The plot below shows the system temperatures of RxA3 observations since 08/01/1998 as a function of CSO Tau.

Apart from the tuning, two main factors that determine the shape of scatter diagram are the CSO Tau and the Elevation of the observations. The lower curve shows an 'eye-ball' to what we can call the zenith behaviour. The linear line shows an 'eye-ball' dependency for the 'average' observation. Remarkably, the line shows some evidence of intelligence on part of some observers to stay at higher elevations when the weather gets worse, at least for taus up to about 0.36. Beyond that a 'the-devil-may-care' attitude appears to take over.

Anyway, based on the these lines we can make the following table:

Except at the two extremes the observing times listed are in reasonably good agreement. A number of conclusions:

1. At tau = 0.25 an observer will need two times the integration time at tau = 0.15
   and more than 2.5 times the integration time at tau = 0.1.
2. Somewhere around tau=0.32 the required integration time becomes 3 times longer
   than at 0.15 and 2 times longer than at tau = 0.2.
3. Smart observers who restrict their observations to the higher elevations at tau = 0.4
   will still need 3 to 4 times the integration time at tau = 0.15.

Where to draw the line? Obviously, for prime programs it may well be worth it to be observing in tau = 0.4 weather and to take the factor of 3 or 4 penalty. However, for BACKUP projects 0.32 seems like a good cutoff when the integration time starts exceeding the one you can get in reasonable RxA3 conditions by a factor of ~2.5. However, under these conditions the atmosphere is likely not to be very stable and the instantaneous pointing and focus are probably going to be very marginal, a factor which has to be taken into consideration when selecting suitable projects.

The plot shows the system temperatures of RxA3 observations as a function of CSO Tau. Selection criteria: Tsys of the center DAS band (unless only one) for observations since 08/01/1998 and below 240 GHz and a Trx between 50 and 150 K.