thomasorb
commented
6 years ago Dear @Klaus130161,
I have counter checked the results returned by my module with the results returned by the module astropy (http://docs.astropy.org/en/stable/coordinates/velocities.html) with the following code
import astropy.units as u
from astropy.time import Time
from astropy.coordinates import SkyCoord, EarthLocation, FK5
keck = EarthLocation.from_geodetic(lat=19.828333*u.deg, lon=-155.47833333*u.deg, height=4160*u.m) # those are exactly the coordinates of the observatory returned by IRAF (observ.lat, observ.lon)
sc = SkyCoord('15:46:43 +37:49:45', FK5, unit=(u.hourangle, u.deg))
obstime = Time(2454220.865771, format='jd') # exactly the number of JD returned by IRAF (and my module)
barycorr = sc.radial_velocity_correction(obstime=obstime, location=keck)
print barycorr.to(u.km/u.s)
heliocorr = sc.radial_velocity_correction('heliocentric', obstime=obstime, location=keck)
print heliocorr.to(u.km/u.s)which returns
0.565774937224 km / s # barycentric (barrycorr.html: 0.567)
0.563097179679 km / s # heliocentric (rvcorrect.py: 0.566 / IRAF: 0.54)It means that, in terms of barycentric velocity, astropy and the barycorr function (http://astroutils.astronomy.ohio-state.edu/exofast/barycorr.html) return comparable values within the 3 m/s precision error reported in both implementations. And, in terms of heliocentric velocity, both astropy and my implementation are giving similar results. I thus believe that there might be a precision problem in IRAF's calculation or some other subtlety I am not aware of.
For the sign problem, in IRAF's function, the longitude is positive towards west which is the opposite for other conventions.
You pointed out a good problem which is the use of this module rvcorrect.py in ORCS which might have not been completely checked and compared for a large range of input. In fact I will remove it and consider using the astropy implementation which is certainly far more robust as it is checked by numerous people ;)
Thank you !
@Klaus130161 Please tell me if it answers your questions so that I can close this issue
Klaus130161
Dear Thomas,
I found your Python code RVCorrect.py on the Internet and have a question about the accuracy of the result and another one about the sign of the geographical length of the observation site.
First of all, I searched the IRAF. net forum for an example that was calculated with IRAF: http://iraf.net/forum/viewtopic.php?showtopic=138738
The example was:
The object: (year, month, day, UT, RA, DEC) 2007 4 30 08:41:59 15:46:43 37:49:45:45
latitude = 19:49.7 longitude = 155:28.7 !!!! sign wrong ??? altitude = 4160.
The result of IRAF (Frank Valdes): HJD VOBS VHELIO VLSR VDIURNAL VLUNAR VANNUAL VS. SOLAR 2454220.86577 0.00 0.54 17.96 0.239 -0.002 0.299 17.428
Then I calculated the same example with your code and I received the following result:
VOBS: 0.000000 km. s-1 HJD: 2454220.865771 VHELIO: 0.566202 km. s-1 VLSR: 18.003718 km. s-1 VDIURNAL: 0.239563 km. s-1 VLUNAR: -0.002047 km. s-1 VANNUAL: 0.328686 km. s-1 VSOLAR: 17,437515 km. s-1
The difference between the two calculations is 0.566-0.54 = 0,026km/sec.
Do you know the reason for this discrepancy?
I have also done a comparison calculation with barycorr http://astroutils.astronomy.ohio-state.edu/exofast/barycorr.html Here is the result: 567.467035794 m/sec. The difference in this case is only 1.265 m/sec !!
The problem, however, is that the sign of longitude has to be accepted positively for this result. Is that correct?
Thank you in advance for your effort Klaus