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HAWK-I User Manual
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42
between the individual windows (such gap occur always because of the gaps between the detectors;
extra gaps occur if DET.WIN.NX is smaller than 128 and if DET.WIN.NY is smaller than
2048). The Burst sub-mode (describe here below only for historic reasons and to highlight the
di↵erences with jitter mode) generates a single fits file, containing a single cube; the FastJitt
generates as many files (each containing a cube) as the number of the jitters in the OB.
The cubes contain one extra slice (i.e. NDIT+1 instead of NDIT) because the last slice is the
average of all NDITs.
In Fast Jitter mode, the sliced in the generated cubes are:
1st DIT (a di↵erence between the 2 reads, separated by DIT seconds)
2nd DIT (a di↵erence between the 2 reads, separated by DIT seconds)
...
INT (an averaged frame of all previous slices)
Therefore, if no frames are lost, the generated cube contains:
N AXIS3 = N DIT + 1
(1)
slices.
In Burst mode, the slices in the generated cubes are:
2nd read of 0th DIT (not an actually useful integration)
1st read of 1st DIT (the first useful integration begins),
2nd read of 1st DIT (the first useful integration ends),
1st read of 2nd DIT (the second useful integration begins),
...
2nd read of NDIT-th DIT (the last, NDIT-th useful integration ends),
1st read of NDIT-th + 1 DIT (not an actually useful integration begins)
INT (an averaged frame)
Therefore, if no frames are lost, the generated cube contains:
N AXIS3 = 2 ⇥ (N DIT + 1) + 1slices.
(2)
Frame loss, that plagues some other “fast” instruments, has not been noticed during the typical
applications of the HAWK-I fast photometry modes. Most likely, because of the slowing down of
the detector read speed, that increased the minimum DITs. This problem usually occurs when
the product of NX and NY is relatively large, and DIT is close to MINDIT, so the IRACE has to
transfer large data volume quickly. To check for frame losses, verify that NAXIS3 header keyword
is equal to estimates, given above. A “feature” of unknown nature causes a loss of two frames in
the first cube after changing the Burst from True to False. It is recommended to take a short bust
after such a change, before starting the actual science observations.
The HAWK-I fast mode is subject to a maximum cube limitation, similarly to ISAAC and SofI. The
bu↵er size in this case is 512 Mb. If the cube size exceeds the 512 Mb limit, the observations will
be split into multiple file extensions, but the headers of all extensions will contain the DATA-OBS
information for the start of the observation, not for the start of the given extension. Each extension
will also have its own smaller header. This poses a problem if the aim of the program is to achieve
high timing accuracy because the data transfer time and the fits header merging time are subject
to variations depending on the load on the local network and on the instrument workstation. These