Burst and FastJitter Modes

Since Period 79, two fast imaging modes are offered: the Burst mode and the FastJitter mode. They are intended for fast relative photometry, from a few ms to a few tenths of ms. Originally implemented (and tested) for lunar occultations, they are suitable for any fast variable phenomena where time resolution is a must.

Principal characteristics:

Imaging is possible in three SW filters: J+Block, H and Ks, using the Aladdin array (located in the ISAAC's LW arm).
Hardware windowing of the array allows very short DITs.
The Burst and FastJitter modes are offered both in VM and in SM.

The two modes differ only in the way an image is processed and transferred from IRACE (InfraRed Array Control Electronics) to the instrument workstation. This difference affects the technical capabilities of each mode. Specifically:

  • The shortest DITs possible are 3.2 ms (Burst mode) and 12ms (FastJitter) with the smallest window size of 32x32 pixels (corresponding to a 4.7"x4.7" FOV).
  • The readout mode is Double Correlated Read (Reset-Read-Read): in this mode, each image, or DIT unit, is made of two reads. The data are stored in single fits files, each containing a data cube:
    • The Burst mode produces a data cube comprised of single reads, which have to be combined two by two to reconstruct an image (DIT). A script which allows image reconstruction is available. For the smallest window, the size of the data cube is therefore 32x32x2xNDIT.
    • The FastJitter mode usually produces a data cube of DITs (i.e. already reconstructed images). For the smallest window the size of the data cube is therefore 32x32xNDIT.
  • In Burst mode the telescope is staring at the target for the duration of the integration (INT=NDITxDIT) and only one data cube is produced. In FastJitter mode, the telescope can jitter in the sky and several data cubes can be produced within one template.
  • In Burst mode it is possible to set the absolute time on which the observation has to be centered. For example, if one wants to observe an event at time T and sets a total integration of 60 seconds, the template will start to collect data at time=T-30 and end at T+30.

Both modes are subject to some limitations:

  • The data cube can contain a maximum of 32000 planes (frames), i.e. the maximum NDIT is 16000 in Burst mode and 32000 in Fast Jitter mode.
  • The maximum data cube size is ~262Mb. Once the window size has been selected, this limits the number of frames/reads and vice versa.

A summary table with the hardware windowing characteristic is given below:

  Burst mode FastJitter mode
Window Size [pxs] Field of View [''] min DIT [ms] max NDIT min DIT [ms] max NDIT
32x32 4.7x4.7 3.2 16000 12 32000
64x64 9.5x9.5 6.4 16000 * 12 16000
128x128 19x19 14 4000 ** 14 4000
256x256 38x38 37 1000 ** 37 1000

* It is observed that the burst observations with such a detector set up are prone to IRACE errors/failures. Therefore, a smaller NDIT (10000-12000) is recommended.
** The burst observations in these cases, too, are prone to IRACE failures. Therefore, if the maximum data cube size is required, the FastJitter mode should be used.
Note that for DIT longer equal or larger than 12 ms there is no real point in using the Burst mode, the only difference being the EVENT time entry in the Burst template.

The detector characteristics are the same as for SW imaging with the LW array (see the User's Manual). The maximum and minimum observable magnitudes can roughly be estimated through the ETC by scaling the obtained counts to the actual DIT.

The table below reports some technical information for the Burst mode.

Window Size [pxs] DIT [ms] NDIT N. Frames Execution time [s]* Data cube Size [Mb]
32x32 3.2 1000 2000 20 4.1
32x32 3.2 2000 4000 37 8.2
32x32 3.2 3000 6000 42 12.3
32x32 3.2 4000 8000 81 16.4
32x32 3.2 5000 10000 112 20.5
32x32 3.2 6000 12000 109 24.6
32x32 3.2 7000 14000 139 28.7
32x32 3.2 8000 16000 174 32.8
32x32 3.2 9000 18000 216 36.9
32x32 3.2 10000 20000 377 41.0
32x32 3.2 13000 26000 431 53.3
32x32 3.2 14000 28000 495-501 57.4
32x32 3.2 15000 30000 562 61.5
32x32 3.2 16000 32000 636-642 65.6
32x32 4.0 7000 14000 143 28.7
32x32 4.5 6000 12000 114 24.6
32x32 5.0 1000 2000 20 4.1
32x32 5.0 2000 4000 29 8.2
32x32 5.0 3000 6000 42 12.3
32x32 5.0 4000 8000 61 16.4
32x32 5.0 5000 10000 84 20.5
32x32 5.0 6000 12000 112 24.6
32x32 5.0 8000 16000 183 32.8
32x32 5.0 10000 20000 374 41.0
32x32 6.0 7000 14000 143 28.7
64x64 6.4 1000 2000 12 16.4
64x64 6.4 2000 4000 31 32.8
64x64 6.4 3000 6000 47 49.2
64x64 6.4 4000 8000 67 65.6
64x64 6.4 5000 10000 92 82.0
64x64 6.4 6000 12000 122 98.4
64x64 6.4 10000 20000 270-300 163.9

*These execution times are just indicative and vary depending on the net connection.

The table below reports some technical information for the FastJitter mode.

Window Size [pxs] DIT [ms] NDIT N. Frames Execution time [s]* Data cube Size [Mb]
32x32 12 1000 1000 18 4.1
32x32 12 2000 2000 30 8.2
32x32 12 3000 3000 41 12.3
32x32 12 4000 4000 54 16.4
32x32 12 5000 5000 66 20.5
32x32 12 6000 6000 77 24.6
32x32 12 7000 7000 89 28.7
32x32 12 8000 8000 102 32.8
32x32 12 9000 9000 113 36.9
32x32 12 10000 10000 126 41.0
32x32 12 11000 11000 137 45.0
32x32 12 13000 13000 162 53.3
32x32 12 14000 14000 174 57.4
32x32 12 15000 15000 187 61.5
32x32 12 16000 16000 198 65.6
32x32 12 20000 20000 246 82.0
32x32 12 32000 32000 388 131.1
64x64 12 16000 16000 199 262.2
128x128 14 4000 4000 114 262.2
256x256 37 1000 1000 37 262.2

*These execution times are just indicative and vary depending on the net connection.

The figure belows shows how the execution time depends on the NDIT value for both the Burst and the FastJitter mode.



Filled squares are for Burst mode executions (black is for window=32x32 and DIT=3.2ms;
blue is for window=32x32 and DIT=5ms; red is for window=64x64 and DIT=6.4ms).
Empty squares are for the FastJitter mode executions (window=32x32 and DIT=12 ms).