NIKA: Alignment and pointing model

It is recommended that the pointing session is performed at night time, at least one day after the antenna is running normal observations with good weather condition, no high wind and the antenna deicing off. We aim with this step to get the antenna and tower well stabilised in temperature

Based on the tutorial "How to run a pointing session and a pointing model parameters fit" by Juan Peñalver et al. (2008).

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Alignment

• Optical Alignment of NIKA with laser
• Radio Alignment of NIKA with absorbers

The pointing session

• Enter into any project of the bolometer pool to run the pointing session (check with the pool manager) and start pako.
• Start the program xephem under the bolometer pool account to identify the more convenient sources for pointing.
• Set Nasmyth offsets to 0/0 in pako
• Set Nasmyth offsets to 0/0 in the NIKA pointing model
• Before starting the pointing session move the antenna more than 60 degrees in azimuth to update the inclinometer information. To see that new inclinometer values are obtained check in any VME machine the last entrance of the file /home/penalver/INCLIN/p4p5yymm.prn where yy indicates the year and mm the month.
• Do a first POINTING and FOCUS scans to confirm the good bolometer operation and the good starting corrections for the pointing and focus
• Then carry out among 20 and 30 POINTING scans well spread in the sky to cover all the azimuth range and the elevation range between 20o and 75o. After each POINTING scan update the pointing corrections. Try to do all the POINTING scans in the shorter time to minimise the possible drift of the pointing parameters.
• After, more or less, 10 POINTING scans do a FOCUS scan to update the focus correction
• After each valid pointing scan write down the antenna azimuth and elevation together with the correction in both coordinates. The information will be used later to fit th optimum pointing parameters.

The pointing model

• It is strongly recommended that the pointing model parameters are changed by who knows the physical meaning of the changes. If necessary get the advice from A. Sievers or J. Peñalver. And finally keep informed of the modifications done to J. Peñalver.
• In the computer mrt-lx3, under the account pointing change to the directory fit (path mrt- lx1/vis/pointing/fit)
• The pointing corrections from the pointing session must be entered in the file FITIN.DAT. This file keep the old structure used in the VAX computers, but the necessary information is entered only in the columns 3 to 6, one line per valid pointing scan, with the antenna azimuth (degrees), elevation (degrees), correction in azimuth (arcsec) and correction in elevation (arcsec) respectively. A model of the file FITIN.DAT with only nine lines of nine pointing scans could be seen in the file FITIN_model.DAT
• In the file FITDES.DAT select the pointing model parameters to fit. The valid parameters to fit are P1 to P3 and P7 to P9, identified in the file as VECT 1 to VECT 3 and VECT 7 to VECT 9. The way to enable the fit of any parameter is removing the exclamation character ! at the starting of the line. The file FITDES_model.DAT shows how to prepare the file to fit only the pointing model parameters P2 (VECT 2) and P7 (VECT 7)
• To fit the selected pointing model parameters, first delete the file FITOUT.DAT (to create a new one later automatically), then run the program ./fit.exe The result is shown in the terminal with the Value to correct and the Error of that parameter fitted. The information is also written in the file FITOUT.DAT
• Usually several combinations of pointing model parameters have to be selected and tested in FITDES.DAT until the optimum combination is chosen. Good corrections are for those parameters where Value is higher than three times Error. But once again, any pointing parameter represents some physical imperfection of the antenna, under that criteria consider if the correction to apply have sense or not
• You could see graphically with greg the result of the fit. Go to the directory fit and follow the steps:

   $ gag_mar08
   $ greg
   GREG> @hu_ini
   !answer the questions
   GREG> COVERAGE
   !to display the pointing scans coverage in the sky
   GREG> SCATTER
   !to display the scatter previous and after the fit

• you can implement the new pointing model parameters modifying the file pointing-bolometer.30m. To do that enter in mrt-lx1 as user mrt (ask the password to the operator) and follow the steps indicated in the wiki page https://mrt-lx1.iram.es/mainWiki/OperationNotesOperatorsHowToChangePointingModel.

• Finally, check the pointing performance on several sources over the sky.