Thermal Lag between JCMT Dish and Legs

Temperature data from the dish (backing structure) is occasionally taken by Fred Baas for use in the surface upgrades program, and analysis of some of these data and contemperaneous leg temperatures provides insight into any possible thermal lag.

For this purpose, a recent large set of dish data, taken from about 8pm April 11 to about 11am April 14, 1997 (during weekend observations of Comet Hale-Bopp) was analysed. One temperature probe was identified at random as yielding a continuous stream of uncorrupted data - this is unit #25 in Fred's scheme of things. It's thermal behaviour is shown in Fig.1 - (ignore the label "#3"). The absolute temperature scale is also, at this stage, unknown, but is of less importance just now than the cyclic behaviour. Samples are collected approximately every 4 minutes - this data stream represents exactly 1001 such measurements.

For comparison we show in Fig.2 the behaviour of the mean leg temperature over the entire 4-day interval starting at 1997 April 11.0   . The profiles for the periods during which these data overlap are remarkably similar and allow a meaningful comparison of any phase lag :

Fig.1	Fig.2

If I can get to it, I ought to superimpose these data in one diagram.

Results
The times of 2 maxima and 2 minima were determined from each dataset :

                    Dish        Legs
                  d  h  m     d  h  m
       Max 1     12 15 48    12 17 30
       Min 1     13 07 30    13 07 30
       Max 2     13 15 02    13 16 00
       Min 2     14 06 49    14 07 45  

Times are accurate to approximately 10 minutes for the dish data, 15 minutes for the leg data.
The average phase lag for maxima = 80 minutes, for minima = 28 minutes.

As mentioned in the previous report, a further upgrade to these pointing algorithms could be necessary. This may require that we

• estimate the future temperature of the antenna based on the current temperatures of the legs, with some knowledge of previous profiles and their evolution with time, or
• use the antenna (dish ?, backing structure ?) temperatures as our real-time working temperature.

Return to POINTING REPORTS

Iain Coulson
29 April 1997