Re: (IAAC) filters

>Thanks for another two intriguing observation posts, Yann!
>I was particularly interested in your use of a light blue(?)
>color filter to enhance the contrast of the faint reflection
>nebula IC 432 in Orion! What other "color" filters have folks
>on the list found to be useful for bringing out these elusive
>reflection hazes? Also, do you happen to know the (rough)
>transmission characteristics of that W38A you used - was it
>a special high-transmission filter for visual use?
>Clear skies!
>Lew, gradually drowning under Leonids emails :)


I use blue filter for reflection nebula and also to sort ou central stars
in PN too embedded to see (N6826, N6210, ...) but the results so far are
not enough interesting, but I plan to try different filter.

The Wratten 38A I used is a classical one (I have corrected the curves
described here after in function of the nighttime eye sensitivity, ie
scotopic vision). Under these conditions, the filter transmits 10%
(percent) of light between 440nm and 540nm and has a peak at 35-40% at
about 500nm.

I also have the Wratten 47B which is darker: it has a peak of 10% in
transmission at 450nm; this one should be better for central stars but
maybe too strong for reflection nebulae.

I don't know if I already told the list, but I have found a much cheaper
UHC filter: its the Wratten 65A. He is dark green and transmits 10% between
470nm and 530nm with a peak of 45% at 500nm. It works great: of course the
transmission of 45% can't compete with the 70%+ transmission of the UHC but
at a fraction of the cost, it provides a good contrast gain on nebulae even
in a dark site so should be very profitable for city observers.

happy filtered views, Yann.

Yann Pothier	tel: 01 43 41 43 29
11 impasse Canart, 75012 PARIS, FRANCE
Email: ypothier@abi.snv.jussieu.fr
Site : http://pegase.unice.fr/~skylink/publi/cielextreme

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