Re: [APML] Poor Blue in 400F?

[Wei-Hao Wang <astrophoto@gmail.com>]

Hi Alen,

Thanks for the comments about my picture.

In general, reciprocity failure is a function of incident light,
temperature, pressure, and humidity.  Low incident light 
intensity is the primary cause of reciprocity failure.  Reciprocity
failure gets stronger under high temperature.  For many films, 
reciprocity failure disappears when it's close to dry-ice (solid
CO2) temperature.  This temperature dependence was first 
realized by professional astronomers.  They found that the 
photographic plates taken in summer and winter had different
sensitivities.

In addition, water and oxygen molecules in the emulsion promote 
reciprocity failure.  Therefore, reciprocity failure becomes weaker 
under low humidity and low pressure.

It's true that my picture was taken under a dark sky.  However, 
M45 is so bright that it doesn't request the darkest sky.  In your
case, I believe reciprocity failure is the key factor since you
confirmed that the shot was taken under humid and warm air.

Cheers,
Wei-Hao

On 8/23/05, Alen Koebel <akoebel@rogers.com> wrote:

> That's interesting. I hadn't heard about this
> relationship before. Yes, it was a warm and humid
> August night, whereas the shots on the previous roll
> that did turn out were back in November of last year,
> when it was cold (-8 C) and relatively dry.
> 
> I'd be willing to bet light pollution is also a
> contributing factor. You have very little on Mauna
> Kea; I have a lot where I am. A slightly hazy night
> seems to "intensify" the sky glow.
> 
> I should try the Pleiades again in October or November
> with the cluster positioned as overhead as possible,
> on a very transparent night (assuming one happens
> near new moon), autoguided for 1 hour and push the
> 400F one stop (as per general recommendations here).
> If I don't get a good result then, I'll hang up my
> AP hat! <g>
> 
> 
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-- 
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Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

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