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When moving from a target to another, several actions take place: slewing
the telescope to the new position while rotating the dome, then selecting
a guide star. On a given target, small offsets usually take place between
exposures.
The following list contains the exact timing of these actions: |
When slewing to a new position, the dome rotation is
the time limiting factor in most of the cases.
A camera readout can take place during a slew and dome rotation as long as the prime focus bonnette X-Y stage (guide camera pick-up mirror) is not activated. Otherwise extra noise is injected on the CFH12K images. For example, this property is used by the twilight flat-field sequencer: the telescope is slewed back to zenith during the readout to maximize the time spent collecting photons in this time critical period. The prime focus bonnette Z stage (focus) can be moved during a readout with no impact on the CFH12K images noise characteristics. |
CFH12K actions also require time: selecting a new filter, preparing the
acquisition system to take an exposure, readout out the CCDs.
An important feature of DetCom (the CFH12K data acquisition system) is its capabilities in reducing the camera overheads by running (safely) multiple actions in parallel. For example, as soon as all the pixels are read into memory a new exposure can be started while the previous image is still being written to disk. The actual pixels readout is 58 seconds long but the pre-exposure sequence adds several seconds to configure properly the acquisition system: grab an unique file odometer number, check if the camera voltages are all right, clean the detectors, etc... The filter selection (CFH12K is equipped with a four position filter wheel) takes about 20 seconds but this task is put in the background and the final position will be checked for successful reach when the exposure will be launched. This is why selecting a new filter should always be the first command issued when changing the observing configuration. Commands can also be piped into DetCom and they will be quickly executed when the prompt is returned at the end of the current action (usually an exposure). This allows saving the time typing and validating the commands (a few seconds!). Focusing CFH12K required a mixing of camera and telescope actions. Its total duration is determined by the number of exposures (from 3 to 9) and the individual exposure times (from 10 to 20 seconds typically). Guided offsets take about 5 seconds. The number given in the following list is valid for a seven 15 seconds exposures sequence. An automatic offset is applied when a new filter is selected. Moving the vertical stage of the bonnette is faster than getting to the new filter position. The following list contains the exact timing of these actions: |
The following list contains several examples of observing scenarios timing:
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For example, using the 6 positions dithering pattern (dp6) for a total exposure time of 3600 seconds (1 hour), when one includes the pointing, focusing, and pattern's overheads, the total telescope time on this field ends up being 1 hour and 15 mn (20% overhead). |