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.