AW: AW: [APML] Achromats?

[a.roerig@t-online.de (Andreas Roerig)]

Hi Roland,

Thanks for your detailed explanation. I will probably use the achromat
for H-alpha work only. Actually I bought it just for surfing through the
milky way while I do long exposures with my C11.

Best regards


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-----Ursprüngliche Nachricht-----
Von: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org] Im
Auftrag von Chris1011@aol.com
Gesendet: Mittwoch, 10. Juli 2002 20:49
An: astro-photo@seds.org
Betreff: Re: AW: [APML] Achromats?


Although achromats have different focus points for red, green and blue,
these focus points are only for a very narrow range of these colors. For
blue especially, the focus changes very rapidly from blue-green to blue
to blue-violet. Blue-green focus at 486nm will be long by approximately
.056%, blue at 450nm will be long by 0.1%, blue-violet at 435nm will be
long by 0.24% and violet at 404nm will be long by 0.5%. So there really
is no one blue focus that works because most film is sensitive to all
these wavelengths - CCDs for sure are sensitive to the violet.

Roland Christen 


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