2 Classification with the low-dispersion prism P1

An objective prism acts as a disperser through the effect of differential refraction. That is to say, the refractive index of the material of which it is composed varies with wavelength, and so rays of differing wavelengths are deviated to different extents on passage through the prism. The full-aperture objective prism P1 has an apex angle of 44 arcmin (2400 Å/mm at 4300 Å). The prism can be mounted on the telescope, and the complete assembly can be rotated to give the dispersion direction in any required position angle. The usual "default option'' is to have the dispersion North-South. The dispersions and effective resolutions for the prism P1 are shown in Table 1.

 

 

Table 1: Dispersions and effective resolutions for the prism P1

${\rm Feature}$	$\lambda(\mbox{\AA})$	${\rm Dispersion}~(\mbox{\AA/mm})$	${\rm Eff.~Res.}~(\mbox{\AA})$
$\mbox{H}_{\alpha}$	6563	8088	176
$\mbox{H}_{\beta}$	4861	3470	76
$\mbox{H}_{\gamma}$	4340	2440	53
$\mbox{H}_{\epsilon}$	3970	1852	41

The error in spectral classification of stars is likely to be approximately one spectral class. O and B spectra are not easily separated by a human classifier, so the P1 dispersion objective prism stellar classification contains the six following classes (Nandy et al. [1977]; Krug et al. [1980]; Cooke et al. [1981]; Savage et al. [1985]).

• OB  spectra
  Uniform continuum intensity distribution over the length of the spectrum. The late B spectra show Balmer discontinuity at the UV region of continuum.

• A  spectra
  Uniform continuum intensity distribution up to a strong Balmer jump at 3700 Å. Broad $\mbox{H}_{\gamma}$ absorption line at 4340 Å. The absorption line $\mbox{H}_{\beta}$ at 4861 Å$\;$is evident as a slight narrowing of the spectrum width.

• F  spectra
  Intensity decreases slowly with decreasing wavelength. The only evident absorption is a blend of $\mbox{H}_{\gamma}$ and G-band at 4340 and 4300 Å respectively. Calcium H and K lines at 3970 and 3934 Å$\;$respectively are not seen. This distinguishes F stars from G stars.

• G  spectra
  Intensity decreases with decreasing wavelength. There is absorption at a strong blend of $\mbox{H}_{\gamma}$ and G-band at 4340 and 4300 Å respectively. Absorption appearance at Calcium H and K lines at 3970 and 3934 Å respectively. Usually a G spectrum is shorter than an F spectrum.

• K  spectra
  Rapid decrease in intensity with decreasing wavelength. The majority of spectral energy is between wavelengths 5400 to 4300 Å. There is a broad absorption line of CaI at 4227 Å. Absorption appearance at Calcium H and K lines at 3970 and 3934 Å$\;$respectively that is presented as a broad absorption at 3950 Å.

• M  spectra
  Very rapid decrease in intensity with decreasing wavelength. A comparison between the K and M examples shows a more rapid decrease in the M star. Absorption line of CaI at 4227 Å. Absorption appearance of TiO bands at 5000 and 4800 Å.