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

["k 50" <k010150@hotmail.com>]

Roland,

What are "cheap" vs. "expensive" filters?  Are we talking about the 
difference between $200 and $2000?  Can we tell the difference by looking at 
the colors reflected off the coatings?  I notice that binocular optics seem 
to have widely varying colors coming off the objectives (red on some, green 
on others).

I'm also curious to know what quality coatings are used on the high end 
refractors (such as AP) vs. mid-range scopes (such as Vixen) vs. 
high-quality camera lenses (like Canon).



>From: Chris1011@aol.com
>Reply-To: astro-photo@seds.org
>To: astro-photo@seds.org
>Subject: Re: [APML] AP 130/F6 LPR filter help
>Date: Thu, 22 Feb 2001 12:07:55 EST
>
>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

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