Tasks for Coordinates, Angles and Time

To compute a parallactic anle we need a number of pieces of information related to sky position (RA,DEC) and time (DATE, UT, ST). I've collected a set of scripts and programs that perform the coordinate, angle and time calculations need to manipulate these quantities. I record, in recipe-style, the codes as used here.

The high level Cal_q.sh script.

I start here with the final script that I used to confirm my parallactic angle calculations. I show a sample run, and then the source code.

 

% Cal_q.sh* list.HA list.DEC list.PARANGLE  

To plot the results:
%  qplot.py -10 380 -10 380  

% cat Cal_q.sh

 

#!/bin/bash

# Test the use of mcd_parallactic_angle routine 

# Check that an argument is present
if [ -z "$1" ]
  then
    printf "Usage: Cal_LST.sh list.HA list.DEC list.PARANGLE \n"
    printf "arg1 = file of HA (sex)\n"
    printf "arg2 = file of DEC (sex)\n"
    printf "arg3 = file of Q (expected)\n"
    exit 1
fi
if [ -z "$2" ]
  then
    printf "arg2 = file of DEC (sex)\n"
    exit 2
fi
if [ -z "$3" ]
  then
    printf "arg3 = file of Q (expected)\n"
    exit 3
fi

i="0"
# Read each line in the input files (= $1, $2, $3)
# using file descriptor 4
exec 3<$1
exec 4<$2
exec 5<$3
while read -u3 HAsex 
do

 i=$[$i+1]
 read -u4 DECsex
 read -u5 Qexp 

sex_hours_2_deg.sh $HAsex > junk1
read HAdeg < junk1

sex2float.sh $DECsex > junk2 
read DECdeg < junk2 

# Compute the Q
mcd_parallactic_angle $HAdeg $DECdeg > junk3
read Q < junk3

#echo $HAsex $DECsex $HAdeg $DECdeg $Qexp $Q
echo $Qexp $Q

done 


# Clean up 
\rm -f calc_parangle.log junk1 junk3 sex_hours_2_deg.log
\rm -f junk2 pars.in sex2float.log sexprep.log

To compute a parallactic angle from the command line (assuming your are at McDonald Observatory:

 
% mcd_parallactic_angle -20.206375122 0.129916667 
Usage: mcd_parallactic_angle -20.20637499 16.808833333 
arg1 - HA in units of degrees  
arg2 = DEC in units of degrees

A simple script to recall the McDonald information: 
% mcdlonlat 
Usage: mcdlonlat Y 
Usage: mcdlonlat  
arg1 - Y flag for verbose output
long = -104.0216667 degrees (- means WEST)
lat  = +30.6716667 degrees (+ means NORTH)

% mcdlonlat N
-104.0216667  +30.6716667

The more general script (where you feed a latitude): 
% calc_parangle.sh +30.6716667 -20.206375122 0.129916667 
  149.694625463

Notice that my script requires parameters that are expressed as ANGLES in units of DEGREES. Quantities like RA, DEC, and Time rarely are provided this way. How do we obtain them?

Handling sexigecimal strings.

I have a number of routines that convert Time/Coordinate strings in sexigecimal (sex) format. The first (sexprep) is one that is mundane, but extremely useful:

 
% sexprep.sh list.RA > new_file    
arg1 - name of ASCII input file with sexigecimal lines

% cat list.RA    
          14:34:12.27
   14:27:07.61
14 27 06.98
         14h55m17.62777s
   14h07m07.17s
          +14:49:45.5488488
     -14:49:46.0334
08:45:32
    08:45:33.23222
 -14:27:15.14


% cat new_file    
14:34:12.27
14:27:07.61
14:27:06.98
14:55:17.62777
14:07:07.17
+14:49:45.5488488
-14:49:46.0334
08:45:32
08:45:33.23222
-14:27:15.14

The corrected (standardized) strings are written to standard out, but in a script application (like for Cal_q.sh above) they are usually directed to a file (in the case above we direct to file "new_file"). The output strings should now be terminated properly, and any signs attached to the input strings should be preserved.

Once the sexigecimal strings are standardized, then we usally want to convert to a floating point angle in units of DEGREES. If the string is expressed in HOURS (like is often done with things like RA, HA, ST) we must multiply by a constant (15.0 in most cases). The tools I have for these sorts of tasks are shown below (they all write to standard out):

 

Convert sex string directly to floating point degrees:
% sex2float.py 14:34:12.27
14.570075000
% sex2float.sh 14:34:12.27
   14.570075000

Convert sex string in HOURS to floating point degrees:
% sex_hours_2_deg.py 14:34:12.27
218.551125000
% sex_hours_2_deg.sh 14:34:12.27
  218.551132202
% sex_hours_2_deg.sh -14:34:12.27
 -218.551132202

Notice that these routines preserve the sign of a quantity. The HA value will often have a sign that is important to preserve (i.e. a negative HA is used for an object that is observed WEST of the meridian). With these routines in place, it is easy to write something that computes hour angle (HA):

 
% cal_from_sex_HA.py 14:34:12.27 11:52:58.9416 
-40.305535000

where, 
14:34:12.27   = the object right ascension (RA) 
11:52:58.9416 = the local sideral time (LST)

How to compute local sidereal time (LST).

Compute the Apparent Local Sideral Time given an input UT date/time in isot format.

%  mcdLST.py 2008-05-03T04:27:12.36 

Usage: mcdLST.py 2008-05-03T04:27:12.36

The ISOT format: 
   yyyy mm dd hh mm ss.ss
   2008-05-03T04:27:12.36 

NOTE: I looked for quite some time throught the Astropy.Time 
      documentation, but could not learn what ISOT stands for. 

Time:
2008-05-03T04:27:12.360
Julian Date          = 2454589.685560
Modified Julian Date = 54589.185560
12h16m54.548s
12:16:56.09     (Expected LST)

Acually, the above run is a special case. For practical concerns I have turned off the JD and MJD outputs, and I do not print the "Expected LST" unless it is recognized for this one case (i.e. 2008-05-03T04:27:12.36). Hence, a more normal run would be:

 
%  mcdLST.py 2011-05-23T14:18:12.36 
23h25m31.4784s

Now it is clear why something like "sexprep.sh" is useful: it can be used to strip out the "h", "m", and "s" characters and then produce a value in degree units.

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