Specialist)
Last modified: 27 July 2014