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Problem: As I am getting extremely weird answers when calculating RV of known stars, I decided to manually verify the calculations FXCOR is doing. The equations listed in the manual are: ref_rvobs = catalogue_vel_of_template - H(temp) # obs. vel. of temp.
VREL = C * (10 ** (wpc * shift) - 1.) # relative vel.
VOBS = ((1+ref_rvobs/C)*(10**(wpc*shift)-1)) * C # observed vel.
VHELIO = VOBS + H(object) # heliocentric vel.Questions: 1. If my file has 1000 pixels in the x-direction, is wpc simply "wavelength range (for a given aperture) / 1000 pix" ? Is there an IRAF command that will give me the wpc directly?
2. The heliocentric correction for temp. and object will come by running the RVCORRECT?
3. Where would I incorporate corrections in vobs which also comes from RVCORRECT?
4. Is there a way to measure the shift using an IRAF command? I thought of overplotting the two spectra and counting the pixels by hand. Is there a better way?Sorry for tons of questions and thanks a lot for your help!

[quote:cfa298995d]Questions:1. If my file has 1000 pixels in the x-direction, is wpc simply "wavelength range (for a given aperture) / 1000 pix" ? Is there an IRAF command that will give me the wpc directly?[/quote:cfa298995d]The 'wpc' value used is actually in log space, specifically the difference between the central pixel and the one next to it (something like "(log(N+1) - log(N))"). If you're spectra are already log-binned then the SLIST task will print out the approriate values in log space, otherwise this is an internal calculation.[quote:cfa298995d]2. The heliocentric correction for temp. and object will come by running the RVCORRECT?
3. Where would I incorporate corrections in vobs which also comes from RVCORRECT?[/quote:cfa298995d]Corrections are also done internally but using the same procedures as in RVCORRECT. The corrections are applied automatically and rely on certain header keywords being correct, the observatory being defined etc. The template velocity you put in the header is assumed to be heliocentrically corrected already. You don't say exactly what the differences are, but if there are problems at this stage the log file (you can view this with the 'v' key) will have warnings about missing information.[quote:cfa298995d]4. Is there a way to measure the shift using an IRAF command? I thought of overplotting the two spectra and counting the pixels by hand. Is there a better way? [/quote:cfa298995d]Turn on the 'pixcorr' param and FXCOR will compute only the pixel shift derived from the correlation. The advantage here is that things like centering in the padded array, apodization and the fit to the peak will all be the same.-Mike

Hello Mike, Thanks for the quick response. When running FXCOR, I have no errors and the log file is clean. This is exactly my concern. Below is what I mean. The object GJ 905 has a published VHELIO of -77.95 km/s. I have two epochs for it. To test the accuracy of the RV calculations, I cross-correlate the same object at a different epoch. I pick one of the epochs as template by adding heliocentric velocity to its header while the other epoch is left as an object. When cross-correlating I get the following (these are the average values): Aperture | V_r | V_o | V_h |1 -0.51 -65.92 -57.84
2 -0.91 -66.32 -58.85
3 +0.50 -66.04 -58.57
4 +0.20 -65.21 -57.74
5 -0.70 -66.12 -58.67
6 +0.16 -65.23 -57.78
7 -0.14 -65.55 -58.07We do see consistency and in fact V_r is within the error, but somehow V_o and V_h seem to be going totally wrong. The result from the V_r is not translating correcting into V_h.

Here is how the header in the template looks like: Please note that I have manually added the keywords needed for FXCOR. They appear at that end of the header. The header for the object is exactly the same but without the VHELIO keyword. ecl> imhead GJ905june5-6.fits lo+
GJ905june5-6.fits[1000,11][real]: GJ905
No bad pixels, min=0., max=0. (old)
Line storage mode, physdim [1000,11], length of user area 7047 s.u.
Created Fri 14:25:01 26-Dec-2008, Last modified Wed 11:36:07 28-Jan-2009
Pixel file "GJ905june5-6.fits" [ok]
EXTEND = F / File may contain extensions
ORIGIN = 'NOAO-IRAF FITS Image Kernel July 2003' / FITS file originator
DATE = '2008-12-26T19:25:01' / Date FITS file was generated
IRAF-TLM= '11:36:07 (28/01/2009)' / Time of last modification
TELESCOP= 'Keck II ' / Telescope
OBSERVER= 'Greene Zapatero Os' / Observer
OBJECT = ' GJ905 ' / Object name
COMMENT = 'Obs # 1, Exp' / Comment
ROOTNAME= 'jun25s ' / File Root Name
FILENUM = 145 / Frame Number
FILENAME= 'jun25s0145.fits ' / Name of file
OUTDIR = '/sdata601/nirspec/2007jun25/spec/ ' / Data Directory
SCRIPTNA= 's25Jun0050.csh ' / Name of current script
FILNAME = 'NIRSPEC-3 ' / Filter Name
SLITNAME= '0.432x12 ' / Slit Name
SLITPA = -233.380 / Slit position angle
SLITX = 134.500 / X pixel of the center of the slit
SLITY = 125.500 / Y pixel of the center of the slit
SLITANG = 15.100 / Slit angle in the scam. CCW from hor.
