LWS Overheads

Types of Overheads

Fortunately, compared to the other instruments, the LWS overheads are fairly straight forward. Here is a list:

Acquisition Overhead - 180 seconds. If an AOT is not concatenated to the previous AOT, then the observer will be charged 180 seconds to acquire this position. If the source is concatenated to the previous AOT, then the observer should instead add 20 seconds for the acquisition of the position. To estimate the total (Target Dedicated) time of an AOT, the observer should add this additional time, i.e., 180 seconds, to the output of the time estimator. Even if the position of a new AOT is the same as the position of the previous AOT, the user must add in the 180 seconds of overhead if it is not concatenated to the previous AOT. The reason for this is that the new AOT may not actually follow the previous AOT in the satellite's schedule.

Note that the concatenation of one AOT to the next is a bit of a dicy subject. While it does save acquisition time for the observer, it represents an unwanted constraint to the satellite schedulers at ESTEC. Therefore, the policy of ESTEC, roughly speaking, is that concatenations of any type, (e.g., LWS02 to LWS02, LWS01 to LWS02, SWS02 to LWS02) require scientific justification to be accepted, not just the "it saves time" reason. All the PGA-MDB files have had all their concatenations broken and all concatenations must be reestablished. Some proposal's concatenations were disallowed in the individual technical assessments of the proposals. (For US observers, these assessments are available from IPAC.) Making new concatenations, not originaly part of one's Phase 1 proposal, requires a scientific justifications, explained at IPAC to the support scientists and writing into the "exit survey" regarding your observation.

What are valid reasons? If the science requires you that acquire all of several map positions on a source for a conclusion, then the points may be concatenated. If the science requires that you have the output of several instruments at one position to make your conclusions, e.g. an SWS and LWS line ratio, they may be concatenated. But you should be aware that the more concatenations you require, the harder it is to schedule your observation! For example, you will be less likely to win in a competition to get data from an oversubscribed location if your many positions in this region are concatenated. Likewise, there may be some orbits where just one instrument will be used (this remains a possibility). Concatenated AOTs from different instruments would be disallowed from these orbits.

Mapping Overheads - user calculates. The lws-te does compute SLEW TIME in its time estmates [new revelation, 30 March]. The user does not need add any further overheads. It has been asked if time can be saved on instrument overheads by slewing and taking care of instument set-up at the same time, either when acquiring a source or when making a raster map. The answer is "No." Essentially, nothing happens with any instrument when you're slewing to a target so one can't save time by setting-up an instrument while slewing. Within a raster map the instruments basically do not reset, and therefore require minimal overhead each time they hit a point in the raster. In fact, the instruments are continually "observing," and they simply synchronize the slewing with start of data taking.

LWS Overheads - there are overheads associated with all LWS observations. These, very roughly speaking, are about 6 minutes, in the sense that one cannot do anything with any AOT in less than this time. The observer is encouraged to use the time estimator to explore this issue. (Your IPAC computer account will be made available ahead of your visit with access to all time estimators if you request this from iso@ipac.caltech.edu). The efficiency improves when you do multiple lines with the LWS02 and LWS04 AOTs, and when you do faint sources in general. The efficiency improves dramatically when you use raster maps. Maps, however carry the constraint that all positions are observed with the same instrument procedure, and so the user can not optimize the observation as per the anticipated source flux.

Strategies for Reducing Overheads

As mentioned above, there is significant overhead in making a single observation, so the strategy should be to get everything one can out of it. A broad LWS01 range scan can be somewhat non-optimal in that it per force uses the same integration time at each grating position (wavelength). Since the detector sensitivities increase greatly from the short to the long wavelengths, one can reach quite different sensitivities at the two ends of the scan (e.g., unnecessarily high sensitivities may be reached at the long wavelengths in a full wavelength scan). If one is fishing for faint lines anywhere in an LWS01 range, one might break the range into segments to achieve more uniform sensitivities across the range (trading off against the additional overheads of doing two AOTs). One nice attribute of the time estimator, is that it will tell you the achieved (as well as the requested) signal to noise.

If one has requested an LWS01 range scan, one might reconsider identifying the lines that are of interest inside the range and doing them in the LWS02 line AOT, spending, within a single AOT, the optimal time on each line. This is allowed if no new science (wavelengths and very different sensitivities) are reached, with permission from ESTEC (helpdesk).

If one has an observing strategy where one repeats an observation at an off position, one might consider doing the on and off measurement as a two position map. This is a little risky however because LWS beamshape is not yet well known.

If one has a bright source (>10 Jy) that one is mapping, then using a raster map within a single AOT can save a great amount of time over individual AOT pointings, in all AOTs, since the observations are overhead dominated and using the same instrument parameters for all positions, (those that pertain to the faintest position) will not incur very much additional times, while losing the overhead for each position will save much time.