Questions about wavelength calibration with wide slit

[Jacques Montier]

Hello,

I am studying the Christian Buil absolute calibration tutorial (part 2)
http://www.astrosurf.com/buil/calibrati ... ion_en.htm
Two spectra (ref and target) have to be made with wide photometric slit (ie 230um).
So how can i run precise wavelength calibration with ISIS ?
ISIS needs the X coordinate of the 5852.49A line, smile informations (Y and radius).

Ne_Ar.png (169.9 KiB) Viewed 2762 times

So i tried this :
- precise wavelength calibration with narrow slit (23um) to get the dispersion polynom
- then i use the same polynom for calibrating the photometric spectra.
It seems to work, but i have to put some wavelength shift in the general parameters.
Is it the right way to go ?

Thank you for your advice and help,

Regards,

[Robin Leadbeater]

Hello Jaques,

You cannot do a very accurate wavelength calibration with a wide slit because the star can move within the slit. The best you can do is assume the star is in the centre of the wide slit and use the slit centre as the reference point. This does not matter in method 1 though (The 4 spectrum method) as you are only using the wide slit spectra to measure what proportion of the light is lost in the narrow slit measurement. This is then applied to the narrow slit spectra to flux calibrate them. (Make sure that the region where you measure the flux is reasonably level and free of lines so that small wavelength errors will not affect the result. If there is an obvious wavelength offset between the wide and narrow spectra you can also move the wide slit spectrum to correct the wavelength error until the lines match up before measuring the flux)

In method 2 (the two wide slit spectra method) the resulting spectrum is flux calibrated but the resolution will be lower and wavelength calibration not as accurate as when a narrow slit is used. The best you can do is to assume the centre of the slit is the reference point and accept that the resolution will be affected by seeing and drift in the position of the star

Cheers
Robin

[Jacques Montier]

Hello Robin

Your response is very clear to me.
Thank you very much

Cheers,