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ATNF ATUC MEMORANDUM

To: ATUC
From: Bob Sault
Date: 31 May 2004
Subject Flexible scheduling for ATCA millimetre observing


Background

The ATCA can make scientifically useful observations in all weather
conditions except severe thunderstorms and high winds. For example, 20cm
observing is largely unaffected by weather. This capability is essentially
unique amongst 3-mm instruments.

Because of the potential impact of poor weather on 3mm observations, the
ATCA has used a "swap" system of flexible scheduling for these
observations. This has been running since winter 2002. This system partners
a 3mm program with a centimetre program, with the two typically scheduled
two days apart. The first program that is observed is determined on the day
by the weather and the weather forecast. An analysis of historical weather
data suggested that this should increase the probability of a successful
3mm observation from about 65% to 85% during the winter. Experience over
the last two years suggests that the weather is somewhat better than the
analysis suggests, although the analysis is broadly correct. Since the
introduction of the scheme, 9 out of a possible 54 slots have had swaps
initiated, two 3mm slots have been ruined by poor weather, and a number
carried out in less than ideal weather. Although swaps would also be useful
for 12mm observers, the pool of available partners has usually meant that
this cannot be supported.

A basic difference of the swap system from backup schemes is that both the
3mm and centimetre programs have been well ranked by the TAC, and both are
schedulable in their own right.

Since 2003 service mode observing has been offered to centimetre swap
partners. This was both to encourage more proposals that could be used as
swaps and to reduce the burden of swapping (the swap system tends to
discriminate against the centimetre observer). To date, no proposals have
taken advantage of this offer.

With the swap scheme, it had been hoped that a number of observing teams
would have effective self-swapping proposals - i.e. the one team had a well-
ranked centimetre and a 3mm observing slot that could be interchanged.
However this has not often happened: the science that observers do tend to
make them either centimetre or millimetre oriented, with few spanning the
gap. At the same time, self-swapping proposals are simple and effective:
they should always be supported where they are possible.

A key requirement of the swap system is that there is an adequate pool of
centimetre observations that can be used as swap partners.

In 2002 the scheme worked reasonably well, whereas in 2003 it was less
effective. With the current winter (the first season with a complete 3mm
system), the swap scheme is showing that the pool of centimetre proposals
(in the array configurations and LST ranges of interest to 3mm observers)
is quite inadequate. The system as it was intended is quite unworkable.

In the current winter, swaps have been achieved by providing a generous
allocation of Directors Time, and by several slots of a given proposal
vying for the one Directors Time swap slot. In the H75, H168 and EW214
arrays, 9% of possible observing time has been specifically left as
Directors Time for possible swaps. This fraction is approximately equal to
the amount of 3mm time that typically would be lost to weather in these
arrays. Hence it is likely that a significant fraction of lost weather time
can be made up for. Note that if all 3mm slots were to be given a swap
slot, the fraction of Directors Time would increase significantly. This
winter, the swap slots (whether a swap is initiated or not) will be
allocated to observers following the pre-existing practise of handling
Directors Time.

Possible Future Approaches
In light of the usage pattern that is emerging, the swap scheme needs to be
re-assessed. Some options are:
. Continue the practise of this winter of allocating swap partners where
possible, and allocating Directors Time swaps otherwise. The 3mm
proposal could use only one slot, with the partnering slot being
allocated using the existing Directors Time rules. Leaving the swap
slots unscheduled well in advance has the advantage that it gives
flexibility to make the most of extended periods of good weather. For
example Directors Time could be used for 3mm proposals if there was no
bad weather (this would not be possible if a centimetre swap were
mandated). The amount of Directors Time allocated could be sufficient
either to allow each 3mm slot to swap or just to accommodate the
typical amount of lost 3mm time.
. Abolish the swap system. In poor weather the time allocation of a 3mm
proposal would be lost, and the time would be used for a backup
proposal.
. An alternative to the above options would be for "make-up" time
(Directors Time) to be scheduled at the end of each array
configuration. This time could then be used for 3mm observing time
that was lost to poor weather, as well as more usual Directors Time
requests. The time required would amount to the typical 3mm observing
lost to weather. This may make better use of the available time and
simpler to administer. Generally it would be less convenient for
observers as they may be required to return to Narrabri for a second
observing run.

Many other alternatives have been considered in the past, and have not been
pursued for sound reasons. Previous memos on these alternatives are
available if ATUC wishes to reconsider them.

Centimetre proposals
Regardless of the way the 3mm proposals are handled in poor weather, there
is a basic fact that an appreciable fraction of time in compact arrays will
not be useable for 3mm observing. At these times the telescope should be
used at centimetre wavelengths. In the current term, there were inadequate
centimetre proposals (in the right array configuration and LST range) to
fill this time. We need to be actively attracting centimetre proposals,
whether they are treated as "fillers", "backup proposals" or "swap
partners" that can be used in compact arrays. Although there has been some
discussion on "key projects" that could be done in this time, no firm
proposals have materialised.

The current practise of allocating Directors Time might need to be made
more rigorous if this time was to be used significantly in any scheme. It
may be appropriate for a delegate of the TAC may be involved in allocating
Directors Time.

Advice sought
ATUC's advice is sought on the following issues/questions:
. Does ATUC have suggestions for the method for flexible scheduling of
3mm observations. In particular, how does ATUC view the three
suggestions made above.
. What approach would ATUC recommend to ensure that there is adequate
centimetre requests (either as backups, swaps, fillers etc) for the
compact arrays favoured by 3mm observers.