SCAMPA = -158.480 / Scam position angle
ITIME = 20.000 / Integration time per coadd
COADDS = 1 / Number of coadds
SAMPMODE= 3 / 1=Single; 2=CDS; 3=MCDS
DETBIAS = 450 / Detector bias setting (0-4096)
MULTISPE= 16 / Number of multiple reads
SAMPRATE= 250 / Spec sampling rate in KHz
Q1OFFSET= 3240 / Aladdin Quadrant 1 preamp offset
Q2OFFSET= 3240 / Aladdin Quadrant 2 preamp offset
Q3OFFSET= 3240 / Aladdin Quadrant 3 preamp offset
Q4OFFSET= 3240 / Aladdin Quadrant 4 preamp offset
GAIN.SPE= 4 / Preamp gain setting 1-4
FREQ.SPE= 4 / Preamp filtering frequency setting 1-4
CRYOTEMP= 30.340 / Aladdin cold head temperature
FIL1POS = 11 / Filter 1 Position
FIL2POS = 14 / Filter 2 Position
SLITPOS = 2 / Slit Position
ECHLPOS = 63.000 / Echelle position
DISPPOS = 34.080 / Cross Disperser position
IROTLOC = 65751 / Step position of Image rotator
CALMPOS = 0 / Cal. Mirror Position, 0=out, 1=in
CALCPOS = 0 / Cal. Cover Position, 0=out, 1=in
CALPPOS = 0 / Cal. Pinhole Position, 0=out, 1=in
NEON = 0 / Neon lamp: 1=on, 0=off
ARGON = 0 / Argon lamp: 1=on, 0=off
KRYPTON = 0 / Krypton lamp: 1=on, 0=off
XENON = 0 / Xenon lamp: 1=on, 0=off
ETALON = 0 / Etalon lamp: 1=on, 0=off
FLAT = 0 / Flat lamp: 1=on, 0=off
UTC = '14:41:28.82 ' / coordinated universal time (h)
AIRMASS = 1.13518 / air mass (1.14)
AXESTAT = 'tracking ' / axes control status
AZ = 26.70867 / telescope azimuth (26.71 deg)
CALOCAL = 0.00767 / collimation azimuth local (27.
CELOCAL = -0.00986 / collimation elevation local (-
CURRINST= 'NIRSPEC ' / current instrument
DATE-OBS= '2007-06-25 ' / universal date of observation
DCSSTAT = 'standby ' / drive and control status
DCSVERS = '6-1-5 ' / drive and control version
DEC = 44.17573 / telescope declination (+44:10:
DOMEPOSN= 26.73000 / dome azimuth (26.73 deg)
DOMESTAT= 'tracking ' / dome status
EL = 61.72482 / telescope elevation (61.72 deg
EQUINOX = 2000.00000 / telescope equinox (2000.0)
FOCALSTN= 'rnas (right nasmyt' / focal station
HA = -17.31611 / telescope hour angle (+22:50:4
LST = '22:33:01.68 ' / local apparent sidereal time (
MJD-OBS = 54276.61214 / modified julian date of observ
PARANG = -143.83567 / parallactic angle astrometric
PARANTEL= -143.84015 / parallactic angle telescope (-
PONAME = 'REF ' / pointing origin name
POXOFF = 0.00000 / pointing origin x offset (0.00
POYOFF = 0.00000 / pointing origin y offset (0.00
POXPOS = 0.00066 / pointing origin xposition (0.6
POYPOS = 0.00121 / pointing origin yposition (1.2
PONAME1 = 'REF ' / pointing origin name 1
POXPOS1 = 0.00066 / pointing origin xposition 1 (0
POYPOS1 = 0.00121 / pointing origin yposition 1 (1
PONAME2 = 'SLIT ' / pointing origin name 2
POXPOS2 = 0.00187 / pointing origin xposition 2 (1
POYPOS2 = 0.00006 / pointing origin yposition 2 (0
PONAME3 = 'REFA ' / pointing origin name 3
POXPOS3 = -0.00406 / pointing origin xposition 3 (-
POYPOS3 = 0.05239 / pointing origin yposition 3 (5
PONAME4 = 'DREF ' / pointing origin name 4
POXPOS4 = 0.00039 / pointing origin xposition 4 (0
POYPOS4 = -0.00090 / pointing origin yposition 4 (-
PONAME5 = 'DREFA ' / pointing origin name 5
POXPOS5 = -0.00057 / pointing origin xposition 5 (-
POYPOS5 = -0.05447 / pointing origin yposition 5 (-
PONAME6 = 'HNOD1 ' / pointing origin name 6
POXPOS6 = 0.00342 / pointing origin xposition 6 (3
POYPOS6 = 0.00057 / pointing origin yposition 6 (0
PONAME7 = 'HNOD2 ' / pointing origin name 7
POXPOS7 = -0.00318 / pointing origin xposition 7 (-
POYPOS7 = -0.00113 / pointing origin yposition 7 (-
PONAME8 = 'LNOD1 ' / pointing origin name 8
POXPOS8 = -0.00005 / pointing origin xposition 8 (-
POYPOS8 = 0.00536 / pointing origin yposition 8 (5
PONAME9 = 'LNOD2 ' / pointing origin name 9
RA = 355.48016 / telescope right ascension (23:
ROTCALAN= 0.00000 / rotator calibration angle (0.0
ROTMODE = 'position angle ' / rotator tracking mode
ROTPPOSN= -76.29774 / rotator physical position (-76
ROTPOSN = 201.58494 / rotator user position (201.58
ROTREFAN= 0.00000 / rotator reference angle (0.00
SECFOCUS= 0.00066 / secondary mirror focus raw (0.
