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

["westergren" <westergren@netzero.net>]

Brian,

I think you have to focus for the specific filter you are going to use.  The
focus shift for H-Alpha filters or #23 filters depends more on the thickness
and index of refraction of the filter than on the color in the visible
spectrum.  Many camera lenses have a different focus marked for infra-red,
but that is beyond the visible spectrum.

Don


> Hi Bill,
> I was going to wait for more replies, but I think ill
> step in regarding the calibration process and what I
> have done so far.
>
> Ive calibrated the 105mm already for H-a work using
> the 2 hole mask.
> This is the only lens in question.
> Would that calibration already be enough for this #23?
> filter?
> Do I even need to worry about calibration since its
> not an H-a filter?
>
> Ok.. maybe im just lacking some sort of understanding
> as far as the differance between H-a and non H-a
>
> So ok....Ive calibrated that lens..is that where I
> should use it at with any type of filter in the red
> spectrum?
>
> Pardon my lack of understanding.
>
> One more clear night and a moon thats at 97.1% ... I
> might just go for it since my clear nights are rare
> and have some results tonight of the Orion complex.
> I think I might really be pushing my chances, but
> hey.. why not eh' it will be warmer tonight in the mid
> teens<g>.
>
> Brian
>
>
> > 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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