More calibration notes

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Mike Harlow

I’ve attached my recent objective prism spectrum not because it shows anything new but to illustrate a trick used for line identification. (Taken on 12th April 2021).

I usually image and take spectra through an Astrodon luminance filter which has a sharp cut-off at 700nm.  The result is that all spectra cut-off at that wavelength giving an internal reference on all the spectra in the field. Coupled with the dispersion equation for the prism this enables reasonable estimates for line positions. In the spectrum of SN2021hpr the first absorption below 700nm is the Si II line as shown nicely in Kevin Gurney’s spectrum (of another type-Ia SN) and those above.

I can’t claim any originality here, this was a trick used by professional astronomers conducting objective prism surveys in the 1960s, 70s and 80s.  Some of the photographic plates used had sharp spectral cut-offs imprinting a known wavelength on their spectra.  And when you have a Schmidt photographic plate with 1000s of spectra on it that’s really useful!