SECTHETX= -0.00030 / secondary mirror thetax (-62.0
SECTHETY= -0.00104 / secondary mirror thetay (-215.
TARGDEC = 44.17500 / target declination (+44:10:30.
TARGEPOC= 2000.00000 / target epoch (2000.000000)
TARGEQUI= 2000.00000 / target equinox (2000.000000)
TARGFRAM= 'FK5 ' / target frame
TARGNAME= 'GJ905 ' / target name
TARGPLAX= 0.00000 / target parallax (0.0 arcsec)
TARGPMDC= 0.00000 / target proper motion dec (0.00
TARGPMRA= 0.00000 / target proper motion ra (0.000
TARGRA = 355.47792 / target right ascension (23:41:
TARGRADV= 0.00000 / target radial velocity (0.0000
TELESCOP= 'Keck II ' / telescope name
TELFOCUS= 0.00137 / telescope focus compensated (1
TUBETEMP= 3.33422 / tube temperature (3.33 degC)
UTC = '14:41:28.82 ' / coordinated universal time (h)
WCSDIM = 2
CTYPE1 = 'MULTISPE'
CTYPE2 = 'MULTISPE'
CD1_1 = 1.
CD2_2 = 1.
LTM1_1 = 1.
LTM2_2 = 1.
WAT0_001= 'system=multispec'
WAT1_001= 'wtype=multispec label=Wavelength units=angstroms'
WAT2_001= 'wtype=multispec spec1 = "1 65 0 11468.767155959 0.16448002671807 100'
BANDID1 = 'spectrum - background fit, weights none, clean no'
CDELT1 = 1.
CDELT2 = 1.
WAT2_002= '0 0. 97.75 107.75" spec2 = "2 64 0 11643.821390994 0.16723670833899 '
WAT2_003= '1000 0. 162.46 172.46" spec3 = "3 63 0 11824.387014591 0.17004387097'
WAT2_004= '54 1000 0. 228.95 238.95" spec4 = "4 62 0 12010.730706841 0.17290395'
WAT2_005= '72571 1000 0. 297.37 307.37" spec5 = "5 61 0 12203.136635055 0.17581'
WAT2_006= '956998631 1000 0. 367.81 377.81" spec6 = "6 60 0 12401.90791103 0.17'
WAT2_007= '87934854854 1000 0. 440.49 450.49" spec7 = "7 59 0 12607.368196518 0'
WAT2_008= '.18182866830201 1000 0. 515.48 525.48" spec8 = "8 58 0 12819.8634747'
WAT2_009= '7 0.18492828743531 1000 0. 593.01 603.01" spec9 = "9 57 0 13039.7640'
WAT2_010= '08561 0.18809573427038 1000 0. 673.27 683.27" spec10 = "10 56 0 1326'
WAT2_011= '7.466508011 0.19133464243411 1000 0. 756.46 766.46" spec11 = "11 55 '
WAT2_012= '0 13503.396534883 0.19464890981716 1000 0. 842.80 852.80"'
DCLOG1 = 'REFSPEC1 = nclamps5-6'
EPOCH = 2000.
EXPTIME = 20.
OBSERVAT = keck
UT = 14.69134
VHELIO = -77.95
Using the 'v' command in FXCOR gives me the following: Obj = ` GJ905june5-6[2]' star = `_GJ905'
Temp = ` GJ905oct7-8[2]' star = `_GJ905'
Deltav = 4.274 Km/sec Tempvel = -77.950 Km/secFit Parameters:
Function = `center1d' Width = INDEF
Height = 0. Minwidth = 3.
Peak = no Maxwidth = 21.
Weights = YES Background = 0.
Wincenter = INDEF Window = INDEF Number of points fit = 0 Mean Residual = 0.9637693
Sigma of Residuals = 0.0131762
Maximum of cross-correlation is in bin = 0.
Variance of cross-correlation = 0.
HJD of observation = 2454277.11430 MJD = 54276.61225
Object sample used in correlation = `*'
Template sample used in correlation = `*'
Tonry&Davis R value = INDEFVelocity Results:
Shift of peak = -6.6748 pixels
Correlation height = INDEF
FWHM of peak = INDEF Km/sec (=INDEF pixels) Velocity computed from shift = -28.5289 Km/sec
Observed velocity = -93.9371 Km/sec
Heliocentric velocity = -86.8221 +/- INDEF Km/secComments:rv>