Re: [APML]: Color Film and Filters

[sdilling@ford.com (Stephen Dillinger)]

>   In the sky, astronomical objects are not always from a single source,
> but from many sources (billions of suns in a galaxy).  Also, colour in
> astronomical objects is due not to reflected light, but emmitted light.

Sort of... stars have color tints, but *strong* colors are from emission
objects.  For stars, as long as the relative strengths of each color layer
remain the same does it really matter if all of the light gets there?

>    Therefore, photo filters for  daylight earth use are based on a
> totally different situation that astro-photo filters.  Have I got it
> right?

Right, the essential difference between *strongly* colored objects in
terrestrial scenes and astronomical scenes is between emission of discrete
wavelengths and reflection of a moderately broad band.

The white card test will give you an indication of the color shift of an object
that radiates (ie - stars).  As long as the filter passes the primary emission
lines from, say a diffuse nebula, there will be negligible color shift.  The
broadband filters in use pass H-alpha as well as the lines around the O-III so
nebulous objects should not be affected except by transmission differences in
the different wavelength bands.

.................... Steve