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

[Philip Perkins <philip@astrocruise.com>]

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
Philip Perkins - philip@astrocruise.com
Wiltshire UK & Luberon France
Astrocruise - http://www.astrocruise.com

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