source: trunk/tutorials/tutorial_5.rst@ 2283

===========================
Tutorial #5 - ASAP advanced
===========================

.. sectionauthor:: Maxim Voronkov

**Main goal:** To understand how to handle the data manually by building a quotient
without using mx quotient

Files
-----

* 2009-04-02 2240 MMB-MX-11.9-0.13333.rpf data file 

* Script 1: tutorial.py general reduction script

* Script 2: systemquotient.py script doing built-in quotient (to be copied and modified)

* Script 3: userquotient.py the result (you’re not supposed to look into itunless desperate)


Instructions
------------

1. Load the data from RPFITS file

2. Play with various methods to get access to the data

3. Run the reduction script using the built-in quotient

4. Copy the template script doing the quotient

5. Modify makeQuotient method to replicate the functionality of the built-in mx quotient

**Help:** Use help method in asap to get information about parameters, etc

.. code-block:: python

   %run tutorial.py

Copy *systemquotient.py* to a new name, change the import statement at the
top of *tutorial.py* to load your file instead of the *systemquotient.py*.

.. code-block:: python

   from myquotient import *

Now your version of `makeQuotient` will be called from *tutorial.py*.
Use the following to load the data into a scan table called sc

.. code-block:: python
   
   sc=scantable("2009-04-02_2240_MMB-MX-11.9-0.13333.rpf")

Inspect the content.

.. code-block:: python

   sc.summary()

The file contains 7 scans. Each scan corresponds to observations of a source
with a different beam of a 7-beam receiver. We want to use the median
spectrum from all other scans as a reference when constructing the quotient
using the formula

.. code-block:: none

   Result = Toff * (On/Off - 1),

where Toff is the system temperature measured during the reference scan, On
is the signal spectrum, Off is the reference spectrum. We want to construct
Result for each individual beam (out of 7 beams available) and average all
these spectra together.

To do the selection

.. code-block:: python

   sel=sc.get_selection()
   sel.set_beams(0)
   sel.set_scans(0)
   sel.set_polarisations(0)
   sc.set_selection(sel)
   selected_sc = sc.copy()
   selected_sc.summary()

To average or compute the median use one of

.. code-block:: python

   ref.average_time(weight="median")
   scans.average_time()

To merge the scans together (i.e. individual quotients for each beam) build
a python list first and then use merge and average time

.. code-block:: python

   res=[]
   for b in range(7):
       res.append(myFunctionReturningAScanTable(beam))
   averaged_scantable = average_time(merge(res))

Note that merge will fail if the list has only 1 element. An if-statement may
be necessary
To scale the scantable with the constant factor use::

   scan.scale(factor,tsys=False, insitu=True)

To add a constant use::

   scan.add(constant_to_add, insitu=True)

To get the tsys use::

   scan.get_tsys()

Note that some selection of data is usually necessary. Otherwise `get_tsys()`
returns too many numbers.
To divide two spectra simply divide one scan table to another::
   
   quotient = signal / ref