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

[Matthias.Felsch@t-online.de (Matthias Felsch)]

----- Original Message ----- 
From: <Chris1011@aol.com>
To: <astro-photo@seds.org>
Sent: Thursday, February 22, 2001 6:07 PM
Subject: Re: [APML] AP 130/F6 LPR filter help

> 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.

Thanks for explaining this!
So in fact it's the filter....

Is it a reflection inside the filter (between the two surfaces), or is it rather one 
between the
filter and the field flattener (or both?)? 

Does the place of the filter in the light path or the distance between filter and 
flattener
have any influence on the amount of ghost images? 

By the way, I did not suspect the Tokai/IDAS filter being a cheap one... 
hm.... which one should I buy?
On the other hand, if ghost images only show up on 3 mag stars (and brighter),
it's something I might be able to live with ....

Matthias





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