Re: [APML] Bill Fletcher's Tri-color work(was 23a filter)

["Brian and Leau Larmay" <brileau@earthlink.net>]

----- Original Message -----
From: Philip Perkins <philip@astrocruise.com>
To: <astro-photo@seds.org>
Sent: Thursday, June 15, 2000 8:13 PM
Subject: Re: [APML] Bill Fletcher's Tri-color work(was 23a filter)


> Brian,
>
> >Personally I dont get why its any harder, in fact, I want to try it once
to
> >see what it is like also.
>
> I have never tried tri-colour photography but I believe I have learnt a
bit
> from listening to Chuck Vaughn.  I think you will find that tri-colour
> photography will be much harder.  Firstly you have to obtain a set of
> filters with the correct spectral response.  Then you have to calculate
> exactly how long each exposure needs to be, dependent on the spectral
> response and the transmission of the filter.  Then you have to take three
> separate exposures instead of one.  And each of those exposures will need
> to be much longer than a white light image (because each filter only lets
> through a small part of the spectrum, hence less light, hence longer
> exposure).  Then, unless you have quite a fast scope, the exposures may
> become so long as to be impractical (for example with an f/10 SCT the
> exposure times will be *literally* impractical).  Then because each
> exposure will be much longer it will show up flexure, focus, and guiding
> problems much more than a short exposure.  Finally, when you have got your
> three exposures you need to combine them in a program like Photoshop, and
> as part of that process you will need to balance them to obtain the
correct
> colour response.
>
> But don't let that put you off.. :)  The reason people do it of course is
> that there's nothing to beat it.  But it is something to be tackled once
> you have mastered the basics (which is why I haven't tackled it yet :)
The
> fact that there are so few people doing it is perhaps an indication of the
> difficulty - the results are fantastic, so if it was easy perhaps there
> would be lots of people doing it?
>
> >Tech Pan...thats a really fast film right?
> >How fast is fast?
>
> It's intrinsically a slow film, but when it is hypered it becomes a fast
> film (to put it simply).  Be aware that in this respect it is very
> different from modern day colour films - some of these don't need hypering
> at all, whereas Tech Pan absolutely has to be hypered to be at all usable
> for astrophotography.
>
> >Would the standard red, green, blue filters work by Cokin, or do they
have
> >to be a certain kind of RGB filters?
>
> No, the Cokin filters are intended only to give a colour cast to an
image -
> they would be nothing like accurate enough for tri-colour
astrophotography.
>  The filters need to have a very specific spectral response and the curves
> need to be right also.  I think that Chuck indicated that the filter set
> sold by SBIG is quite good in this respect.
>
> Regards
> --Philip

Hi Phil,
Thanks for the informative response....thats another thing I have to put
off, being that I am still trying to learn what the basics are.
Till then, I'll have to race around with the other things Im in the process
of learning.

Thanks again,
Brian



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