Robin Leadbeater wrote:James Ley wrote:
The common reference is, in fact, the dispersion, which doesn’t change. I begin by building a polynomial using the Balmer lines from an A-type star.
The dispersion alone NOT sufficient to calibrate the spectrum of a slitless system. You are guessing at the wavelength of one of the features in the spectrum ie you need to know in advance something about the object you are measuring.
Robin
In my very limited experience with bright stars and planetary nebulae, I have found that advance knowledge is not necessary either to make a guess or to confirm it.
Very roughly, if I guess that the large absorption slightly blueward of the middle of the spectrum is H-beta and, unknown to me, if the star is O-type through F-type, then I am rewarded with all the Balmer and Ca II K and H lines of a reference spectrum aligning with my spectrum, confirming my guess. The Ca II H and K lines also make an easy guess if the pair is visible. Finally, the Na I D line, is often a large absorption in the middle of the spectrum.
I am not suggesting that such a method is appropriate for all users under all circumstances.
The educational aspect is also interesting. In half an hour I have taught many students who had never even seen a telescope before to calibrate a star analyzer spectrum using the zero order. It was easy. They didn't have to think about the absorption features or the spectral type of the star. They just had to click on the dot. Certainly there are lots of occasions where this would be desired. But perhaps, with appropriate "unknown" targets, students could learn a bit more with the above method where they had to look for what features they can find in the raw spectrum, see the consequences of guessing correctly and incorrectly and compare how different spectral types have characteristic absorptions.
Shelyak Alpy 600 (spectroscope, guiding module and calibration module) | GSO 8" f/4 | Orion Atlas Pro | ZWO ASI178MM-Cool (mono) | ZWO ASI290 Mini (mono)
"think like a photon" -- Steve Shore