CFH12k Queued Observing Programme: Preliminary Description.


1. Introduction.

 The CFH12k CCD mosaic imaging camera will incorporate 12 MIT Lincoln Labs CCDs, each of 2048x4096 pixels arranged in a 6x2 mosaic to produce a total imaging area of 8,192x12,288 pixels (each pixel is 15 microns square). CFHT is currently finding the UH mosaic camera, the UH8k, to be exceedingly popular with proposing observers. This demand is expected to increase with the enhanced sensitivity of the CFH12k and the latest semester's proposals reflect this. Investigators have tended to propose programmes which make strong demands on the capabilities of the instrument. In particular, the better seeing available at CFHT is often quoted as required by the aims of the proposal. Clearly, this is not always what is obtained and highly ranked proposals may suffer as a result of poor weather conditions whereas lower ranked, but scheduled, programmes may be completed in a run of good weather. Overall, we do not see this as applying the scientific priorities set to CFHT by the Telescope Allocation Committees (TAC).

To address this and other related issues, the Science Advisory Committee (SAC) has recommended that CFHT begin a study of the CFH12k mosaic imaging camera as an instrument suitable for the queued mode of observing.

In the queued observing mode, the specific observations to be made at any given time are a function of:

  1. Local sidereal time.
  2. Current weather and observing conditions.
  3. Portion of each programme already completed.
  4. Relative rank of accepted proposals as assigned by the Telescope Allocation Committee.
That is, the telescope and instrument are used so that feasible, more highly ranked proposals are selected "on the fly" according to current conditions and are pursued while the necessary conditions are maintained and until they are completed. When conditions degrade, the observer moves on to a programme which has less stringent requirements. When a programme is completed, another, of lower rank, but requiring the same conditions, takes its place. Thus a highly ranked programme, which requires exceptionally good seeing, may interrupt a lower-ranked programme which requires less exceptional seeing in the event that the seeing improves. Work will proceed on the higher ranked programme until seeing degrades again, the target objects set, or it is completed. Depending on the programme rules adopted, it is also possible that a lower-ranked programme, requiring excellent seeing, may interrupt a higher ranked programme when the seeing improves, particularly if the former is nearer completion than the latter. However, programme ranking will necessarily determine which of the feasible programmes is started next, when a window of opportunity opens.

The queued observing mode is intended to replace the classical mode in which programmes are granted fixed allotments of time and are vulnerable to poor weather conditions, instrument failure and other effects which may cause the partial or complete loss of the run. Queued observing will result in a greater fraction of successfully completed programmes by matching programme requirements with current conditions. Secondarily, the more highly ranked programmes among those which require the same observing conditions will be more likely to be among those completed, thus more closely achieving the aims set by TAC ranking.

Since CFHT's imaging quality is so high, frequently acheiving images at prime focus of 0.5" FWHM, queued observing is also a method of maximising the scientific return from such valuable conditions because any given programme will be executed in seeing which is only as good as required and no better (or worse).

Observing at CFHT will be rendered more efficient by queued observing. Under classical observing it is entirely possible, even likely, that two programmes, scheduled back-to-back, both obtain identical calibration observations, thus wasting telescope time which could be used to scientific advantage. Queued observing will eliminate this wastage. Furthermore, calibrations obtained in queued observing will be better, on average, than in classical observing: the data will always be obtained by an experienced observer, more images from a wider variety of fields can go into a superflat, and greater numbers of different calibration images will be available to each investigator. Finally, and also for these reasons, we will obtain better internal consistency of all data.

A commitment to queued observing has a number of significant consequences:

  • Investigators will not be able to make their own observations, since there is no guarantee that it will be possible to obtain their observations within any given short time window. Moreover, since the observer will participate in the choice of programme to execute at any given time, he or she must be impartial to the aims of the programmes themselves. Thus, suitable service observers will need to be found.
  • Investigators must be prepared to share calibration data, and possibly science data (if two programmes overlap) and CFHT must protect the scientific interests of competing investigators.
  • The proposal procedure will be revised. This document proposes that we learn from others' efforts and split the procedure into two tiers, first a proposal to TAC which concentrates on the scientific justification, then a second document which details the technical requirements of the observing programme.
  • Investigators must be prepared to put significantly greater effort into proposal preparation than is currently the case, defining the sequence of required observations and calibrations and necessary conditions for each, so that the observer has all the information needed to execute the programme. Note that it is not in the investigator's interest to request better conditions than he needs, since this will make it less likely that his observations will be performed.
  • CFHT will establish an infrastructure capable of supporting an observer who is relatively ignorant of the scientific aims of an observing programme, but who is an expert in the use of the CFH12k. This can be achieved with thorough and complete preliminary documentation from the investigator and a suitably designed database engine.
  • An algorithm will be defined which, on an exposure-by-exposure basis, synthesises the rank of a proposal, the current observing conditions, and all other relevent characteristics into a decision with respect to which programme to pursue.
  • CFHT itself will incur a greater cost per observing run, in terms of an increased need for time and energy input by the astronomy department. Since the current resident astronomers are already working at full capacity, the implication is that the community may need to fund further positions. This expense will be compensated by, for example, fewer intercontinental airfares. This document presents an outline of the proposed plan of action and how queued observing will be performed at CFHT. Section 2 discusses the proposal process for the queued observing programme. Section 3 discusses the inclusion of calibration images into the queue. Section 4 discusses the factors which affect the sequence in which the queue is executed. Section 5 discusses the rules by which these factors actually determine the sequence. Section 6 presents decision trees intended to illustrate the algorithm used by observers to decide which exposure to take next. Section 7 defines the form of the Tier 2 proposal and provides an example. Section 8 briefly describes the manpower needed by the programme during execution. Section 9 reviews some policies to be adopted by the programme. Appendix 1 defines some terms used in this document.



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