Re: [APML] AP 130/F6 LPR filter help

[Chris1011@aol.com]

In a message dated 2/21/2001 6:32:20 PM Pacific Standard Time, 
Matthias.Felsch@t-online.de writes:


> Regarding reflections, there was only one:
> I found a single blue reflection (a kind of halo) in a picture of M42, 
> around jota orionis.


Plane parallel glss such is used in a filter will produce a ghost image. A 
blue reflection indicates that the filter has only a simple mag-fluoride 
coating. To eliminate reflections, filters need a high quality multi-coating 
with less than 0.2% reflectivity. Coatings must be put on both surfaces. Do 
not be misled by a manufacturer's claims of multicoating. There are plenty of 
cheap multicoats that have high reflectivity. The coating should be specified 
to have less than 0.2% from 400 to 600 nm. These coatings are expensive, and 
you will not find them commonly used on cheap filters.

Roland Christen