Indianaradio wrote:

Because they can and people will pay for a peice of glass as they think it'
is sooooooo good (Not)



Because to make a filter narrow band enough to work, and also to be
temperature stable in passband is extraordinarily difficult ?

There ARE other methods - the oscillating slit spectrohelioscope is on
my projects list for the future, but what you gain versatility, you will
lose in size, and it isn't going to be cheaper.

Steve