Brightness of the Solar Disk with a
Spectrohelioscope
Fredrick
N. Veio, August 2002
The brightness of the solar disk
depends upon various paramenters as eyepiece focal length in mm,
passband in angstroms, % solar continuum of wavelength employed
and % eye sensitivity at employed wavelength. The grating
efficiencty and F ratio of the SHS are left out for simplicity.
The Veio spectrohelioscope will be the
standard for visual reference with an F 44 system, classicial
32x30mm ruled grating of 1200 gr/mm at 5000A wavelength, eyepiece
120mm f.l., H alpha 0.6A passband and latter with 16% of solar
continuum and eye sensitivity at 10% in the H alpha line.
Instrument published in Sky and Telescope, January of 1969.
The basic formula is
BSD = new eyepiece f.l./120mm f.l.
squared X new passband/0.6A X new solar continuum
wavelength/16% X new eye wavelength/10% =
Wavelength
|
|
Eyepiece
|
0.6A Std
|
Solar Cont 16%
|
Eye Sens 10%
|
BSD
|
Comments
|
3934A
|
Ca II
|
120mm
|
6.0A
|
6%
|
1%
|
0.37
|
Acceptable
|
4861A
|
H beta
|
120mm
|
0.4A
|
14%
|
20%
|
1.17
|
Bright
|
5167A
|
Mg b4
|
120mm
|
0.05A
|
9%
|
40%
|
0.19
|
Faint
|
5167A
|
Mg b4
|
180mm
|
0.05A
|
9%
|
40%
|
0.38
|
Acceptable
|
5896A
|
Na I
|
120mm
|
0.1A
|
5%
|
70%
|
0.37
|
Acceptable
|
6563A
|
H alpha
|
120mm
|
0.6A
|
16%
|
10%
|
1.00
|
Medium Bright
|
6563A
|
H alpha
|
120mm
|
0.2A
|
16%
|
10%
|
0.33
|
Acceptable
|
The violet Ca II line was observed
with 0.5A passband, but the solar disk was too faint. One must use
a wider passband, for the human eye has low sensitivity to violet
light.
The green 5167 Mg line can be used
for visual sun spot polarity work but a few changes are
needed. Use about 180mm f.l.eyepiece instead of 120mm f.l. Will
brighten up the solardisk by two times. Employ a larger grating
of about 50x50mm to 64x64mm in order to have more light. The
Polaroid used will absorb about 40% of the incoming light.
Few calculations,
3934A Ca II 120/120mm
sq x 6.0/0.6A x 6/16% x 1/10% = 0.37
6563A H alpha 120/120mm sq
x 0.6/0.6A x 16/16% x 10/10% = 1.00
I have observed in various
wavelengths. The above is an excellent guide. The solar
continuum values are from the Meudon Observatory. The eye
sensitivity values are from Edmund Scientific booklet on
spectroscopes. The passbands are per Veio.