Re: [APML] RE: Exposure formula. Was: Oh No! Kodak "Improves" Royal Gold

[Roland Roberts <roland@astrofoto.org>]

>>>>> "Bert" == Bert Katzung <katzung1@attbi.com> writes:

    Bert> How does your rule of thumb formula work for slower scopes?
    Bert> For f/7 it seems a bit on the long side: 14 x 8 = 112
    Bert> minutes? I assume this is _without_ an LPR filter?

Yes, it is without an LPR filter.  FWIW, my rule-of-thumb is not much
different than others I've read, e.g., the f^2 rule.  And while it
does seem rather slow, I can say that the one shot I took at f/8 for
20 minutes was way underexposed.

My shots last month (30 minutes at f/2.8) may be a little on the long
side for the site; they are definitely lighter than I expected, but I
do not think they are too far wrong.  Even so, you can do okay within
a factor of 2 on the exposure.  The f^2 rule would indicate 49
minutes; I tend to think twice that long is fine provided the skies
are dark.  With an LPR filter you can also go longer without
increasing the sky fog.

I'm new enough at this and haven't yet tried enough different
exposures to know for sure.  Perhaps next month I will try some longer
exposures; I have a couple of telephotos (135mm and 200mm) that will
need to be stopped down to at least f/5.6 and maybe f/8, so I'll have
to try a few long exposures to find out.

What I really hate is when you get those really *good* nights a few
exposures is all you can manage before the sun comes up.  By the time
I get the kids to sleep and then make sure I'm well aligned, it's
midnight.  At least the nights are getting longer and the kids start
going to sleep a little faster when it's already dark....

roland
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Roland B. Roberts, PhD                             RL Enterprises
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