Re: [APML] Red filters and the need to focus

["William R. Mattil" <wrmattil@ix.netcom.com>]

Brian larmay wrote:
> 
> Does anyone know whether I need to calibrate my lens
> for a deep red filter that is not H-a?
> 
> Im guessing mine is a #23 since there are no #'s on it
> besides R-2.
> Customer support is auomated and offers no help.
> 
> When does one ned to calibrate, and when does one not
> need to calibrate?

Brian,

I just ran a series of tests using an #82A filter to focus through and
then I substituted an Ha filter before starting the exposure. This was
at prime focus so that may have made it more critical ? or less so
depending on who you talk to :^) this was on an f/4 optical tube that is
nortorious for being picky in this regard. But actually determining the
exact point of focus with a camera lens is pretty easy using a label and
a two hole mask and then doing a series of shots to determine where the
best focus really is. John Mirtle has some information on this on his
web site IIRC.

But I think the real answer to your question is ... yes. It will make a
difference.


Regards

Bill


-- 
William R. Mattil	|                              
wrmattil@ix.netcom.com	| http://celestial-images.com      
(972) 399-4106		|
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