Radial velocity measurements with LowSpec

[drjolo]

After careful preparation I made another attempt to radial velocity measurements with LowSpec spectroscope. Currently it works at low resolution (600 lpm grating, R~1400) and I expect accuracy at the level of 1/10 R. With 600 lpmm grating it maps to about 20 km/s. So I picked some fast star for the first test - BD+37 1458. And slow HD43094 for comparison. Here are both candidates imaged with EV (Evostar 72, IMX224, 30x15s):

Here are both star spectra already with Relco SC480 (light grey plot). Both stars exposure time was 3x180s. It can already be noticed, that Ha lines of both stars are shifted. But rightmost absorption is at the same position - this is atmospheric Fraunhoffer B line. 

After zooming into Ha line I have made several measurements in BASSProject software:

And here are the calculations. Last step was correction to Earth orbital and rotational motion (calculated in BASSProject Tools).

The actual values in SIMBAD database are 242 and 38 km/s. So the error is more less equal to my expectations (about 1/10 R). One thing that I should do is to determine systematic error. But I will do another attempt when 1800 lpmm grating will arrive (it is already ordered). I am pretty happy with these results at the moment  Although I think spectrum SNR is too low for this kind of work, and it probably also negatively affected the accuracy. Another thing to improve is position of the line of interest. I should have put Ha line in the centre of FOV and focus there. 

http://simbad.u-strasbg.fr/simbad/sim-id?Ident=BD%2B37+1458

http://simbad.u-strasbg.fr/simbad/sim-id?Ident=hd43094

Edited February 25, 2020 by drjolo

[robin_astro]

Nice result. Next step where the stars have similar features (or you have a template) or are you are following the same star for binary orbits for example could be to use cross correlation  which gives potentially a much higher precision and is less sensitive to SNR. The technique is very powerful but  it only gives a differential result though unless one of your stars has a known RV. ISIS has a tool for this and the procedure is described here for example in the workshop tutorial "Observing with a LISA spectrograph" here

https://www.britastro.org/downloads/15701

where a 1 sigma precision  ~1/40 of the resolution was achieved with careful attention to detail

Cheers

Robin

 

 

Edited February 25, 2020 by robin_astro