Corrections for system temperature are carried out offline. For the recent SMA data, the corrections for T use smafix :
smafix vis=050911H30a_rx0.usb out=050911H30a_rx0.usb.tsys \ device=/xs xaxis=time yaxis=systemp nxy=2,4 \ options=tsyscorr
Here, 050911H30a_rx0.usb is the Miriad formated data that is directly converted from SMA archival data and 050911H30a_rx0.usb.tsys is the output file which has been applied for T to the fringe amplitude of the visibility.
For the older SMA data, since bad stablity of the total power detectors in T measurements, the values of system temperature were corrupted. a repair for the corrupted T measurements might be required prior to applying the T corrections to the visibility data. smafix provides features for fixing bad T measurements.
For old SMA archival data, here is a recommended procedure for T correction in Miriad :
1. Polynomial Fit to T and Apply T Correction
smafix% inp Task: smafix vis = gc_rx1.lsb % name of the visibility data file; out = gc_rx1.lsb.tsys % name of output file; device = /xs % pgplot device,X-windows here; xaxis = time % time in xaxis; yaxis = systemp % system temperature in yaxis; nxy = 2,4 % 2 plots in xaxis and 4 in yaxis; yrange = 100,1500 % plot range from 100 to 1500 in yaxis; dofit = 2 % polynomial fit; options = tsyscorr,dosour,tsysswap % apply tsys corrections, do source based fitting, replace the raw Tsys with fitted values.
A few fitting options have been provided to derive the T corrections. Fig.2.1 shows a plot of antenna-based system temperature. Each panel stands for one antenna. A second order source-based polynomial fit to T (solid lines) are carried out in order to get rid of the bad T measurements. If options=tsyscorr, then the polynomial curves are replaced in the correctionis for system temperature. Careful users might like to check up the polynomial curves. The fitted T is stored as a variable systmp in the uv data in the defualt options. If options=tsysswap, the fitted T will swap with the orignal T, i.e. the fitted T is stored as a variable systemp and the original T goes to variable systmp.
2. Check and Plot Fitted T
One can plot systmp or systemp using varplt or smavarplt (the sources are coded in different color; see Fig. 2.2):
smavarplt% inp Task: smavarplt vis = gc_rx1.lsb.tsys device = /xs xaxis = time yaxis = systemp % the variable for fitted Tsys nxy = 2,4
3. Replacement and Flagging of Corrupted T
Occationally, an antenna might be corrupted in T measurements. Users can choose a good antenna in T measurements from the rest of the antennas to replace the bad antenna. The keyword bant in smafix can specify the id of the corrupted antenna and gant assigns a good antenna to replace the bad one. Here is a usage:
Task: smafix vis = gc_rx1.lsb out = gc_rx1.lsb.tsys device = /xs xaxis = antel yaxis = systemp nxy = 2,4 bant = 4 % ant 4 is bad. gant = 1 % Tsys values of the good ant 1 will replace those of ant 4. dofit = 2 options = tsyscorr,dosour,tsysswap
Alternatively, the systemp values of ``bad antennas'' can be replaced with those of a good antenna without performing polynomial fitting:
Task: smafix vis = visdata out = visdata.tsys device = /xs xaxis = time yaxis = systemp nxy = 2,4 bant = 1,8 % ants 1, 8 are bad. gant = 3 % Tsys values of the good ant 3 will replace those of ants 1, 8.
If only a few data points are corrupted, one may use smacheck to flag the data outside a specified range for the T or other variables. By default, smafix will only take the unflagged (good) data in fitting, plotting and correction. There is an options of all to allow to use all (flagged and unflagged) data in the T correction.