4. THE GRATINGS EFFICIENCY
The efficiency of a grating is a parameter which gives the percentage of energy concentrated in a given order and in a given wavelength. More this efficiency approach 100% better is the grating for the wavelength considered.
This page gives the efficiency of various transmission gratings, not very expensive, and usable in the convergent beam of the telescope for low spectral resolution observation. I measured this efficiency by using an Helium-Neon laser and an Audine camera as detector. So, the efficiency is given for the wavelength of 632.8 nm.
The table below shows the result for 4 gratings
easy to find: the Cokin grating, the Jeulin grating, the Edmund
Scientific grating and the Rainbow Optics grating. Order higher
than 2 are ignored in this study because of their weak contribution
in general. But because of this approximation one can to estimate
that all the values of efficiency are overestimated from 2 to 3
The blue extended spectral sensitivity of the KAF-0401E is very usefull for spectrography. I tested this CCD in an Audine camera by observing SAO83560, a star of the spectral type A0V and of magnitude 6.9 (RA=14h52m07.37s, DEC=+20°17' 27.6"). The dispersive element, in the convergent beam of the 190 mm telescope, is the Jeulin grating of 100 lines/mm. The grating is located 52.8 mm in front of the CCD. The final image is the result of the combining of two 90 seconds frames.
Spectral profile of star SAO83560. The spectrum is not calibrated, but some lines were identified. The mark on the left is towards 3200 A. The green rectangle highlights the additional part of the spectrum which it is possible to observe with the KAF-0401E compared to the KAF-0400. Dispersion is 16.9 A/mm.
of the blue part of the spectrum of SAO83560. The line H-epsilon
would be problematic to observe with a standard KAF-0400.
The H&K lines of CaII are accessibles. These lines, which have
a great astrophysical importance, are very easily accessible with
the KAF-0401E. The green arrow indicates the position of the Balmer
discontinuity at 3647 A.