Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 01/11/2024 - 01:43
The presentation at last night's meeting on atmospheric differential refraction and slit rotatiion has been posted to the web page of the Spectroscopy Observing section. Included is the companion Slit Loss Calculator tool (Excel).
Members might also be interested in these observations by Christian Buil on the combined effects of atmospheric dispersion and chromatism in the telescope optics on the flux calibration
http://www.astrosurf.com/buil/dispersion/atmo.htm
Cheers
Robin
Just a quick comment on the effect of atmospheric dispersion for slitless spectroscopy eg Star Analyser users.
Distortion of the spectrum continuum due to sampling at the slit is not a problem of course (provided you open up your binning zone to include the effect of any curvature of the spectrum) but if you don't run with the parallactic angle vertical in the field, a component of the atmospheric refraction adds to the grating dispersion (a bit like a grism) and distorts the spectrum dispersion making it slightly non linear. You may then notice that your calibration based on the zero order and linear dispersion does not work so accurately at low elevations.
Cheers
Robin
Hi Robin,
StarAnalyser 200 newbie here. Could you elaborate on how to avoid issues when attempting to do linear calibration on the zero order? In my very few initial tests, I've seen this issue on my spectra.
Thanks for all your help.
Lucas
Hi Lucas,
In theory the wavelength calibration should be close to linear including the zero order with the low dispersion angle of the Star Analysers but in practise it is only approximate (For me it is usually pretty accurate but occasionally for reasons I dont fully understand is a bit out but still close enough to allow to identification of the key features in the spectrum.)
There are a couple things that you can check that might help,
For targets close to the horizon, rotate the camera plus grating so the spectrum is parallel to the horizon to avoid the additive effect of atmospheric refraction
Measuring the exact position of the Balmer lines in the raw continuum shape can be difficult so roughly rectifying the spectrum (dividing it by a smoothed continuum) so the Balmer lines stand out more clearly against a flat background can help.
The distortion of the line shape (chromatic coma) inherent in the simple converging beam setup can introduce offsets in measurement of the position of the lines, increasing towards the red which can make the dispersion appear non linear.
If you are looking for higher accuracy and the non linearity is repeatable then you could consider using a non linear fit instead using the zero order and several Balmer lines .
Cheers
Robin
I also missed the meeting thinking it was going next week. The slides are super helpful!
Thanks,
Lucas.