COS maintains two on-board data buffers, each with a capacity of 9 MBytes (2.35 × 10
6 counts). The
BUFFER-TIME is the time that it takes to fill one of these buffers. COS uses the
BUFFER-TIME to establish the pattern and timing of memory dumps during a
TIME-TAG exposure. For the first
BUFFER-TIME of an exposure counts are recorded in the first COS data buffer. At the end of this time data recording switches to the second data buffer and the first buffer is read out while the second is being filled.
For all external TIME-TAG observations a value of the
BUFFER-TIME must be specified in the Phase II proposal. The
BUFFER-TIME is 2.35
× 10
6 counts divided by the anticipated count rate in photons per second. The
BUFFER-TIME calculation should include counts from the detector dark current and stim pulses (for FUV) as well as the detected photon events, and factor in the instrument quantum efficiency and dead time. It is strongly recommended that the
COS ETC be used to compute an accurate value of the
BUFFER-TIME.
In addition, to prevent the loss of data, should your target be brighter than specified in the ETC calculation, give yourself a margin of error of about 50%; i.e., multiply the ETC BUFFER-TIME by 2/3.
If the BUFFER-TIME is overestimated the buffer may fill before input switches to the other buffer. Subsequently-arriving photons will be lost, leaving a gap in the data. The pipeline will correct the exposure times for any such gaps, so flux calibrations will be correct, but the overall S/N will be lower than expected. If
BUFFER-TIME is underestimated input will switch to the second buffer before the first buffer is full. No data will be lost, but the resulting drain on spacecraft resources could preclude other activities, including parallel observations. There are, however, some instances when it is advantageous to use a slightly smaller
BUFFER-TIME to minimize the overhead when the buffer is being read (see the discussion below).
Figure 5.11 shows a flow chart that illustrates the process of selecting a
BUFFER-TIME value for all possible combinations of exposure time and
BUFFER-TIME. Each of the options is discussed in detail in
Sections 5.4.1 through
5.4.5.
For exposures where 2/3 of the BUFFER-TIME returned by the
ETC is larger than the Exposure Time and larger than 110 s the
BUFFER-TIME should be set to 2/3 of the value returned by the
ETC. In this case the time allocated to read the last buffer dump is proportional to the number of events on the buffer, and some overhead can be saved if only a portion of the buffer needs to be read out. By specifying 2/3 of the
BUFFER-TIME returned by the
ETC a margin of error of 50% in the observed count rate is used. However, if the observed count rate is actually higher than the 50% margin of error then those events will not be read out of the buffer and will be lost. If there is a concern that this may happen the
BUFFER-TIME should be set to the exposure time. This will ensure that the entire buffer is read out at the end of the exposure.
In this case 2/3 of the BUFFER-TIME returned by the
ETC should be used. As a special case, to minimize the overhead associated with reading the buffer, the
BUFFER-TIME can be specified such that there are only between 100 and 110 s left of exposure for the last buffer dump. This new
BUFFER-TIME can be calculated using
BUFFER-TIMEnew = (Exposure Time
− 110) /
n, where
n is the value of (Exposure Time
− 110) / (2/3
× ETC Buffer Time) rounded to the next higher integer. For example, suppose that the Exposure Time is 2300 s and that 2/3 of the
BUFFER-TIME returned by the ETC is 2/3
× 1050 = 700 s. Then (2300
− 110) / 700 = 3.13, which is rounded up to
n = 4 and
BUFFER-TIMEnew = (2300
− 110) / 4 = 547.5 s, which we round up to 548 s. This means that the buffer will be read out every 548 s, and after four buffer reads there will be 2300
− 4
×548 = 108 s left in the exposure. This last buffer read has a lower overhead and allows the next exposure to start sooner.
It takes 110 seconds to empty a COS data buffer. A BUFFER-TIME of 110 seconds corresponds to a count rate of 21,000 counts per second. If the count rate exceeds this value, then the second data buffer will be filled before the first buffer has been completely read out. In this situation, you have two options. You can either shorten your exposure, or you can accept gaps in the recorded data stream. In either case
calcos will compute the actual exposure time and will calculate fluxes correctly, but the total number of collected counts, and hence the S/N, will be limited by the 21,000 counts per second rate.
As an example, suppose that 2/3 × (
BUFFER-TIME returned by the
ETC) is 100 seconds, and you want an exposure time of 360 seconds.
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With Option A, you would specify BUFFER-TIME=100. Because it takes longer than that to read out the buffer, the APT limits you to an exposure time of 2 × 100 = 200 sec. In this case COS records all the events that arrived during the exposure.
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With Option B, you would specify BUFFER-TIME=111. Since the COS buffer may be full after the first 100 seconds the last 11 seconds of data may not be recorded, and are lost each time the buffer fills. With this option you will get a series of data blocks as follows: 100, 11, 100, 11, 100, 11, 27, where the bold numbers represent periods when the data are recorded, and the italic numbers represents periods when the data are lost. The COS shutter remains open for the full 360 seconds, and the data are properly flux calibrated by the pipeline.
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The minimum allowed value of the BUFFER-TIME is 80 seconds. This value corresponds to a count rate of 30,000 counts per second over the entire detector, the maximum rate at which the flight electronics are capable of processing counts. If 2/3 of the
ETC BUFFER-TIME is less than 80 seconds, then the source is very bright and should be observed in
ACCUM mode. If your exposure is less than 80 seconds in length set
BUFFER-TIME=80. The buffer will be read out immediately after the exposure ends, and there will be no idle time.
If you use the AUTO-ADJUST feature in the
APT to set your exposure times do it first, then adjust the
BUFFER-TIME of each exposure according to the rules above.
