Bright Object Mode

The solution adopted to this problem for NICMOS is the provision of a bright object mode which enables targets to be observed which are ~600 times brighter than is possible in the other modes without saturating. In BRIGHTOBJ mode, an ACCUM sequence of operations is performed on one pixel in each quadrant at a time. That is, the pixel is reset, read, integrated, and read again with the difference between the final and initial readouts being stored as the measured signal and the interval between the reads being the exposure time. This process is repeated sequentially for all pixels in each quadrant. Users can think of this as integrating on a single pixel at a time. The smallest integration time which can be used is 1.024 milliseconds. Figure 15.1 illustrates the operation of bright object mode. Initially the detector is reset and the first pixel (solid shading) in each quadrant is read. A reset is then made and the second pixel in each quadrant is read. The process continues until all 16,384 pixels in each quadrant have been read.

Figure 15.1: Bright Object Mode Operation

The time required to take a BRIGHTOBJ mode exposure can be rather long. Since photons are only collected in one pixel per quadrant at an time, the time associated with obtaining the frame is where EXPTIME is the integration time per pixel (i.e. the observation time is approximately (128²) x the exposure time). For example, if an integration time of 0.1 seconds is used to observe a bright target then the actual time required to complete the observation would be around 27 minutes! This means that allowing for acquisition time only two such exposures could be obtained in a single target visibility period. However, it is not always so serious. In the case of Jupiter for example the integration times required per pixel are only of the order of milliseconds and so the total integration time will only be around 20 seconds.

The longest exposure time which is possible in BRIGHTOBJ mode is 0.261 seconds, requiring 4278 seconds in total. Thus it is possible, in the worst case, for a single BRIGHTOBJ mode exposure to use more than an orbit. In general observers are strongly advised to consider the trade-off between relatively long BRIGHTOBJ mode exposures (which take the longest time) and short ACCUM mode exposures (perhaps using a filter and camera combination with lower throughput).

One of the obvious uses of BRIGHTOBJ mode is for solar system targets. Due to the limitations of the Track 51 capability (linear tracking with orbital or planetary parallax correction) HST can only follow a moving target for 2048 seconds, of which 1980 seconds is available for an exposure. This therefore sets the longest integration time that is possible for a moving target in BRIGHTOBJ mode. Proposers will need to judge the real integration time and signal to noise ratio required for the observation time and adjust accordingly.

The advantage of this mode of operation is the ability to observe objects significantly brighter than the normal saturation limit of the detector.

The disadvantages are several:

• Due to the extremely large time penalties involved in this mode operation, it cannot be used to accomplish time resolved observations on shorter time intervals than ACCUM mode.
• Some observations will take a long time. BRIGHTOBJ mode exposures are therefore very sensitive to the quality of the pointing of HST. They should not be obtained using GYRO guiding mode. In addition, if the object changes (planetary rotation) or if the telescope pointing changes it will affect different parts of the image differently.
• The D.C. offset of the detector output is not removed in this mode of operation. In general, the signal is very high and the offset does not matter. In some cases it will and this can be a detriment to the signal accuracy.
• There is also no cosmic ray correction or saturation detection in this mode of operation. Although they are still susceptible to cosmic rays, events should be very rare as the integration time per pixel is very short.