Subject: Arc Line Profile Fitting
From: Orlon Petterson <[email-masked]>
To: [email-masked], [email-masked]
Cc: Orlon Petterson <[email-masked]>, Daniel Pooley <[email-masked]>, Michael Albrow <[email-masked]>, Peter Cottrell <[email-masked]>, Jennifer McSaveney <[email-masked]>
Date: Fri, 04 Aug 2000 15:28:53 +1200 (NZST)

Dear Sir/Madam,

When reducing echelle spectra using your ECHOMOP package (version 3.3-4) we
have noticed a rather curious feature.

The tables have been garnered from the reduction database file.

Table 1
Line Ord  Pixel       Wavelength
No.  No.  No.

1    1    132.5948    5437.388     
2    1    176.9542    5440.602     
3    1    268.0225    5447.154     
4    1    331	      5451.652     
5    1    339	      5452.219     
6    1    434.3124    5458.968     
7    1    473.6672    5461.736     
8    1    551.2589    5467.161     
9    1    603.013     5470.759     
10   1    797.3746    5484.146    
11   1    930.4254    5493.204    
12   1    947.1292    5494.331    
13   1    1004.136    5498.184    
14   1    1020	      5499.255    

1    2    108	      5567.999     
2    2    219	      5576.204     
3    2    262	      5579.358     
4    2    367	      5587.026     
5    2    478	      5595.063     
6    2    610	      5604.516     
7    2    697	      5610.681     
8    2    843.0988    5620.921     
9    2    911.7693    5625.678

Table 2
Line Ord  Pixel       Wavelength
No.  No.  No.

1     1   46.73944     5431.112
2     1   176.9541     5440.602
3     1   268.049      5447.154
4     1   300.5043     5449.479
5     1   331.0103     5451.652
6     1   338.8997     5452.219
7     1   473.6638     5461.736
8     1   486.1268     5462.613
9     1   508.8778     5464.206
10    1   602.9222     5470.759
11    1   723.3015     5479.075
12    1   922.144      5492.643
13    1   930.4376     5493.204
14    1   947.153      5494.331
15    1   1004.119     5498.184
16    1   1020.048     5499.255

1     2   57.71556     5564.201
2     2   108.3163     5567.999
3     2   151.1628     5571.191
4     2   238.9309     5577.685
5     2   261.6746     5579.358
6     2   302.8055     5582.367
7     2   321.8966     5583.762
8     2   477.9442     5595.063
9     2   610.2751     5604.516
10    2   681.7349     5609.58 
11    2   697.3672     5610.681
12    2   717.0639     5612.068
13    2   763.3085     5615.319
14    2   911.7241     5625.678
15    2   978.4145     5630.297

These orders have been fitted with a 4 coefficient polynomial with an RMS
< 0.002. Probably a good identification.

The only difference in process is that the orders were optimally extracted
in the first case and profile extracted in the second (ech_extrct).

The main feature is that the pixel numbers for the optimal case often have
integer values. In the profile case there are also integer pixel values,
but these are only for lines which are saturated. Consequently the arc
line centres are more accurate and lead to fits in general involving
nearly 50% more lines. The standard deviations for radial velocities
calculated from these fits are 1.16 km/s and 0.68 km/s respectively.

The question we have is why does the part of echomop responsible for the
centroiding of arc lines feel compelled to round some pixel values in the
optimal case but not the profile? Are we setting some parameter
incorrectly or is there really a bug in the program?

Yours sincerely

Daniel Pooley & Orlon Petterson

--
Orlon Petterson    email:[email-masked] 
                   Department of Physics and Astronomy 
          University of Canterbury,  Christchurch,  New Zealand
                http://www.phys.canterbury.ac.nz/~physokp/
+-------------------* Our future is among the stars! *-------------------+