COS maintains two on-board data buffers, each with a capacity of 9 MBytes (2.35 ×
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 ×
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).
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
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.
The FUV dark current fluctuates and ocassionally the combined global dark rate for both segments has exceeded 90 count/s. To ensure that all detected events are dumped from the COS buffer, even for a very faint target, the use of BUFFER-TIME
> 20,000 s should normally be avoided.
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 −
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 s to empty a COS data buffer. A BUFFER-TIME
of 110 s corresponds to a count rate of 21,000 count/s. 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 count/s per rate.
As an example, suppose that 2/3 ×
returned by the ETC
) is 100 s, and you want an exposure time of 360 s.
|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 s. In this case COS records all the events that arrived during the exposure.
|With Option B, you would specify BUFFER-TIME=111
s. Since the COS buffer may be full after the first 100 s the last 11 s 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
, 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 s, and the data are properly flux calibrated by the pipeline.
The minimum allowed value of the BUFFER-TIME
is 80 s. This value corresponds to a count rate of 30,000 count/s 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 s, then the source is very bright and should be observed in ACCUM
mode. If your exposure is less than 80 s in length set BUFFER-TIME=80
s. 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.