Radial velocity errors
Posted: Thu Sep 03, 2015 2:53 am
Last night was very hazy but the seeing was good at 1.6 arcsec FWHM so I decided to do an experiment that I have wanted to do for some time.
I have wanted to determine just how sensitive the wavelength calibration is to small position changes of the star image
relative to the center of the slit. My slit width is 23 um or 1.8 arcsec on the sky.
I acquired 4 spectra of P_Cyg while it was near the zenith. Each spectra used 5 x 30 sec exposures with a Relco lamp calibration before and after.
1 - star centered on slit
2 - star offset by 20% of the slit width to the left (1/2 pixel)
3 - star offset by 20% of the slit width to the right (1/2 pixel)
4 - star centered on slit
I established the center of the slit by imaging the evening sky and adjusting the guide camera so that a pixel column was exactly centered.
For each of the 4 conditions above I produced two sets of spectra.
Set (A) had no HRV correction and no removal of the H2O lines.
Set (B) had the HRV correction and the H2O removed (but with no additional shift).
The integrated intensities for the case of the star shifted left or right dropped by only 2 to 4%.
I used the radial velocity tool in ISIS to calculate the velocity differences.
For set (A) a synthetic H2O spectra was used for the reference spectrum (adjusted to match the resolution and intensity of the water lines).
Using the water lines in this way to calculate the errors in calibration produced the following (in Km/sec):
star on center -0.8
star to left 1.37
star to right -2.75
star on center(2) -0.61
For set (B) the spectra #1 "star centered on slit" was used as the reference.
Using the P Cyg lines in this way to calculate relative errors in calibration produced the following (in Km/sec):
star on center 0
star to left 1.82
star to right -1.60
star on center(2) -0.04
Both sets tell the same story. Offsetting the star by only 20% of the slit width produces aprox. 2 km/sec errors!
Set (A) indicates that I have an error of about -0.7 Km/sec when using my Relco lamp to calibrate if the star is perfectly centered. But it is also possible that what I considered to be perfectly centred was actually off by about 7% of the slit width.
Obviously keeping the star precisely centered on the slit is ultra important if one wants accurate radial velocities.
Dithering the star back and forth across the full slit width may be another option but this would reduce the efficiency greatly and the dithering itself would have to be done very uniformly.
Now I am beginning to see just how difficult it is to get accurate radial velocities! Anyway I thought these results may be of interest.
Tim
I have wanted to determine just how sensitive the wavelength calibration is to small position changes of the star image
relative to the center of the slit. My slit width is 23 um or 1.8 arcsec on the sky.
I acquired 4 spectra of P_Cyg while it was near the zenith. Each spectra used 5 x 30 sec exposures with a Relco lamp calibration before and after.
1 - star centered on slit
2 - star offset by 20% of the slit width to the left (1/2 pixel)
3 - star offset by 20% of the slit width to the right (1/2 pixel)
4 - star centered on slit
I established the center of the slit by imaging the evening sky and adjusting the guide camera so that a pixel column was exactly centered.
For each of the 4 conditions above I produced two sets of spectra.
Set (A) had no HRV correction and no removal of the H2O lines.
Set (B) had the HRV correction and the H2O removed (but with no additional shift).
The integrated intensities for the case of the star shifted left or right dropped by only 2 to 4%.
I used the radial velocity tool in ISIS to calculate the velocity differences.
For set (A) a synthetic H2O spectra was used for the reference spectrum (adjusted to match the resolution and intensity of the water lines).
Using the water lines in this way to calculate the errors in calibration produced the following (in Km/sec):
star on center -0.8
star to left 1.37
star to right -2.75
star on center(2) -0.61
For set (B) the spectra #1 "star centered on slit" was used as the reference.
Using the P Cyg lines in this way to calculate relative errors in calibration produced the following (in Km/sec):
star on center 0
star to left 1.82
star to right -1.60
star on center(2) -0.04
Both sets tell the same story. Offsetting the star by only 20% of the slit width produces aprox. 2 km/sec errors!
Set (A) indicates that I have an error of about -0.7 Km/sec when using my Relco lamp to calibrate if the star is perfectly centered. But it is also possible that what I considered to be perfectly centred was actually off by about 7% of the slit width.
Obviously keeping the star precisely centered on the slit is ultra important if one wants accurate radial velocities.
Dithering the star back and forth across the full slit width may be another option but this would reduce the efficiency greatly and the dithering itself would have to be done very uniformly.
Now I am beginning to see just how difficult it is to get accurate radial velocities! Anyway I thought these results may be of interest.
Tim