Exercise 3: A WCS exercise with no image flip.

Here I demonstrate the wcs tools applied to a (PFC) image of NGC 4625 that does not require an image flip.

Get the data and build the wcs_setup.out file.

Here are a few simple steps with only terse explanations.

 

# Collect the data set 
% cp -r $critfils//tdat/n4625_pfcB.fits . 
% ls 
n4625_pfcB.fits

# Get a sky position for NGC 4625 
% rc3gal n4625 
Results of the RC3 search:
Name_RC3       RA_2000    DEC_2000     Type     Input_name
-----------  ----------- ----------   ------    ----------
NGC  4625    12:41:53.00 +41:16:27.0  .SXT9P.   n4625     
dssget 190.470825  41.274166 10.0 dss poss1_red
Btot,DIA(armin),b/a,V:    12.92     2.19   0.871      659.0
Other names: 
   NGC  4625      
   UGC  7861      
   IRAS12395+4132 
   PGC 42607      

# Look at the image and verify the X,Y,Ra,Dec data set  
# I strip out the WCS and do the same thing again 
usno_identify n4625_pfcB.fits 18.0 yellow 18 
  show_wcs_points wcs_setup.out  blue 26 
# I had to manually enter to usno_identify:    12:41:53.00 +41:16:27.0 4.0  
# I displayed X,Y in frame 1 and Ra,Dec in frame 2  
# Note that I wanted to learn the (Ra,Dec) for image. I'll need this after I remove the WCS from the test image. 

% ls 
b.png	 dss.fits  just_text.reg    rc3_list.short  usno_all.cdfp	usno_identify.final_list  usno_targs_V.reg
ds9.png  gals.reg  n4625_pfcB.fits  stars.reg	    usno_all_magR2.reg	usno_targs.cdfp
# The important file we need is:   usno_identify.final_list 
% cat usno_identify.final_list 
 247.42 164.99   12:41:39.4332 +41:16:48.5037   1 
 209.85 56.02   12:41:44.1996 +41:14:20.6827   3 
 61.69 176.26   12:42:01.9080 +41:17:04.9283   10 
 216.29 119.36   12:41:43.3608 +41:15:47.3099   2 

# Make the file = n4625_pfcB.xyrd  
% make_xyrdfile n4625_pfcB.fits usno_identify.final_list 
% cat n4625_pfcB.xyrd 
# Sources collected with make_xyrdfile
# data
     247.420      164.990   190.414305000   41.280139917       1  12:41:39.4332 +41:16:48.5037 
     209.850       56.020   190.434165000   41.239078528       3  12:41:44.1996 +41:14:20.6827 
      61.690      176.260   190.507950000   41.284702306      10  12:42:01.9080 +41:17:04.9283 
     216.290      119.360   190.430670000   41.263141638       2  12:41:43.3608 +41:15:47.3099 

# Make the "standard" wcs_setup.out file 
cutup_xyrdnames n4625_pfcB.xyrd
cat wcs_setup.out 
1
247.420 164.990 190.414305000 41.280139917
3
209.850 56.020 190.434165000 41.239078528
10
61.690 176.260 190.507950000 41.284702306
2
216.290 119.360 190.430670000 41.263141638

I wanted to demonstarte (again!) the tools I would need to gather WCS solution data. However, for the purposes of testing whetehr routines line wcs_trs or wcs_2lin are working, you coul just cut and paste the wcs_setup.out file above, pull the image file from $critfiles as shown above, and you are ready to roll.

This is the image I created with the usno_identify routine:

  % usno_identify n4625_pfcB.fits 18.0 yellow 18

The image on the right is my CCD image to be WCS-calibrated. In the right panel is the corresponding DSS field. Since the input CCD image had no positional header information I had to manually enter the image position (at Ra,Dec = 12:41:53.00 +41:16:27.0) and the field size I wanted for the DSS image (4.0 arcminutes). I have only slightly zoomed and recentered the images above. Note that unlike our previous two examples, in this case, the orientation of the exercise image (left) matches well the standard N-up,E-left orientation of out DSS image. This is not trur for the raw McDonald PFC images, but in reducing these data in IRAF I always always apply the flip in one of the final reduction steps. Hence, I always end up with images that are close to N-up,E-left orientation and a plate scale of 1.35 arcsec/pixel.

A quick verification: run wcs_2lin.

The wcs_2lin routine will usually yield a good solution if we have at least 4 poinnts that are well distributed across the image.

# Derive the WCS calibration  
% wcs_2lin n4625_pfcB.fits N 
# Demonstrate that the WCS header works 
% usno_look_wcs n4625_pfcB_2lin.fits Blue 18.75 1

Although not a fantastic fit, the figure below verifies that we derive a usable solution.

Here we have displayed our calibrated image, named n4625_pfcB_2lin.fits, with the routine usno_look_wcs. This code collects the USNO sources in the area of the image field and overplots them. The command line we used was:

  % usno_look_wcs n4625_pfcB_2lin.fits Blue 18.75 1

With the above command we collected all USNO-B1.0 stars with a Blue magnitude brighter than 18.75 and overplotted them with cyan circles and text labels.

Now we have verified that our set of (X,Y,Ra,Dec) coordinates is approximately valid and we can proceed with testing this data set with wcs_trs.

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