GIX instructions for observations

Updated 060530 /tho

Updated 081219 /maturun

For making the observations you need (besides this instruction):

• User ID for Metsähovi's Linux computers (you need that for Xephem and weather information, not for the actual observations). You can get that from Ari or Timo. (Xephem can be run at Merjas chaos computer and weather information is available on www, you can run them on antimatter computer)
• Sourcelist (nowadays available on web) and observations logbook (usually from Merja or Anne).
• Instructions for using Contobs (the program that does the actual observations).
• Instructions for using Xephem (the interactive source map program) and/or check automap.pl

How to make observations:

• Choose a suitable source from the sourcelist on web, by checking the priorities and using Xephem to see where the source is at the moment. Calibration sources must be observed frequently, the most important one is DR21 , and the secondary calibrators are 3C84 and 3C274. When you start observations after a break, try to observe a calibration source as soon as possible, when the weather conditions are still the same.
  We try to observe sources when their elevation is between 40 and 60 degrees. Some of the sources we observe are so southern that they never rise that high, and therefore must be observed when they culminate i.e. elevation is highest. With the most southern sources that is only about 20 degrees, so you must pay attention to the timing when you observe them. Do not observe in any other directions below 40 degrees (the bushes will affect your results).
• Then choose offsets depending on the direction you observe.
• Choose 1p/5p observation.
• Observe the source.
• At the beginning of the observation, check if the results are reasonable: Is the error small enough, were the offsets reasonable... if it's 5p observation, check the pointing after two rounds (400s at 37GHz, 600s at 22GHz). If the pointing is not good (especially if the middle point is not the highest) stop the observation and change the offsets.
• If conditions are good observe the source until about 1200s (or even 1800s with fainter sources).
• Write down the results. Remember also the weather information, especially if the conditions change (humidity, temperatures and clouds). These are now automatically read from data computer when using read_contobs_data.pl, read_multi_gix.pl or gixwrite.pl to read the results. It is better still to write every few hours the weather information on the logsheet so the data reduction is easier.
• Change the source: choose a source from the same direction and use same offsets, or change direction and check the offsets.

Every few hours and always with a calibration source (before or after) you must do the calibration of the noise diode ("CAL"). Check the error and if it is large (several mV), there is something wrong with the conditions, so check the weather or make a new CAL-observation.

Calibration sources:

The primary calibration source DR21 must be observed many times during an observation shift, and always when the conditions change (if it can be observed, if not, use secondary calibrators instead). You can observe it between 14.30-18.30 and 22.30-01.30 LST, both times 1-2 times. At other times you choose the suitable one from 3C84 and 3C274.

You must get a good result from a calibration observation, so if the pointing is not good, the fluxlevel is low or error is large, all observations with that calibration must be thrown away. So, if you don't get good results from a calibration observation, do not continue observations until you do get a good result! (unless there is no calibration source available and you could observe one after an hour or so, you can continue observations and hope that conditions stay the same). If you have stopped the observations because of bad weather, and when the conditions get better, are thinking of starting them again, it is good to start by checking the fluxlevels of a calibration source.

"A good fluxlevel" depends on the sources elevation and other conditions. You can check earlier results from the log book and then see what is a normal fluxlevel. If the weather is good and the fluxlevel of a calibration source is low, check the offsets and humidity / heater (see next paragraph).

Using the heater:

The heater is used in winter to melt snow from the radome and dry radome after rain. In summer to dry radome after rain and remove dew from the radome.

In summer: dew is falling on the radome during night time, and that lowers the fluxlevels. Even if the weather seems perfect, there will almost always be some dew, and you must try to prevent that. So in the evening when the humidity starts to rise to about 80% you start to use the heater (position 1 from the actual heater and then position "talvi" from the main switch). Check instructions for using heater and autoheat from the instruction list.
You keep it on about 15-20 min and then off about 40-45 min (depending on the humidity). Check the fluxlevels to see if they are changing. If they are rising a lot it might be wise to keep the heater on a bit longer until the flux levels have stabilised. And when the heater is used it is extremely important to write down the temperatures (expecially Tant) because they can change rapidly. After rain the heater is kept on until the radome is (hopefully) dry and then after the temperature has normalized start the observations. It is wise to observe calibration source when you are drying the radome, so you know when the flux levels are normal again.

In Winter: Drying the radome after rain similarily as above, but also melting the snow. Usually you melt the snow with "full power", so the settings are position 2 from the heater and position "Talvi" / "Winter" from the main switch. When the weather is cold it can take time to melt the snow, and you must check the radome if the snow has really melted and is there water dripping from it. After melting the snow it can take hours to the temperature to be normalized again and you can start observing. And when it's really cold (-10C or more) it is almost impossible to melt the snow.

Last modified: Fri Dec 19 16:14:12 EET 2008